Levin The Earth Through Time 10th c2013 txtbk

Dinosaurs: “Terrifying Lizards” b 435

FIGURE 14-24 A Cretaceous street gang. From left, these giant predatory dinosaurs are
Daspletosaurus, Tyrannosaurus, and Tarbosaurus. Tyrannosaurus attained lengths of 13 meters
(42 feet) and weighed over 4 metric tons (the weight of two small pickup trucks). (J. Sibbick)

FIGURE 14-25 The
ostrich-like dinosaur
Struthiomimus. This
Cretaceous dinosaur not
only resembled a
featherless ostrich, but
probably lived in much
the same way. Note the
saurischian triradiate
pelvic bones. Label the
pubis, ischium, and illium
on the drawing.

436 c Chapter 14. Life of the Mesozoic

FIGURE 14-26 Deinonychus (“terrible claw”), another serious predator. Its large, serrated
teeth and a greatly enlarged “terrible claw” on the second digit of its hind feet made this
creature a killer. The size of its claw compelled the animal to run on only two toes. Shown here
are three Deinonychus attacking the large ornithischian Tenontosaurus. (J. Sibbick)

Prosauropods were quadrupeds, but their forelimbs Could these giants have been water dwellers? Sev-
were much shorter than the hindlimbs (Fig. 14-27). eral decades ago, paleontologists speculated that they
This indicates that they were able to rear up on their could not have supported their weight continuously,
hind legs to reach food higher on tree branches. This and might have lived in the buoyant waters of lakes and
ability also gave them a better view of their surroundings streams. But little evidence supports this. Sauropod
and menacing predators that might be lurking nearby. footprints and foot structure indicate they walked on
the tips of the toes on their front feet, with the heels of
Prosauropods lived from Late Triassic to Early their hind feet resting on large pads like those of
Jurassic. During the Early Jurassic they were replaced elephants. Clearly, they were land dwellers whose
by the famous giant sauropods. The most impressive massive limbs provided adequate support on land.
were colossal, long-necked, long-tailed animals that
required a sturdy four-legged stance to support their Being a giant was not without advantages for sau-
tremendous bulk. Apatosaurus (formerly Brontosaurus) ropods. Predators often avoid encounters with huge
is a sauropod favorite of schoolchildren, weighing 30 animals. In addition, great size in cold-blooded ani-
tons (about the weight of four African bull elephants). mals provides a way to reduce loss of body warmth. We
Another favorite is Brachiosaurus (Fig. 14-28). Although all know that a large pot of hot water takes more time
awesome in size, Apatosaurus and Brachiosaurus were to cool than a small pot. Similarly, a large cold-
relative lightweights when compared to the 80–100-ton blooded animal loses heat more slowly than a small
Supersaurus (Fig. 14-29). animal. This is because the ratio of surface area to
volume for an animal decreases as size increases. The
Sauropods were plant eaters. The majority were larger animal has a proportionately smaller surface
tree-top browsers whose long necks permitted munch- over which heat can be lost.
ing the upper foliage of trees. Brachiosaurus not only
had a long neck, but forelimbs that were longer than Sauropods roamed the Earth from Early Jurassic
hindlimbs, providing an even better ability to reach the until the end of the Cretaceous. Their “heyday” (when
topmost foliage. To avoid the burden of a heavy they were most diverse and abundant) was during the
weight on the neck, the heads of sauropods were Jurassic to Cretaceous transition. Three prominent
relatively small. Teeth were nonserrated and either families were the diplodocids, camarasaurids, and
peglike or spatulate (spoonlike) in shape. In spite of brachiosaurids. Diplodocids like Diplodocus had long
their huge dimensions, the brain size of sauropods was slender skulls and slender peglike teeth confined to
nothing to boast about. A large sauropod had a cranial the front of the mouth. Camarasaurids (such as
capacity approximately the size of your clenched fist. Camarasurus) differed from diplodocids in their short,

Dinosaurs: “Terrifying Lizards” b 437

FIGURE 14-27 Plateosaurus, a Late Triassic prosauropod. Very likely it could rear up on its
hind legs to reach food and to spot predators. (J. Sibbick)

FIGURE 14-28 Huge
Jurassic sauropod
Brachiosaurus. Displayed at
the Field Museum in Chicago.
(Harold Levin)

438 c Chapter 14. Life of the Mesozoic

FIGURE 14-29 A colossal
trio: Sauropods, the
largest land animals to
evolve. Seismosaurus (left)
with two Supersaurus. The
Supersaurus at center was
formerly named Ultrasaurus
but subsequently was
identified as a Supersaurus.
( J. Sibbick)

blunt-snouted skulls and spoon-shaped teeth. Brachio- availability for mating or for alerting the herd to
saurids are distinguished by their longer forelimbs dangers. The dinosaur-populated plains and forests
than hindlimbs. of the Cretaceous may have been alive with a variety
of vocal trumpetings, growls, and roars.
Bird-Hipped Plant-Eaters (Ornithischians)
With nearly 100 species already identified, ornith-
The other major dinosaur line, the Ornithischia, opods were a very diverse group. Most species are
evolved near the end of the Triassic and thrived placed into one of four prominent ornithopod families:
throughout the following Jurassic and Cretaceous. All namely the Heterodontosauridae, Hypsilophodon-
of the ornithischians were herbivores. They included tidae, Iguanodontidae, and Hadrosauridae.
both bipedal and quadrupedal varieties, with the biped-
als considered more primitive. Even the most advanced “Heterodontosaur” means “different-tooth lizard.”
quadrupeds, however, had shorter forelimbs than The name reminds us that these dinosaurs had teeth
hindlimbs, indicating their ancestors were bipedal. that differed in size and shape. In particular, there was
a pair of prominent tusk-like teeth in the lower jaw.
ORNITHOPODS (MOSTLY BIPEDAL ORNITHISCHIANS) The Heterodontosaurus was a small (1.2 meters long), bipedal
primarily bipedal ornithischians are known as ornitho- dinosaur that fed on ground-level vegetation.
pods. Their evolutionary history began in the Early
Jurassic and continued for approximately 125 million Hypsilophodonts, like Hysilophondon (Fig. 14-30)
years until the end of the Cretaceous. During the Creta- lacked the tusklike teeth of heterodonts, and were
ceous, they overtook and replaced the sauropods as
dominant terrestrial herbivores. To the delight of ver- FIGURE 14-30 Hypsilophodon. This swift-running and agile
tebrate paleontologists, ornithopods have a splendid ornithopod lived during the Late Jurassic and Early
fossil record. That record not only includes the skele- Cretaceous. The skull of Hypsilophodon was generally similar
tons, which permit reconstruction of the anatomy of to Heterodontosaurus, but lacked tusks. In this John Sibbick
ornithopods, but also eggs, embryos, and juveniles that painting, the group is about to be attacked by the theropod
provide insight into ornithopods’ reproduction and Allosaurus. (J. Sibbick)
growth. Like birds, ornithopods laid their eggs in nest-
like structures and brought food to their hatchlings until
they were able to leave the nests. Ornithopod coprolites
(fossil dung) provide a glimpse of what kind of
plants were consumed, and trackways preserved in
sedimentary rocks reveal that many ornithopods moved
about in large herds. Some ornithopods sported crests
that were probably attractive to members of the
opposite sex. In some crests, tubular openings to the
nasal passages allowed the animal to trumpet their

Dinosaurs: “Terrifying Lizards” b 439

FIGURE 14-31 An adult and juvenile Iguanodon. FIGURE 14-32 Cretaceous duck-billed dinosaurs
Iguanodons were large, heavily built ornithischians that lived (hadrosaurs). Displayed at the American Museum of
during the late Jurassic and Early Cretaceous. Although Natural History in New York. (Harold Levin)
basically bipedal, the animal also spent some of its time
walking on all fours. A distinctive feature of Iguanodon was its The Hadrosauridae are sometimes dubbed the
thumb claw, which was modified into a huge spine that could “duckbill dinosaurs” because of their expanded, tooth-
be used in defense. (J. Sibbick) less ducklike bills (Fig. 14-32). Behind the toothless
forward part of the jaws were hundreds of interlocking,
somewhat larger (3.6 meters in length). They were able to lozenge-shaped teeth cemented together to form a rasp-
run swiftly on hind legs while keeping their counter- like grinding surface. Coupled with powerful jaw
balancing tail horizontal. Remains of Hypsilophodo- muscles, these dental batteries were capable of grinding
ntidae are found in Middle Jurassic to Late Cretaceous very coarse vegetation. Parts of twigs and pine needles
rocks of North America, Eurasia, and Australia. can be recognized in the coprolites of hadrosaurs.

The Iguanodontidae lived during the Late Jurassic The hadrosaurs made their debut during the Middle
and Early Cretaceous. They take their name from Cretaceous. They included herbivores that had crests
Iguanodon (Fig. 14-31), one of the first dinosaurs to on the top of their heads (lambeosaurines) and those
be described scientifically. A country doctor named lacking head crests (hadrosaurines). The crests on the
Gideon Mantell provided the name in 1825. Iguanodon lambeosaurines (Fig. 14-33) contain tubular extensions
is sometimes called the “thumbs-up dinosaur” for the of the nasal passages (Fig. 14-34).
horny spike that substituted for a thumb. Dr. Mantell
made a mistake in his sketch of Iguanodon by placing As noted earlier, these crests could have been used
the spike on the beast’s nose. In general, iguanodonts to catch the eye of potential breeding partners, or as
were huge, robust herbivores with powerful hindlimbs, vocal resonators. Computer simulations suggest that
horselike skulls, and toothless beaks. Leaves and stems air forced through the tubing inside some crests may
were cropped with the forward beaklike part of the have produced a sound rather like that make by a
jaws and passed back to the teeth for chopping and trombone.
chewing. The relatively long and robust forelimbs of
many iguanodonts and the presence of small hooves on Thyreophorans: Stegosaurs and Ankylosaurs
some of the fingers indicate they were facultative quad-
rupeds, meaning that they were comfortable feeding Thyreophorans means “shield bearers.” The name
and walking on all fours when it was necessary. refers to the heavy armor of plates, scutes, and knobs
that protect these animals from attacks by theropods.

440 c Chapter 14. Life of the Mesozoic

FIGURE 14-33 Bellowing resonators? Skulls of B
Cretaceous crested hadrosaurs (“duck-billed”
dinosaurs). (A) Hadrosaur Lambeosaurus skull with a FIGURE 14-34 (A) Skeleton of Parasaurolophus on
peculiar hatchet-shaped crest. (B) Corythosaurus with a display at the Royal Tyrrell Museum of Paleontology,
helmet-shaped crest. These crests may have functioned as Alberta, Canada. (B) Internal structure of the skull crest of
vocal resonators for bellowing. Skulls are approximately 0.75 Parasaurolophus. The bone on the left side of the crest has
meter (30 inches) long. (Courtesy of the U. S. National been removed to expose the left nasal passage (n). Air entered
Museum of Natural History, Smithsonian Institution) the nostrils at (a), moved up and around the partition in the
crest, and from there down and back to openings in the
palate. When models of the passages are constructed, they
can be used to generate sounds like those of a trombone.
Parasaurolophus may have made such sounds to attract a mate.
((A) Francis Gohier/Photo Researchers, Inc.,
(B) J. A. Hopson, 1975, Paleobiology, 1:24)

The bony armor was developed in the skin and was not museum shops. The vertically oriented plates that
attached to the skeleton. Stegosaurs and ankylosaurs are extend along the midline of the neck, back, and tail
the two major groups of thyreophorans. Stegosaurs are the most readily recognized characteristics of Stego-
lived from Middle Jurassic until Early Cretaceous. saurus. These plates probably functioned in the regula-
They were the dominant herbivorous Jurassic ornithi- tion of body temperature. Indeed, their arrangement,
schians. Occasional remains of ankylosaurs have been size, and shape, plus the presence of branching grooves
found in Late Jurassic rocks, but they became much for blood vessels, all favor this interpretation. Heat
more numerous during the Cretaceous. Thyreophorans could be absorbed from the environment when body
had beaks at the front of their jaws that had a horny temperature was low, and radiated back into the envi-
covering and were the main tools for cropping plants. ronment when the animal was overheated.

STEGOSAURS The best known stegosaur is Stegosaurus Stegosaurus also had two pairs of heavy spikes at the
(Fig. 14-35), plastic models of which are best-sellers in tip of its tail. When swung from side to side the spikes
might have impaled an enemy approaching from

Dinosaurs: “Terrifying Lizards” b 441

FIGURE 14-35 “Plated dinosaurs” of the Jurassic. Best known of this group is Stegosaurus
(upper left), weighing 1–2 tons. Its relatively small head terminated in a toothless, narrow beak
suitable for cropping plants. Chewing plant food was the task of leaf-shaped teeth in its cheek
regions. Although a large animal, its brain weighed about 2.6 ounces (the size of a small potato).
Its distinctive diamond-shaped plates were covered with grooves and canals that mark the
location of blood vessels. These plates may have functioned in temperature regulation,
somewhat like solar panels and radiators. Other stegosaurs shown are Tuojiangosaurus (upper
right), Dacentrurus (lower right), Lexovisaurus (lower center), and Kentrosaurus (lower left).
(J. Sibbick)

the rear. The short forelimbs and longer massive are the ankylosaurs. There are two groups of ankylo-
hindlimbs gave Stegosaurus a forward tilt, facilitating saurs, the ankylosaurines and the nodosaurines. Both
its ability to feed on low-growing vegetation. groups were heavily armored, rather like the dinosaur
version of military tanks. When viewed from above, the
Measuring 7 to 8 meters (23 to 26 feet) in length and skull of an ankylosaurine had a distinctively triangular
weighing a couple of tons, Stegosaurus was a huge shape. Triangular horns were present on either side at
animal, yet it possessed a very small brain. If you the rear of the head, and the nasal opening faced toward
were to compare the brain of a living lizard that was the front. A large bony club at the tip of the tail could be
enlarged to the size of a stegosaur, the brain from the swung from side to side like a bludgeon to discourage
“enlarged lizard” would be about twice the size of the predators (see Euoplocephalus, Fig. 14-36).
stegosaur brain. However, the stegosaurs seem to have
gotten along very well with their diminutive brain. As seen in Edmontonia (Fig. 14-36), nodosaurines
lacked the heavy tail club of ankylosaurines. The head
As depicted in Figure 14-35, Stegosaurus was not the was more elongate, and the nasal openings were placed
only stegosaur that roamed Jurassic landscapes. laterally. Unlike the ankylosaurines that had folded,
Among others was Tuojiangosaurus from China and S-shaped nasal passages, the nasal passages of nodosaur-
Kentrosaurus from Tanzania. European stegosaurs ines were simple tubes that ran from the nostrils to the
include Dacentrurus and Lexovisaunis. throat. The more complex nasal passages of ankylosaur-
ines may have helped moisten air being drawn into the
ANKYLOSAURS (ANKYLOSAURINES AND NODOSAU- lungs, or possibly improve the animals’ ability to smell.
RINES) The second major group of thyreophorans

442 c Chapter 14. Life of the Mesozoic

FIGURE 14-36 Ankylosaurs, the heaviest-armored Cretaceous dinosaurs. These bulky, squat
ornithischians wore closely fitted bony plates that protected their entire 6-meter backsides. Their
small heads sometimes were covered with armor. Euoplocephalus (top) had a bony tail club that may
have been defensive, perhaps by damaging the legs of an attacking theropod. Nodosaurids like
Edmontonia (foreground) were somewhat less ponderous and could move about more quickly.
(J. Sibbick)

Marginocephalia: Pachycephalosauria genus Psittacosaurus, the “parrot dinosaur.” The nick-
and Ceratopsia name is appropriate, for Psittacosaurus had a short snout
and beak rather resembling that of a parrot. The frill was
The term Marginocephalia means “rimmed head,” and so small as to be hardly noticeable. Psittacosaurus was a
refers to a rim of bone at the rear of the skull. In pachy- relatively small bipedal dinosaur (a couple of meters
cephalosaurs the rim is very small, but in ceratopsians is long) capable of running swiftly on its long hindlimbs.
developed as a large and very prominent feature.
A piece of preserved skin from Psittacosaurus was
PACHYCEPHALOSAURIA The pachycephalosaurs discovered in 2008 in China. The fossil revealed this
(translated as “thick-headed lizards”) take their name small ceratopsian had a remarkably thick skin, com-
from the amazingly thick bones of the skull roof, giving posed of over 25 layers of tightly packed collagen
it a domelike appearance (Fig. 14-37). These dinosau- fibers. Collagen is a protein that protects the skin
rian bone heads were bipedal Late Cretaceous herbi- against strain, rupture, and puncture. The presence
vores that apparently used their thick skulls to engage of so many collagen layers indicates a tough skin that
in head-butting, a behavior seen in mountain goats, provided protection against predators. Psittacosaurus, it
sheep, and some antelopes. Like these living herbivores, appears, “was one tough cookie.”
head-butting was important in the competition for
mates and territory. Evidence for this interpretation The Neoceratopsia differ from the Psittacosauridae
includes not only the massive boney construction of the in their quadrupedal gait, larger heads, prominent
skull that protected the brain, but the angle of the skull frills, and sharply keeled and pointed beaks. The first
relative to the spinal column, the shortened base of the neoceratopsians to appear are called protoceratopsids,
skull, strong neck vertebrae, and ossified tendons along after Protoceratops from the Late Cretaceous. Unlike
the backbone (to alleviate the impact that would be the earlier Psittacosaurus, Protoceratops was a quadruped
passed to the vertebrae). with massive limbs that were almost equal in length. It
also had a larger skull and much longer frill. Proto-
CERATOPSIA There are two groups of ceratopsians, ceratops lacked the horns for which the ceratopsia are
the Psittacosauridae and the Neoceratopsia. The named. Their skulls differ somewhat in size and shape,
former group is represented by the Early Cretaceous indicating differences between males and females.

During the closing stages of the Mesozoic,
small ceratopsians like Protoceratops were replaced by

FIGURE 14-37 The “bone-head” dinosaur, Dinosaurs: “Terrifying Lizards” b 443
Pachycephalosaurus. Its skull is mostly solid bone, with a tiny
brain space. Perhaps they used their skulls as battering rams rhino-sized dinosaurs with huge skulls, prominent
against one another during competition for territory or shield-like frills, and a variety of horns. Some of the
mates. Abrasions on skulls and tendons that braced the neck best known examples are Triceratops with its three
vertebrae support this interpretation. Similar head-butting is prominent horns and solid frill, Pachyrhinosaurus
seen today in bighorn sheep. (U.S. National Museum of with its more decorative frill, and Styracosaurus
Natural History/Smithsonian Institution) with huge spines along the upper margin of its frill
(Fig. 14-38). Kosmoceratops would win a contest for
bizarre horns. It had two sideward projecting horns on
its brow, and a frill adorned with ten horns curved
downwards like hooks. They would have been of little
use in defense, but may have had a role in attracting
mates. All of these great beasts had complex dental
batteries, a beaklike bone at the front of the snout that
gave their upper jaws a parrotlike profile, and a skull
shape that was narrow at the beak and flaring in the
cheek region.

You can understand the horns having display and
defensive functions, but what purpose was served by
the frills? Might it have protected the animal from a
frontal attack by a large theropod? Judging from the
tooth marks that mark the frill bones of some Tricera-
tops fossils, encounters with theropods was not
uncommon. It is not unlikely, that a huge, horned
Triceratops would not have been easy for a predator to
bring down. Many reached lengths of 8 meters (28
feet) and weighed more than 9 tons.

The bone beneath the skin of many ceratopsian
frills was not a continuous plate. Rather, it contained a
pattern of large and small holes or fenestra. The larger
fenestra would have reduced the frill’s weight and
hence pressure on the animal’s neck (some of the
neck vertebrae in ceratopian were fused together to
help hold the weight of the head). Perhaps the smaller
fenestra accomodated tissue rich in blood vessels
whose function was to help the animal radiate excess
body heat, or if needed, absorb heat from the environ-
ment. Other possible functions for ceratopsian frills
include display for attracting breeding partners, or in
ritualized combat with other males so as to establish
dominance in the herd.

FIGURE 14-38

Ceratopsians: horns, huge
heads, parrotlike beaks.
These beasts take their
name from horns that grew
on the face of all but the
earliest forms. From left: the
well-known Triceratops,
Pachyrhinosaurus, and the
multihorned Styracosaurus.
(J. Sibbick)

444 c Chapter 14. Life of the Mesozoic the body. Dinosaur stance resembles that of mammals
and birds, does this indicate that dinosaurs were endo-
cDINOSAURS: COLD-BLOODED, thermic? Possibly, but dinosaur posture in some
WARM-BLOODED, OR BOTH? groups may have been an evolutionary solution for
supporting enormous weight. Legs held vertically
Since the late 1700s, when they were first studied under the body would support more weight than if
scientifically, dinosaurs were regarded as reptiles, and the legs extended toward the sides.
therefore cold-blooded (ectothermic). With the dis-
covery in recent years that certain dinosaurs share many Bakker and others also use bone histology to support
features with birds, some paleontologists favor combin- their belief in dinosaur endothermy. Bone histology
ing birds and dinosaurs into a group separate from the involves the study under a microscope of thin sections
reptiles which they name Dinosauria. Dinosaurs also of bone. Some dinosaur bones are richly vascular, like
retain many reptilian traits, and so the debate rages over the bones of mammals. This is in contrast to the bones
whether they were ectothermic like modern reptiles, of most living reptiles, which are less vascular, and
endothermic like birds, partially endothermic, or indicative of a poorer supply of blood to bone tissue.
whether the degree of endothermy or ectothermy varied However, once again the correlation is not absolute.
among specific groups. Some bones in living crocodiles, as well as certain
turtles and lizards, have considerable vascularity, yet
Ectothermic animals have little or no ability to main- these animals are primarily ectotherms. Perhaps vas-
tain uniform body temperature through their own cular bone in dinosaurs indicates a more active lifestyle
physiologic processes. However, they may affect their or reflects growth rate rather than degree of warm-
body temperature by seeking either sun or shade in bloodedness.
response to temperature needs. In living reptiles, the
pineal gland may play a role in directing this behavior. Another line of evidence in the dinosaur ectothermy
versus endothermy debate relates to the presence or
In extinct reptiles, certain anatomic features—such as indications of membrane-covered bone or cartilage
the sail in Dimetrodon, the plates on the back of Stegosau- coils in the nasal passages. These structures are called
rus, or the frill on Triceratops—may have served two respiratory turbinates and their function is to reduce
opposite temperature-regulating purposes: to absorb water and heat loss associated with rapid rates of lung
heat from the sun’s rays when the body needed warming, ventilation. Such rapid rates are characteristic of endo-
and to radiate excessive heat when cooling was needed. therms like birds and mammals. These endotherms
also have larger nasal cavities in order to accommodate
In contrast, endothermic (warm-blooded) animals the respiratory turbinates. If dinosaurs had large nasal
such as mammals and birds maintain a constant body cavities, then one might surmise that they possessed
temperature by internal production of heat and the turbinates and were endothermic. However, such
radiation of excess heat away from the body. Mammals dinosaurs as Tyrannosaurus and Ornithomimus have
produce heat by oxidizing food. However, when body narrow nasal passages, implying they were probably
temperature rises, the hypothalamus (part of the brain) ectotherms or near ectotherms.
triggers internal mechanisms to dissipate the heat,
including expansion of blood vessels in the skin, per- Can isotope analysis of bone help solve the prob-
spiring, or (in furry animals), panting. When temper- lem? Cold-blooded animals exhibit large differences in
aures fall, other mechanisms minimize heat loss, such the oxygen isotope content of bones of the extremities
as restriction of blood vessels in the skin and shivering. versus bones of the body core. Warm-blooded verte-
brates do not exhibit this variation. Analysis of bone
In the 1860s, biologist Thomas Huxley (1825–1895) from Cretaceous theropods, ceratopsians, and hadro-
suggested that dinosaurs might have been warm- saurs show isotope variability similar to that in warm-
blooded (endothermic). This idea was forcefully revived blooded vertebrates.
in the 1970s by paleontologist Robert Bakker, who
became an enthusiastic supporter of the warm-blooded Yet another argument for dinosaur warm-blooded-
dinosaur concept. Bakker knew that birds and dinosaurs ness is correlation between the proportions of preda-
have many similarities. If dinosaurs were truly endo- tors to prey in mammals (endotherms) as opposed to
thermic, it would be logical to reclassify vertebrates by living reptiles (ectotherms). Today’s warm-blooded
removing dinosaurs from the reptile category and build- communities are about 3% predators and 97% prey
ing a new classification that would include dinosaurs (plant-eaters). Clearly, it takes a lot of food to fuel the
and birds under one designation. In this reclassification, energy requirements of an endothermic predator such
dinosaurs still “live” today—they are birds. as a lion or wolf. In the cold-blooded community, 33%
of the animals are predators and 66% are prey. Deter-
Bakker supported his hypothesis of warm-blooded mining the proportion of predators to prey among
dinosaurs with several other lines of evidence. One dinosaurs is tricky because the fossil record cannot be
related to the way dinosaurs stood and walked. as precise as data obtained from living communities. At
Today’s lizards and salamanders are cold-blooded present, the evidence is inconclusive.
and have a sprawling stance, with their limbs directed
toward their sides. In contrast, the limbs of warm-
blooded mammals and birds are held directly beneath

Dinosaur Parenting b 445

FIGURE 14-39 Maiasaura nesting area as
interpreted from the remains of nests, eggs,
and juveniles found in the Late Cretaceous
Two Medicine Formation of Montana.
( J. Sibbick)

Clearly, the question of dinosaur endothermy or ecto- after they were laid? Did they nurture the hatchlings?
thermy cannot be universally resolved at this time. It is Discoveries made during the last few decades in
likely that some dinosaurs were primarily ectotherms, Montana and Mongolia show that at least some
some endotherms, and some partially one or the other. dinosaurs cared for their young.

cDINOSAUR PARENTING Dinosaur eggs from Montana occurin the Cretaceous
Dinosaurs reproduced by laying eggs. Clutches of Two Medicine Formation in the western part of the
dinosaur eggs have been found at dozens of localities state. Dubbed “Egg Mountain” by its discoverer John
around the world. But did dinosaurs care for the eggs (Jack) R. Horner, the fossil site includes an entire hatch-
ery of hadrosaurian dinosaurs complete with nests,
clutches of eggs, embryos, and nestlings (Fig. 14-39).

ENRICHMENT

Can We Bring Back the Dinosaurs? sections would be decomposed or destroyed. It would be
necessary to reconstruct missing parts, filling the gaps by
Imagine visiting a zoo or theme park and safely viewing a specifically and repetitively copying segments between
living Tyrannosaurus or Triceratops. Michael Crichton defined nucleotide sequences.
described such a place in his 1990 science fiction novel
Jurassic Park, which appeared three years later as a Holly- If we make the extravagant assumption that we would be
wood movie. The novel describes a dinosaur-populated able to produce a dinosaur gene segment, or even a few
theme park constructed on an isolated tropical island. hundred segments, these would still represent only a tiny
Dinosaurs dwelling in the park are copies of real animals, fraction of the billions of segments that once were present in
produced by cloning of their Mesozoic counterparts. complete dinosaur DNA. Further, there are specific proteins
that coat chromosomes and bind to key locations. These
Cloning has been accomplished in several kinds of living proteins govern how genes are expressed. Without them, the
animals. The process involves transplanting DNA from a chromosomes are ineffective.
somatic cell (a body cell, not involved in reproduction) to
an egg that has been stripped of its nucleus. With DNA from But assume that all of the above problems are solved and
the animal to be cloned, the egg completes development we have well-preserved DNA with correctly linked proteins.
without fertilization. The result is a precise copy of the When the DNA is implanted into the egg of another species,
animal that contributed the DNA. will it develop into a dinosaur embryo? Eggs are not just
passive containers waiting to receive DNA from any provider.
In the Jurassic Park tale, the DNA was obtained from They contain specific directions about how cell division is to
dinosaur blood extracted from Mesozoic mosquitoes pre- proceed and how the embryo is to be positioned within the
served in amber (see Fig. 6-7). Missing DNA segments were confines of a shell of particular size and shape. It is
added from the DNA of frogs. improbable that the egg of a living reptile or bird would so
closely resemble that of a Tyrannosaurus rex to allow com-
The premise of Jurassic Park is clever, but we have no way plete development.
of knowing whether Mesozoic mosquitoes sucked dinosaur
blood. Perhaps they preferred the blood of the small furry We have witnessed some truly astonishing advances in
mammals that were scurrying about. But assuming that they molecular biology over the past several decades. Maybe
did bite dinosaurs, would blood extracted from the belly of an someday, some of the seemingly insurmountable obstacles
amber-encased mosquito come from one species of dino- to dinosaur cloning may find solutions. For the foreseeable
saur, or from two or more? Some spectacular DNA forensics future, however, don’t expect to see dinosaurs on your next
would be required to make the necessary identifications. visit to the zoo.

Another problem is the extreme improbability that DNA
could survive intact for tens of millions of years. Lengthy

446 c Chapter 14. Life of the Mesozoic

There is evidence that hadrosaur babies were nurtured belong to the ceratopsian dinosaur Protoceratops. How-
by their parents, lived within the social structure of ever, on top of the nest that Norell found were the
large herds, were warm-blooded, and in general behaved bones of Oviraptor, so-named because that theropod
more like birds than like today’s reptiles. was assumed to have died while attempting to steal or
eat the eggs. Actually, Oviraptor was falsely accused,
The hadrosaurs hollowed out bowl-shaped nests in for the eggs were its own. It died while protecting or
soft soil and laid about 20 eggs in neatly arranged circles incubating them. As evidence of this, several Oviraptor
within each nest. Plant impressions in the sediment skeletons were recently found squatting over their
suggest that the eggs were covered with decaying vege- nests in precisely the manner of birds.
tation so that its fermentation would provide warmth.
Horner observed that many of the nests contained the cFLYING REPTILES
bones of juveniles that were about a meter long. Thus, This same pattern repeats throughout the history of life:
the babies (which were only about 30 cm long when A small group of animals initially adapts to a narrow
hatched) stayed in their nests where food was brought to range of ecologic conditions. Then their descendants,
them until they had grown sufficiently to fend for through many generations of evolutionary processes,
themselves. In addition, the teeth of these juveniles spread into different environments. As the descendants
exhibited distinct signs of wear, suggesting that they diverge from their ancestral lineage, they change in ways
had been feeding for some time while still in the nest. that make them fit their new surroundings. The process
Horner named these dinosaurs Maiasaura from the is called adaptive radiation.
Greek for “good mother lizard.” Adaptive radiation is well demonstrated by the
Mesozoic vertebrates. We have seen the remarkable
The gentle plant-eating hadrosaurs were not the only adaptive radiation of dinosaurs that resulted in a rich
dinosaurs to “sit” their nests like birds. Predators appear variety of land animals adapted to live on flesh, plants,
to have had the parenting instinct as well. In 1993, Mark or both. Opportunities for making a living, however,
A. Norell discovered a nest of dinosaur eggs in Creta- also existed overhead in the sky, and certain lineages of
ceous rocks of Mongolia’s Gobi Desert. Within the nest reptiles evolved ways to glide and fly.
was the nearly complete skeleton of an embryonic
theropod. The tiny dinosaur was about to hatch and Permian and Triassic Gliders
resembled a miniature adult. It was readily identified as
the embryo of the predatory dinosaur Oviraptor. The first reptiles to attempt flight probably were
gliders and not true “wing-flappers.” They were
The fossil eggs found by Norell are identical to
those found in 1922 in the Gobi Desert in Mongolia
by famous fossil hunter Roy Chapman Andrews
(Fig. 14-40). At the time, the eggs were thought to

FIGURE 14-40 Fossil dinosaur eggs
from the Upper Cretaceous of
Mongolia. Discovery of eggs containing
fully formed embryo skeletons indicates
that the eggs belonged to the theropod
dinosaur Oviraptor. (U.S. National
Museum of Natural History/Smithsonian
Institution)

Flying Reptiles b 447

FIGURE 14-41

Sharovipteryx, a primitive,
gliding, diapsid reptile from
the Triassic of central Asia.
The flight surface in this small,
lightly built archosaur was a
thin skin extending from hind
legs to tail. Sharovipteryx could
manuever while gliding by
changing the position of its
hind limbs. From snout to tail
tip, this early glider was only
about 24 cm long. (J. Sibbick)

similar to present day “flying lizards.” For such ani- terminated in a diamond-shaped vane (Fig. 14-42).
mals, gliding was an efficient way to move from branch The most advanced pterodactyloids were tailless, as
to branch or tree to tree. These aerial acrobats first seen in Pteranodon (Fig. 14-43). Much like large sea
appear in the Permian where they are represented by birds today, Pteranodon probably soared above the
Coelurosauravus. The skin membranes that served as waves, snapping up sea creatures in their toothless
the “wings” in Coelurosauravus were supported by at jaws. Relative to body size, pterosaurs had somewhat
least 22 long, slender bones extending outward from larger brains than their land-dwelling relatives. Perhaps
each side of the body. At one time these thin bones this was the result of a higher level of nervous system
were thought to be ribs, but a fossil discovered in 1916 control and coordination needed for flight.
revealed that they did not attach to the rest of the
skeleton. When not in use as an aerodynamic surface, The first prize for size among pterosaurs goes to the
the “wings” could be closed like a Japanese fan. huge Quetzalcoatlus (Fig. 14-44). With a wingspan of
12 meters (35 feet), Quetzalcoatlus (named after the sky
Mecistotrachelus, recently discovered in Triassic rocks god of the Aztecs) was the size of a small airplane. It
along the Virginia–North Carolina border, had similar was able to catch thermals and glide for thousands of
membrane-covered “wings.” The creature was about miles without expending a lot of energy. Although
the size of a robin, and had a “wing span” of about 30 cm Quetzalcoatlus efficiently soared the skies, limb bones
(about a foot). Another Triassic glider was Icarosaurus, and fossil footprints indicate it was quite capable of
named for Icarus of Greek mythology, who used home- walking on land. Small dinosaurs and reptiles were its
made wings to fly. One of the more unusual Triassic probable prey.
gliders was Sharovipteryx (Fig. 14-41), from Kyrgystan.
Sharovipteryx had winglike skin membranes attached to If Quetzalcoatlus was the largest pterosaur,
the rear edges of both its front and rear limbs. Pterodaustro, with its strainer-teeth, was the most
unusual (Fig. 14-45). Apparently, Pterodaustro fed
Dragons of the Sky: The Flying Pterosaurs itself by dipping its curved beak into the water to
filter out crustaceans and other small aquatic crea-
The pterosaurs were highly successful aerial reptiles, tures. The smallest pterosaur was a tiny, toothless
dominating the skies for over 100 million years—from creature named Nemcolopterus crypticus from
the Late Triassic to the Late Cretaceous. The most China. This tiny flyer had a wingspan of only 25 cm
familiar Jurassic and Cretaceous pterosaurs were long- (10 inches).
jawed, with large heads and eyes. In most forms, the
jaws were lined with thin, slanted teeth. The bones of Some pterosaurs had a covering of soft hair,
the fourth finger were lengthened to help support the prompting the hypothesis that they were warm-
wing, whereas the next three fingers were of ordinary blooded. This would be an important adaptation
length and terminated in claws. The wing was a sail of for flying vertebrates. Without a regulated body tem-
skin, stretched along the lengthy fourth digit, sides of perature, cold air would limit their power of exertion,
the body, and (in most groups) the hindlimbs. and they might have difficulty staying aloft. In one of
these flyers, Sordes pilosus (“hairy devil”), the fur is
There were two general groups of pterosaurs. The longest on the animal’s underside, prompting specu-
rhamphorhyncoids evolved first, with long tails that lation that it also served to incubate eggs or insulate
hatchlings.

448 c Chapter 14. Life of the Mesozoic

FIGURE 14-42

Pterosaurs, most famous
of the flying reptiles.
Eudimorphodon (right
foreground) was a
rhamphorhyncoid with
long, sharp teeth to catch
and hold slippery fish. The
creature was about 60 cm
long. Also shown is
Peteinosaurus (left), with a
rudderlike membrane or
vane at the end of its tail.
(J. Sibbick/Salamander
Picture Library)

cDRAGONS OF THE SEAS water, paddle-shaped limbs to replace feet, and highly
Nothosaurs and Placodonts efficient lungs to hold air while submerged.

The marine habitat is one in which the archosaurs Marine reptiles with paddle-shaped limbs were
were not notably successful. Only one archosaurian already present during the Triassic Period. One group,
group, the sea crocodiles, was able to invade the the nothosaurs, were just beginning to show adapta-
oceanic environment (Fig. 14-46). However, other tions that would be perfected in their descendants, the
groups adapted well to life in the sea: ichthyosaurs, plesiosaurs.
plesiosaurs, mosasaurs, and sea turtles (Fig. 14-47).
Many adaptations were needed to change a land animal Nothosaurs were joined in the Triassic by a group
into an ocean dweller, including the evolution of of mollusk-eating flippered reptiles known as placo-
streamlined bodies for efficient movement through donts (Fig. 14-48). These bulky animals had distinc-
tive pavement-like teeth in the jaws and palate. The
teeth were perfectly adapted for crushing the shells of
the mollusks on which they fed.

FIGURE 14-43 The Cretaceous pterodactyl Pteranodon. Plesiosaurs
Pteranodon had a large wingspan over 7 meters (23 feet), but
its body was only goose-sized. Its skeleton was lightly Plesiosaurs were paddle swimmers with large, many-
constructed, as required of any aerial vertebrate. The crested boned flippers and slender curved teeth that seemed
point at the back of the skull had branching channels thought perfect for ensnaring fish. Their bodies were short and
to have contained blood vessels. Blood flowing through the broad, and in some species the neck was extraordi-
crest could have been cooled as air rushed over the crest or narily long. Elasmosaurus, a widely portrayed long-
warmed in the morning sun. Thus, the odd-looking necked Cretaceous plesiosaur (Fig. 14-49), attained
pterosaur crest may have functioned in temperature an overall length of 12 meters (40 feet).
control. (Joe Tucciarone/Science Photo Library/Photo
Researchers, Inc.) Conversely, some plesiosaurs had short necks that
supported large heads. It is likely that the short-necked
models were aggressive divers. Kronosaurus, a giant,
short-necked form from the Lower Cretaceous of
Australia, had a 3-meter-long skull that probably holds
the record for any known reptile.

Ichthyosaurs

The most fishlike of marine reptiles were the Triassic
to Early Cretaceous ichthyosaurs (Fig. 14-50). In

The Rise of Modern Birds b 449

FIGURE 14-44 The gigantic Cretaceous pterosaur Quetzalcoatlus. With an astonishing
wingspan of about 12 meters (40 feet), Quetzalcoatlus probably flew much like a modern condor,
using thermal air currents and winds to help keep it aloft. It was named for an Aztec god who
took the form of a feathered serpent. (J. Sibbick/Salamander Books)

many ways, they were the reptilian counterparts of Sea Turtles
present-day toothed whales. Ichthyosaurs had fishlike
tails, boneless dorsal fins to help prevent sideslip and Less spectacular than the mosasaurs, but far more
roll, and paddle limbs for steering and braking. persevering, were the sea turtles. This group tended
toward giganticism; for example, the Cretaceous turtle
Ichthyosaurs were active predators with highly Archelon was nearly 4 meters long (13 feet). As an
adapted vision and high-speed swimming ability. Their adaptation to its aquatic habitat, the marine turtle’s
large eyes (some larger than a dinner plate) let them see carapace (back shell) was greatly reduced, and its limbs
in the vanishing light at great depths. A ring of bony were modified into broad paddles.
plates surrounding the eyes protected them against high
water pressure. Their heads were a pointed wedge, cTHE RISE OF MODERN BIRDS
suitable for cutting rapidly through water. From the time of Darwin, naturalists have recognized
the structural similarities between birds and reptiles.
Mosasaurs They prompted Thomas Huxley to remark that birds
are “glorified reptiles” that evolved wings and feathers
The mosasaurs were a highly successful group of Cre- and lost their teeth. But Huxley’s comment depreciates
taceous giant marine lizards. They had large, sharp the marvelous attainments of birds, including their
teeth, elongate bodies, and porpoiselike flippers. The superior power of flight and high level of endothermy.
lower jaw had an extra hinge at mid-length (Fig. 14-51), Both of these attributes are related to the evolution of
allowing the animal to open its mouth widely to grasp
large prey.

450 c Chapter 14. Life of the Mesozoic

ENRICHMENT

The Archaeopteryx Controversy Richard Owen (who gave us the name “dinosaur) insisted that
transitional species needed to prove gradual evolutionary
In 1861, a worker extracting stone from the Solnhofen Com- change did not exist. Darwin agonized over not finding transi-
munity Quarry located about halfway between Nuremberg and tional species, calling it “the most obvious and gravest objec-
Munich found a splendidly preserved feather. The fossil was tion which can be urged against my theory.” He correctly
somehow obtained by the prominent German paleontologist surmised that absence of fossils of transitional species was
Hermann von Meyer, who recognized it as a bird’s flight merely because fossils of those creatures had either not been
feather. Later in the same year, Meyer announced the discov- found, were not preserved, or had been obliterated by the
ery of a nearly complete skeleton bearing feathers on its many destructive forces operating throughout geologic time.
forelimbs and long, lizardlike tail. Meyer named the specimen
Archaeopteryx lithographica (Archea, meaning “ancient,” But then came the discovery of Archaeopteryx. It
and pteryx, meaning “winged”). The species name, lithog- appeared to be the perfect transitional fossil, bridging the
raphica, refers to the Late Jurassic host rock, which was used gap between the Class Reptilia and Class Aves. However,
extensively in lithography. Richard Owen insisted Archaeopteryx was simply a bird and
not a transitional species. Darwin’s champion Thomas Hux-
As described earlier, Archaeopteryx possessed both bird- ley disagreed. He argued that Archaeopteryx clearly filled the
like and reptilelike traits. The creature had true feathers, a gap between reptiles and birds. He even countered Owen’s
feature that defines birds. In most other respects, the argument by citing examples of small birdlike dinosaurs that
skeleton closely resembled that of a small, theropod dino- were themselves transitional to primitive birds like Archae-
saur like Velociraptor. The jaws of Archaeopteryx bore teeth. opteryx. According to Huxley, so-called “missing links” do
It did not have the horny beak of modern birds. not exist. History has shown Huxley was correct. Since the
days when he debated evolution with Owen, hundreds of
Reports of the discovery of Archaeopteryx came only two transitional fossils have been discovered, including transi-
years after the publication of Darwin’s On The Origin of tional forms between fish and amphibians, amphibians and
Species. In his famous book, Darwin stated that evolution reptiles, and reptiles and mammals.
involves gradual changes over time with transitional species
having anatomical features of both their ancestors and
descendants. Darwin’s opponents, including the influential

FIGURE 14-45 The distinctive pterosaur Pterodaustro. Pterodaustro had long, curved jaws.
Its upper jaw held rounded teeth for crushing the shells of invertebrate prey. The lower jaw
had more than 400 flexible, wire-thin teeth for straining tiny organisms from water. While
feeding, Pterodaustro probably folded back its wings, dipped its curved snout into the water,
and swept the lake bottom for mollusks and crustaceans. On lifting its head, water would drain
through its mesh of teeth, straining out the food behind to be swallowed. It was found in
Cretaceous lake sediments in Argentina. (J. Sibbick/Salamander Books)

The Mammalian Vanguard b 451

FIGURE 14-46 Toothy skull of Jurassic marine crocodile modern birds (Fig. 14–52). The feathers are distinctly
Geosaurus. The sea crocodiles were the only archosaurian preserved, but unlike modern birds, its jaws bore teeth
group able to invade the oceanic environment. Length of this and it had a long, feathered, but otherwise lizardlike tail.
skull is about 45 cm (1.5 feet).
On the wings of modern birds, the bones of the
feathers from reptilian scales. The first feathers to digits coalesce for greater strength. However, the
appear in the fossil record did not function in flight, primitive wings of Archaeopteryx retained claw-
but for insulation, display, or possibly camouflage. bearing, free fingers for climbing and grasping. The
sternum lacked a keel, indicating that the sturdy
As described earlier in this chapter, birds evolved muscles needed for sustained flight were lacking.
from a subgroup of theropods called deinonychosaurs.
Birds and deinonychosaurs were similar in their Also, the feather shafts in Archaeopteryx were much
bipedal stance, as well as the structure of their limbs, thinner and weaker than those in modern birds of the
feet, shoulder girdles, and skull. same size. Those flimsy feathers would have buckled or
snapped during strong flapping. It is likely Archaeop-
You may wonder why birds evolved from a member teryx used its wings to glide from branch to branch or
of the Saurischia (reptile ¼ hipped) rather than the to slow its descent to lower branches or the ground.
Ornithischia. After all, doesn’t Ornithischia mean
“bird-hipped”? It so happens that the birdlike pelvic Small, delicate, hollow-boned animals with tiny
structure in which the pubis is rotated backward against bones are not readily preserved, and thus the fossil
the ischium occurred twice in evolution, first in the record for early birds is not good. Fossils of larger
Ornithischia and later in the Saurischian (theropod) an- birds, like the Cretaceous aquatic bird Hesperornus (see
cestors of birds. It is an example of convergent evolution. Fig. 13-35), are more common. Nevertheless, the
fossil record is sufficient to indicate that many different
Archaeopteryx, the first undisputed bird, is a close kinds of birds lived during the Cretaceous Period.
evolutionary link between deinonychosaurs and
cTHE MAMMALIAN VANGUARD
While Mesozoic reptiles had their heyday, small, furry
animals were scurrying about in the undergrowth and
unwittingly awaiting their day of supremacy. These

FIGURE 14-47 Stratigraphic ranges of
Mesozoic marine reptiles. Ichthyosaurs,
plesiosaurs, mosasaurs, and sea turtles
adapted well to life in the sea.

452 c Chapter 14. Life of the Mesozoic

FIGURE 14-48 Triassic
nothosaurs and
placodonts. Nothosaurs
such as Ceresiosaurus (top
left) and Nothosaurus (right
foreground) were
medium-sized marine
reptiles with long necks
and jaws set with sharp
teeth for snaring fish.
Placodonts, such as
Placodus (bottom left), were
massively constructed
reptiles adapted for
plucking mollusks from
the seafloor. Placodus was
about 3.5 meters long.
(J. Sibbick)

shrewlike descendants of the mammal-like reptiles bone and teeth remains. Here is some of the evidence
were the first primitive mammals. Among the earliest we have that these creatures were mammals:
were Megazostrodon, Eozostrodon, and the more widely
known Morganucodon (Fig. 14-53) from Upper Triassic Differentiation of teeth is a mammalian trait.
rocks of southern Wales. Like us, these tiny creatures had incisors, canines,
premolars, and molars. Further, the teeth grew
We know these early mammals from all three from “baby teeth” like we have, suggesting that
systems of the Mesozoic, based on rare finds of tiny the young were suckled.

FIGURE 14-49 Cretaceous long-necked plesiosaurs. On Reptiles have a single ear bone (the stapes). But
viewing the skeleton of a long-necked plesiosaur, Thomas more efficient hearing was achieved in these early
Huxley said the animal reminded him of “a snake threaded mammals by two additional ear bones, the mal-
through a turtle.” Earliest remains of plesiosaurs occur in leus and incus.
strata of Jurassic age. (INTERFOTO/Alamy)
Whisker pits on the upper jaw bones indicate a
covering of hair.

The articulation of the jaw to the skull was
mammalian, and the lower jaw was functionally
a single bone, the dentary, as in mammals.

Tooth morphology is of particular importance in
identifying early mammals (Fig. 14-54). The group
called docodonts, for example, had multicusped molar
teeth, which suggests they may have been the stock
from which evolved present-day monotremes (egg-
laying mammals). Very likely, the docodonts fed on
insects, as did many of these primitive mammals.

Symmetrodonts had molars constructed on a more
or less triangular (“symmetrical”) plan. Multituber-
culates had teeth with many tubercles or cusps on
their molars. Their chisel-like incisors and a gap
between the incisors and molars gave them a rodent-
like appearance (Fig. 14-55). The multituberculates
were a persevering lineage. They appeared during the
Late Jurassic and were present until the Eocene epoch.

Triconodonts are recognized by their cheek teeth,
which have three cusps aligned in a row. The brain-
case, vertebrae, and pelvis in these early mammals are
distinctly mammalian in form.

Although Mesozoic mammals are generally
regarded as being subordinate to dinosaurs, recent

The Mammalian Vanguard b 453

FIGURE 14-50 Ichthyosaurs were the supreme marine reptiles of the Mesozoic. Readily
recognized by their dolphin-like bodies, Ichthyosaurs were the reptilian counterparts of present-
day toothed whales. Exceptional vision and swimming ability made them powerful hunters. Shown
here is the large Jurassic ichthyosaur Brachypterygus, about 4 meters (13 feet) long. There were
giants among the ichthyosaurs, including one from British Columbia measuring 23 meters (more
than 75 feet) long. Many ichthyosaurs had very large eyes. The 15-foot-long Opthalmosaurus had
eyes that were 8 inches in diameter. Large eyes in ichthyosaurs gave them good vision at very low-
light levels. They were able to pursue their prey at very great depths. (J. Sibbick)

FIGURE 14-51 Cretaceous mosasaur. These giant marine lizards attained lengths over
9 meters (30 feet). They were primarily fish-eaters, but some frequently dined on large
ammonites. Puncture wounds on the shells of ammonite fossils precisely match the dental
pattern of mosasaurs. (U.S. National Museum of Natural History/Smithsonian Institution)

454 c Chapter 14. Life of the Mesozoic

FIGURE 14-52 The Jurassic bird Archaeopteryx. (A) Archaeopteryx in Germany’s Solnhofen
Limestone, which formed from lime mud deposited on the floor of a tropical lagoon. This
extraordinarily preserved specimen resides in the Berlin Museum of Natural History.
Archaeopteryx has a mixture of both reptile and bird characteristics. Its teeth and long bony tail
are reptilian, but its feathers and the presence of an avian “wishbone” are characteristic of birds.
((A) Jason Edwards/National Geographic RF/Getty Images, (B) Illustration by John Sibbick.
# The Natural History Museum/The Image Works.)

discoveries from China suggest they were more abun- that, in the competition between dinosaurs and early
dant and diverse than once believed. A furry, semi- mammals, it was always the mammals that provided
aquatic mammal named Castorocauda, for example, occasional meals for hungry theropods. But fossilized
fished the waters of Jurassic lakes and lived much like stomach contents of R. robustus contain the bones of a
a modern beaver or otter. Later, during the Early young dinosaur Psittacosaurus, about 6 inches long.
Cretaceous, two species of a land dweller named Repe- Juvenile dinosaurs beware, mammals were on the prowl.
nomamus roamed the Earth. The larger species (R.
robustus) was about the size of a modern badger (Fig.
14-56). As indicated by strong jaw musculature and
sharp teeth, the larger species was well adapted for
catching, holding, and tearing prey. You might surmise

FIGURE 14-53 Morganucodon, an early mammal from FIGURE 14-54 Molars of Mesozoic mammals. Side views
the Late Triassic of Wales. This shrewlike descendant of of lower molars (as viewed from inside the mouth) and top
mammal-like reptiles was among the earliest of the primitive views of the oral surfaces.
mammals.

Sea Plants and Phytoplankton b 455

FIGURE 14-55 The rodentlike multituberculate
Taeniolabis. Chisel-like incisors and the gap between the
incisors and the cheek teeth gave them a rodentlike
appearance and indicate that they may have been plant-eaters
and probably had rodentlike gnawing habits as well.

Mammal Types

Taxonomists divide mammals into two groups. The FIGURE 14-57 Fossil remains of the small Cretaceous
first are called prototherians and include the trico- mammal Eomaia. From uncurled tail to tip of snout, the
nodonts, multituberculates, and the monotremes. creature was about 16 cm long. Eomaia was discovered in the
Monotremes are represented today by the platypus Lower Cretaceous Yixian Formation of Liaoning Province,
and spiny anteater of Australia. China. (From: Anne Weil, “Mammalian Evolution:
Upwards and Onwards,” Nature (416):798–799, Fig. 1,
The second group consists of therians. Marsupials 2002)
(also called eutherians) and placental mammals
(like us) are therians. Both prototherians and therians For the mammals, the Mesozoic was a time of
were present by Middle Jurassic time. The earliest evolutionary experimentation. They lived among the
known placental mammal was recently discovered in great archosaurs while steadily improving their ner-
Lower Cretaceous beds in China. Named Eomaia vous, circulatory, and reproductive systems. With
(“dawn mother” in Greek), the tiny fur-covered animal their ability to control body temperature, they could
(Fig. 14-57) had feet well adapted for climbing among thrive in both cold and warm climates. As the dinosaur
the branches of trees 125 million years ago. population declined near the end of the era, mammals
quickly expanded into vacated habitats.

FIGURE 14-56 In this Cretaceous scene, two large cSEA PLANTS AND PHYTOPLANKTON
Cretaceous predatory mammals named Repenomamus Animal life on Earth ultimately depends on plant life.
can be seen at the lower right. About 130 million years ago, This generalization was as valid during the Mesozoic
such mammals preyed on small dinosaurs (and probably each as it is today. Then, as now, plants made up the broad
other). Species of Repenomamus ranged in size up to 3 feet base of the food pyramid. Their nutritious starches,
long. (J. Sibbick) oils, and sugars made possible the evolution and con-
tinuing existence of animals.
Plants are a fundamental part of Earth’s essentially
self-sustaining ecologic system. The operation of the
system depends on oxygen and carbon dioxide. Animal
respiration provides the carbon dioxide needed for
plant photosynthesis, whereas plants, by means of
photosynthesis, supply the oxygen needed by animals.
In the geologic past, variations in plant productivity
may have caused corresponding changes in the amount
of carbon dioxide and oxygen in the atmosphere. Such
variations may have favored the evolution of some
animals over others and may have been responsible
for the demise of particular groups.

456 c Chapter 14. Life of the Mesozoic

FIGURE 14-58 Geologic
distribution and abundance of
phytoplankton. The most
abundant Mesozoic
phytoplankton groups were the
coccolithophorids and
dinoflagellates. (Tappan, H.,
and Loeblich, A.R., Jr., 1970,
Geobiologic implications of fossil
phytoplankton evolution and
time-space distribution, in
Kosanke, R.M., and Cross, A.T.,
eds., Symposium on Palynology of
the Late Cretaceous and Early
Tertiary: Geological Society of
America Special Paper 127: 257)

Marine Phytoplankton Coccolithophorids

Photosynthetic organisms that live suspended in water The coccolithophorids also began their expansion
do not require the vascular and supportive systems that during the Early Jurassic. These calcium carbonate-
characterize land plants. Most of these organisms are secreting organisms have a splendid fossil record.
unicellular, although they may grow together in Their abundant remains formed many of the extensive
impressive colonies and aggregates. They are part of chalk deposits of the Mesozoic and early Cenozoic.
that vast realm of floating organisms termed plankton. Today, they are frequently present in the deep-sea
sediment known as calcareous ooze.
Those having internal organelles called chloro-
plasts can photosynthesize their food and are gener- The coccolithophorid organism is one of several
ally called phytoplankton. The more familiar of these varieties of unicellular golden-brown algae. These
are members of the Protista. The geologic record of algae deposit calcium carbonate internally on an
the most important fossil phytoplankton groups is organic matrix and construct tiny, shieldlike structures
shown in Figure 14-58. called coccoliths. Once formed, the coccoliths move
to the surface of the cell and form a calcareous armor
Dinoflagellates (Fig. 14-60).

Fossil dinoflagellates are important aids in Mesozoic Coccoliths have the right traits to be extremely
and Cenozoic stratigraphy. From the Jurassic forward, useful in stratigraphic correlation of Cretaceous to
they were among the primary producers in the marine Holocene rocks: They are abundant fossils, have
food chain. For propulsion, they used two flagella: one undergone frequent evolutionary changes through
longitudinal and whiplike, the other transverse and time, and are widely dispersed by oceanic currents.
ribbonlike.
Silicoflagellates and Diatoms
During their life cycle, dinoflagellates develop a
mobile planktonic form and a cyst phase that is formed Silicoflagellates are flagella-bearing organisms that
within the mobile organism. Only dinoflagellate cysts, secrete delicate siliceous skeletons in simple latticelike
which have an organic covering that is extremely frameworks. Radiating spines characterize most gen-
resistant to decay, are known as fossils (Fig. 14-59). era (Fig. 14-61).

Land Plants b 457

FIGURE 14-59 Dinoflagellates. (A) The modern dinoflagellate Peridinium as it appears
during the mobile plantonic stage of its life cycle. Note the two flagella, one in an encircling
horizontal groove, and one extending downward. Magnification 700Â. (B) Scanning
electron micrograph of a fossil dinoflagellate cyst. Magnification l300Â. (Biophoto Associates/
Photo Researchers, Inc.)

Diatoms also secrete siliceous coverings (Fig. 14- Cretaceous, and then all groups expanded again into
62). The covering is called a frustule. In the past, a the early epochs of the Cenozoic.
proliferation of diatoms was often associated with vol-
canic activity. Apparently, silica supplied to seawater as cLAND PLANTS
fine volcanic ash stimulated diatom productivity. The Mesozoic is sometimes called the Age of Reptiles.
With equal validity, the era could be dubbed the Age of
The earliest known silicoflagellates and diatoms Cycads. These are seed plants lacking true flowers
appeared in the Cretaceous. Along with other phyto-
plankton, they experienced a decline at the end of the

FIGURE 14-60 Scaning electron
micrograph of a Cretaceous
coccosphere. Although coccospheres
measure only 0.002 to 0.01 mm in
diameter, an electron microscope lets us
discern their intricate construction.
Their skeleton consists of elliptical or
round plates that are concave on one
side to fit snugly around the surface of
the spherical cell. Each plate is
composed of smaller elements
uniformly arranged in a circular, radial,
or spiral plan, this image is magnified
approximately 6000 times. (Andrew
Syred/Photo Researchers, Inc.) What
are the individual plates that surround the
coccosphere called?

458 c Chapter 14. Life of the Mesozoic

FIGURE 14-61 Silicoflagellates from the Mid-Atlantic FIGURE 14-63 Cycads. Cycads are seed plants that lack true
Ridge. They range in size from 0.02 to 0.1 mm. flowers. The fossil stumps of cycads are among the most
Silicoflagellates and diatoms, along with the abundant plant fossils of the Jurassic. The pineapple-like
coccolithophorids, are members of the phylum Chrysophyta. structures at the top are pollen cones. Cycads occur today in
Image is magnified 3800 times. (Harold Levin) Southern Hemisphere tropical forests, as well as in Florida.
(Dr. Carleton Ray/Photo Researchers, Inc.)
(Fig. 14-63). Jurassic cycads included tall trees with
rough branches marked by the leaf bases of earlier 3. The Early Cretaceous appearance of angio-
growths and by crowns of leathery pinnate (feather- sperms—species having enclosed seeds and flow-
like) leaves. Cycads experienced a marked decline in ers. Pollen grains produced by angiosperms
the Late Cretaceous, and only a few have survived to provide the earliest evidence of these flowering
the present time, such as the sago palm, a common plants.
house plant.
Gymnosperms
There were three important episodes in the evolu-
tion of land plants: Among the nonflowering, pollinating seed plants, all
but seed ferns have living representatives. Six groups of
1. Development of the spore-bearing plants, like conifers were present during the Jurassic and Creta-
ferns. ceous, including large numbers of pines. In 1994, the
remains of 39 huge pines were discovered in Wollemi
2. Evolution of the gymnosperms—nonflowering, National Park, Australia. Dubbed the “Wollemi
pollinating seed plants like the cycads, seed ferns, pines,” many of the trees are species known only
conifers, and ginkgoes (Fig. 14-64). from this locality.

FIGURE 14-62 Modern marine diatoms. Diatom shells FIGURE 14-64 Ginkgo biloba, the ginkgo or maidenhair
(tests) are composed of an upper part (the epitheca) and a tree. Note the naked, fleshy seeds that grow on the female
lower part (the hypotheca) that fit together like a lid and a trees. Fossils of these plants that are over 200 million years
box. The covering frustule may be circular, cylindric, old are nearly identical to those living today. (# Marli
triangular, or a variety of other often beautiful shapes. Bryant Miller)
(M. Abbey/Photo Researchers, Inc.) In what taxonomic
kingdom are diatoms placed?

Late Cretaceous Catastrophe b 459

dinosaur consumption, leaving over-browsed areas
that could be invaded quickly by the evolving lineages
of shrublike angiosperms.

Unlike gymnosperms, angiosperms reproduce, dis-
perse, and grow rapidly. Thus, they are highly resistant
to over-browsing. The presence of this expanding
angiospermal food supply undoubtedly influenced
the evolution of the ornithischians that were so numer-
ous during the Cretaceous.

The duckbills took on habits not unlike today’s
antelope and bison, whereas ceratopsians may have
lived rather like rhinoceroses. The new plants pro-
vided nutritious fruits and nuts for these dinosaurs, and
the dinosaurs in turn aided in the dispersal of angio-
sperm seeds by passing them unharmed through their
digestive tracts.

FIGURE 14-65 Fossil sassafras leaf, Cretaceous Dakota cLATE CRETACEOUS CATASTROPHE
Sandstone, Ellsworth, Kansas. The slab is about 12.5 cm Just as the end of the Paleozoic was a crisis for animal
(5 inches) wide. By Middle Cretaceous time, such life, so also was the end of the Mesozoic—specifically,
angiosperms had become widespread. (Harold Levin) at the end of the Cretaceous. Primarily on land, but
also at sea, extinction overtook many seemingly secure
Angiosperms groups of vertebrates and invertebrates. Altogether,
the Late Cretaceous catastrophe eliminated about
By the Middle Cretaceous, angiosperms had become one-fourth of all known animal families.
widespread. Forested areas included stands of birch, In the seas, the plesiosaurs, mosasaurs, and the once
sycamore, magnolia, holly, palm, maple, walnut, beech, highly successful ammonites perished. Entire families
poplar, willow, and sassafras (Fig. 14-65). Before the of echinoids, bryozoans, planktonic foraminifers, and
period came to a close, angiosperms had surpassed the calcareous phytoplankton became extinct.
nonflowering plants in both abundance and diversity. On land, the most noticeable loss was among the
Flowering trees, shrubs, and vines expanded across the great clans of reptiles. Gone forever were the magnifi-
lands and, except for the absence of grasses, gave the cent dinosaurs and soaring pterosaurs. The only rep-
landscape a modern appearance. tiles to survive the great biologic crash were turtles,
snakes, lizards, crocodiles, and New Zealand’s reptile
Angiosperms provide many examples of coevolution tuatara (Sphenodon).
with Mesozoic insects, dinosaurs, mammals, and birds. What caused this spectacular decimation of animal
Coevolution occurs when two or more different orga- life at the end of the Mesozoic? Scores of hypotheses,
nisms develop a close, reciprocal relationship in which some scientific but many preposterous, have been
the evolution of one organism is partially dependent on offered. The most credible hypotheses attempt to
the evolution of the other. explain the simultaneous extinctions of marine and
terrestrial animals and seek a single or related sequence
The coevolution of insects and flowering plants is a of events as a cause.
classic example. Angiosperms, by encouraging insect The hypotheses fall into two broad categories:
visits, use insects as delivery agents for their pollen.
This provides a far more efficient means of pollen An external or extraterrestrial event, such as an
dispersal than random wind pollination. The selective encounter with an asteroid or comet. Such events
competition for efficient insect pollinators has induced areconsidered catastrophicbecause theireffectsare
evolution of constantly changing variations in both concentrated within a relatively short span of time.
plants and insects. In the angiosperms, the need for
each plant to be recognizably different results in a An event or trend right here on Earth, such as
spectacular floral variety (color, shape, scent, height) extreme volcanism.
that has persisted from the Cretaceous to the present.
Did a Bolide Impact Cause
In addition to their interdependence with insects, the Mass Extinctions?
early angiosperms likely developed coevolutionary
relationships with birds, mammals, and even dino- Since geologists first became aware of the extinctions
saurs. Many of the great sauropods subsisted largely at the end of the Cretaceous, they have speculated
on ferns, horsetails, tree-sized club mosses, and coni-
fers. But such plants are slow to grow and regenerate.
At times, regeneration may not have kept pace with

460 c Chapter 14. Life of the Mesozoic

analyzed, with startling results. The samples contained
approximately 30 times more of the metallic element
iridium than is normal for Earth’s crustal rocks.

FIGURE 14-66 The iridium-rich clay layer (yellow sign) Evidence: Iridium and Shocked Quartz
separates Cretaceous and Paleogene rocks near Gubbio
Italy. (Peter L. Kresan Photography) Where could this high concentration of iridium have
come from? Iridium probably is present in Earth’s core
about collisions with asteroids or comets, or lethal and perhaps the mantle, but how could the metal from
cosmic radiation. But tangible evidence was lacking so deep a source find its way into a clay layer at the
until 1977. Geologist Walter Alvarez discovered a thin boundary between Cretaceous and Paleogene beds?
clay layer near Gubbio, Italy, at the boundary between Volcanism is an obvious possibility.
the Mesozoic’s Cretaceous Period and the Cenozoic’s
Paleogene Period (Fig. 14-66). Alvarez sent samples of But iridium also occurs in extraterrestrial objects
the clay to his physicist father Luis, who had the clay such as asteroids and meteorites. For this reason, the
father-and-son Alvarez team favored an extraterrestrial
hypothesis for the iridium in the clay layer. They pro-
posed that an iridium-bearing asteroid crashed into
Earth at the end of the Cretaceous Period (Fig. 14-67).

The explosive, shattering blow from the huge body
(presumed to be over 10 km in diameter) would have
thrown dense clouds of iridium-bearing dust and other
impact ejecta into the atmosphere. Transported by
atmospheric circulation, the dust might have formed a
lethal shroud around the planet, blocking the Sun’s
rays and killing marine and land plants on which all
other forms of life ultimately depend. As the dust
settled, it would have formed the iridium-rich clay
layers found at Gubbio and subsequently at many
other places around the world (Fig. 14-68).

In addition to the iridium-rich clay layer found at
many sites around the world, there is other evidence
for the impact of a large extraterrestrial body at the end
of the Cretaceous. There is a widespread occurrence of
shocked quartz in the boundary layer (Fig. 14-69).

FIGURE 14-67 Artist’s conception of
an asteroid colliding with Earth 65.5
million yeas ago. (David A. Hardy/
Science Photo Library/Photo
Researchers, Inc.)

Late Cretaceous Catastrophe b 461

FIGURE 14-68 Occurrences of iridium-rich sediment layer at the Cretaceous–Paleogene
boundary. (After W. Alvarez et al., 1990, Geol. Soc. Amer. Special Paper 190:305–315)

These mineral grains are recognized by distinctive rare, dense, high-pressure silicate mineral known as
parallel sets of microscopic planes (called shock lamel- stishovite, found at Meteor Crater in Arizona and
lae) that are produced when high-pressure shock other known impact structures, is also found in the
waves, such as those emanating from the impact of a boundary clay. It is taken as evidence of sudden
large meteorite, travel through quartz-bearing rocks. extremely high pressures, such as those that would
be associated with the impact of an asteroid. Finally,
In the stratum containing shocked quartz grains, sediment in the boundary layer often includes a carbon
there also are tiny glassy spherules thought to repre- soot that may be the residue of forests burned during
sent droplets of molten rock thrown into the atmo- the firestorm caused by the impact.
sphere during the impact event. These are tektites. A
Any large extraterrestrial object that explodes upon
striking Earth is called a bolide. Many bolides have
collided with Earth in the geologic past, but the scars
they left on continents have mostly been obliterated by
weathering and erosion. Nevertheless, a few can be
discerned on photographs taken from spacecraft or
during geologic investigations. For example, geologists
have discovered a large, Jurassic-age bolide impact
crater on the seafloor north of Norway. Named the
Mjølnir Crater, it is about 40 km (25 miles) in diameter.
Sediment within and around the crater contains
shocked quartz and high concentrations of iridium.

FIGURE 14-69 Shocked quartz grain. It was found in Haiti The Chicxulub Structure
and thought to have been ejected during the Chicxulub
asteroid impact. Note the parallel microscopic planes, called But the Mjølnir Crater is of Jurassic age. Where is the
shock lamellae (a lamella is a thin, flat layer). Maximum grain crater produced by the bolide that allegedly caused
diameter is 0.3 mm. (Dr. David Kring/Science Photo mass extinctions at the close of the Cretaceous?
Library/Photo Researchers, Inc.)
Currently, the best candidate is a buried crater in
the Gulf of Mexico just offshore from the Yucat an
Peninsula, named for the nearby town of Chicxulub
(Fig. 14-70). At this location is a buried circular

462 c Chapter 14. Life of the Mesozoic fossils at all, suggesting that plankton had been exter-
minated. At the top of the core, fossils again appear,
FIGURE 14-70 Location map for the Chicxulub indicating that several thousand years later, recovery
structure. was beginning to take place.

structure about 180 km (112 miles) in diameter, Does the timing of the proposed impact correlate
revealed by magnetic surveys, gravity surveys, and with extinctions that were widespread at the end of the
oil well cores and logs. The southern rim of the crater Cretaceous? Rocks that had been melted by the event
is visible on radar images taken by the space shuttle were found by isotopic dating to be 65.2 Æ 0.4 million
Endeavor. Andesitic rock in the central core of the years old. In addition, while these rocks had been
structure has an isotopic composition similar to that melted by the heat of impact, they acquired remanent
of tektites that are abundant in the Cretaceous- magnetism indicating that they solidified during the
Cenozoic boundary layer at many locations in the episode of reverse geomagnetic polarity known to exist
Caribbean region. at the time of deposition of the boundary layer.

Further evidence of the bolide impact is in core If the Chicxulub structure is indeed an impact
samples of rocks penetrated during oil drilling in and crater, it is among the largest on Earth. The bolide
around the Chicxulub structure. Prominent in the core that produced it had an estimated diameter of about
samples are coarse breccias that occur both above and 10 km (6 miles). As it splashed down, it would have
interbedded with the andesite. The breccias contain produced a tsunami-like ocean wave over 34 meters
shocked quartz and appear to be part of a blanket of (112 feet) high that would flood land areas around the
shattered rock, such as a massive impact would produce. Carribean and Gulf of Mexico. Geologists have found
evidence of tsunami devastation in the form of termi-
If a large asteroid impacted our planet at the end of nal Cretaceous sediments that have been churned up
the Mesozoic, there should be layers of sedimentary and chaotically disturbed. Such deposits, called tem-
rock containing the fossil remains of some of the pestites, are found in the Mississippi Valley as far
asteroid’s victims. In 1996, paleontologist William north as Missouri. Nearer the impact, rocks would
Zinsmeister reported such a discovery near the tip have been vaporized, and the atmosphere would
of the Antarctic Peninsula on Seymour Island. There, become a suffocating cloud of dust, water vapor,
in strata immediately above the iridium-rich boundary and carbon dioxide emitted from melted limestones.
clay, he found the remains of a huge fish kill. Their
doom would appear to be directly related to the event Did Global Volcanism Cause
that produced the iridium-rich boundary clay. the Mass Extinctions?

Not only fish, but ocean plankton would have been On first examination, the bolide-impact hypothesis
affected by the impact. In 1997, a core sample was seems a tidy way to account for the extinction of
taken from beneath the ocean floor about 320 km (200 dinosaurs and many of their animal and plant con-
miles) east of Jacksonville, Florida. The core repre- temporaries. Like all hypotheses, however, it must
sents the entire time span of the bolide event. At its stand the rigorous test of scientific scrutiny. Were
base, there is a layer of white sediment containing the the Alvarezes correct in assuming that the iridium
shells of millions of microfossils (mostly foraminifera). was derived from an impacting asteroid? Or could
They represent life just before the impact. Above the there be another explanation?
fossiliferous white layer is a green clay thought to
represent the dust and ash from the impact explosion. Geologists have found evidence that iridium in
The green layer is capped by a red clay that contains no clays such as that at Gubbio might have its source
in Earth’s mantle, from which it can move to the
surface by way of conduits and blast into the atmo-
sphere as iridium-rich volcanic ash and dust. Volcan-
ism was prevalent during the late stages of the
Cretaceous. Especially significant were the tremen-
dous outpourings of lava at the end of the Cretaceous
that formed the Deccan Traps in India. Intensive
volcanic activity at the end of the Cretaceous also
was vigorous in western North America, Greenland,
Great Britain, Hawaii, and the western Pacific.

Volcanoes produce dust and aerosols, such as sul-
furic acid, that block solar radiation and thus cause
temperatures to decline. Sulfuric acid in the atmo-
sphere generates acid rain, and such precipitation
could change the alkalinity of the oceans, placing lethal

Late Cretaceous Catastrophe b 463

ENRICHMENT

Bolides and Modern Day Catastrophism America, Eurasia, South Africa, and Australia. These, and
the probability of other craters yet to be discovered, suggest
In the early decades of the nineteenth century, Georges that bolide impacts were a potent force in causing biological
Cuvier promoted his theory of catastrophism. Cuvier believed catastrophes of great magnitude.
periodic mass extinctions were caused by global flooding like
the deluge in the biblical story of Noah. One need not invoke Knowing about mass extinctions in the past leads us to
supernatural forces, however, to explain causes of mass think about what may happen in the future. Imagine the
extinctions. There are plenty of natural causes. A short damage to our planet inflicted by even a meteor of modest
list would include geologically sudden and severe climate size, like the one that produced Meteor Crater in Arizona.
change, sea level fluctuations, colossal volcanic events, That meteor had an estimated diameter of only about 30
changes in the composition of the atmosphere and oceans, meters (100 feet), yet it excavated a crater 1.2 km wide,
and last but not least, the impacts of meteors and comets. releasing energy equivalent to a 20-megaton nuclear bomb.

You read in Chapter 12 that our planet has experienced A meteorite from 50 to several hundred meters in diameter
at least five major mass extinctions (there were at least 12 is believed to strike Earth every 200 to 300 years. In 1908,
more of lesser magnitude). Those that study these biological one such object, thought to have been about 60 meters in
catastrophes are constantly on the alert for evidence of some diameter, entered Earth’s atmosphere and exploded above the
sort of cosmic encounter as the cause. They scour the Earth for Tunguska Valley in Siberia. The explosion, which was heard
extraterrestrial materials like iridium, rocks and minerals thousands of miles away in London, flattened and burned
exhibiting shock metamorphism, impact melt rocks, and, of vegetation across 50 square kilometers of forests.
course, the impact craters themselves. Unfortunately, craters
do not lend themselves very well to preservation. Especially for The Tunguska Valley event, however, would be trivial
impacts older than Cenozoic, craters are destroyed by erosion, compared to the catastrophe produced by a 1000-meter-
hidden by infilled sediment, or obliterated by volcanism or wide impacting body. A body that size would release the
tectonic forces. Meteor Crater of Arizona (Fig. A) looks so energy equivalent of a billion tons of TNT. It would eject a
pristine because it is a mere 49,000 years old. gigantic cloud of fiery rock and dust into the atmosphere.
Firestorms would sweep across the continents. Dust and
Although often difficult to find, geologists have now smoke would blot out the sun and throw the world into total
recorded the existence of over 160 impact craters on Earth. darkness. The loss of sunlight for months would prevent
They range in age from Proterozoic to the Holocene. Most of photosynthesis and cause mass extinctions of plants and
the craters were found within the stable cratons of North animals both on land and in the ocean. At the very least, the
event would wipe out over a quarter of the planet’s human
FIGURE A Meteor Crater (also called Barringer Crater) in Arizona. population.
It was formed by the impact of a meteor about 30 meters wide (100
feet) approximately 49,000 years ago. (Peter L. Kresan What can be done to avoid such a calamity? An easy
Photography) answer is “not much.” Scientists are currently examining
ways to detect, track, and intercept big rocks headed our
way. They have set up a global network of telescopes for early
detection of approaching large meteorites. If the incoming
body is spotted sufficiently early, perhaps it might be possi-
ble to push it into a nonthreatening trajectory by exploding a
nuclear device off to its side. On the side nearest the
explosion, heat from radiation would vaporize the surface
of the meteorite, and the resulting jet of vapor might act like
a rocket engine and nudge it off course. It would be impor-
tant that the bomb not strike the meteorite directly as the
fragments would send a lethal rain of shrapnel down upon
the Earth. Other plans are still being formulated, as space
scientists grapple with possible ways to safeguard civiliza-
tion against a catastrophic encounter with a bolide.

stress on plankton, other invertebrates, and life higher considered reasonable for global volcanism. If the
in the food chain. Thus, volcanism can be considered iridium were of bolide origin, one would expect to
harmful to animals both on land and in the sea. find it confined to a thin layer of sediment.

Proponents for a volcanic source for the iridium in Finally, those favoring the volcanic hypothesis
the boundary clay note that the element is often also note the presence of antimony and arsenic in
distributed across sedimentary thicknesses of 30 to some of the beds containing the iridium. Although
40 cm. This suggests deposition of ash-derived iridium common in volcanic ash and lava, these elements are
over a span of several thousand years, an interval rare in meteorites.

464 c Chapter 14. Life of the Mesozoic have been able to adapt to the harsher conditions.
One by one, species would have met their end. As is
Did Environmental Change Cause the Mass characteristic within the complex ecologic web, their
Extinctions? demise would affect other organisms, resulting in
waves of extinctions among dependent species.
Several hypotheses that seek to explain the demise of
plants and animals near the end of the Cretaceous Did a Combination of Factors Cause
propose that events on Earth upset the ecologic bal- the Mass Extinctions?
ance between organisms and the environment to which
they had become adapted. Recall that continents dur- Still other hypotheses account for the Late Cretaceous
ing the Cretaceous were extensively covered by shal- extinctions, but the examples provided above illustrate
low, warm, epeiric seas. Many geologists believe these the complexity of the problem. The debate between
transgressions were caused by displacement of ocean those favoring sudden extraterrestrial causes and those
water when midoceanic ridges were raised as a conse- supporting purely terrestrial hypotheses will continue
quence of accelerated seafloor spreading. Whatever for decades. Whatever the outcome, it is a fact that
the cause, the warm inland seas that resulted were very hard times near the end of the Mesozoic doomed the
favorable for marine life and helped to moderate and dinosaurs, pterosaurs, ammonites, and over three-
stabilize climates on the continents as well. Under such fourths of the known species of marine plankton.
conditions, plants and animals experienced remarkable
increases in variety and abundance. However, these It is important to note that the extinctions did not
favorable conditions were soon to change. happen simultaneously for all groups. Many groups
died out sporadically over an interval of 0.5 to
Studies of the stratigraphic sequence across the 5 million years. This is an argument against a bolide
Cretaceous-Paleogene boundary indicate a global low- impact.
ering of sea level at the end of the Mesozoic. Perhaps
this was caused by a slowing of seafloor spreading Perhaps the real cause of extinctions will be found in
rates. Whatever its cause, the change in sea level a combination of detrimental factors that left popula-
spelled disaster to animals and plants of formerly tions debilitated. The victims of the hard times may
extensive shallow coastal areas, especially to the phy- have been dealt a coup de grace by an impact event.
toplankton adapted to the shallow sea environment. Fortunately, some mammals, birds, lizards, crocodiles,
turtles, many fish groups, certain mollusks, and decid-
Without the moderating effects of vast epiconti- uous plants survived the hard times and expanded
nental seas, landmasses also would have experienced during the following Cenozoic Era.
harsher climatic conditions and more extreme seasonal
changes. Many families of organisms that were
adjusted to the previous environment might not

SUMMARY

Climates of the Mesozoic were generally mild and equa- The Mesozoic is noteworthy as the era during which
ble, except for occasional intervals of aridity and an mammals and birds appeared. The birds, amply feathered
episode of cooler conditions near the end of the era. for insulation and flight, may have evolved from
small carnivorous dinosaurs (theropods). The oldest
In the widespread Mesozoic seas, coccoliths and diatoms unquestionable remains of a true bird are Archaeopteryx
flourished, as did invertebrate groups such as ammonoids, from the Jurassic.
belemnites, oysters, and other bivalves, echinoderms, cor-
als, and foraminifera. Primitive mammals debuted during the Triassic and had
become common by Cretaceous time. One group of
On land, gymnosperms (cycads and conifers) were common primitive prototherian mammals gave rise to marsupial
in Triassic and Jurassic forests. In the Cretaceous Period, and placental mammals during the Cretaceous.
the angiosperms (flowering plants) expanded and with them
coevolved a multitude of modern-looking insects. Like the Paleozoic Era, the Mesozoic Era closed with an
episode of extinctions. Dinosaurs, pterosaurs, plesiosaurs,
Dinosaurs were the ruling vertebrates of the Mesozoic. Both mosasaurs, ammonoid cephalopods, many groups of
carnivorous and herbivorous varieties occupied a variety of bivalves, and large numbers of planktonic foraminifera
habitats. Based on difference in pelvic structure, we recog- became extinct.
nize two groups of dinosaurs, the Saurischia and Ornithis-
chia. Certain dinosaur species, as well as the pterosaurs, may An impressive body of evidence indicates that a large
have been endothermic (warm-blooded). This is indicated meteorite or asteroid struck Earth about 65.5 million
by bone structure, posture, predator-prey relationships, and years ago, and its effects may have caused or strongly
the presence of feathers on some theropods. contributed to the mass extinctions.

Reptiles also were successful in the seas, where ichthyo- Others argue that the extinctions can be attributed to
saurs, plesiosaurs, mosasaurs, and large turtles competed extensive volcanic activity, coupled with loss of epiconti-
successfully with sharks and bony fishes. nental seas, and resulting climatic changes.

Questions for Review and Discussion b 465

KEY TERMS

adaptive radiation, p. 446 foraminifera, p. 425 plankton, p. 456
acetabulum, p. 429 frustule, p. 457 plesiosaur, p. 448
ammonite, p. 423 gastropod, p. 425 prosauropod, p. 434
ammonoid, p. 423 goniatite, p. 423 prototherian, p. 455
Ammonoidea, p. 423 gymnosperm, p. 458 Psittacosauridae, p. 442
angiosperm, p. 459 Hadrosauridae, p. 438 pubis, p. 429
ankylosaur, p. 441 Heterodontosauridae, p. 438 radiolarian, p. 425
anklyosaurine, p. 441 Hypsilophodontidae, p. 438 regular echinoid, p.422
archosaur, p. 427 ichthyosaur, p. 448 respiratory turbinates, p. 444
belemnite, p. 424 Iguanodontidae, p. 438 rhynchocephalian, p. 427
bolide, p. 461 ilium, p. 429 rudistid, p. 421
carnosaurs, p. 429 irregular echinoid, p. 422 Saurischia, p. 429
ceratite, p. 423 ischium, p. 429 sauropod, pp. 434
ceratopsian, p. 442 lepidosaur, p. 427 Sauropodomorpha, p. 429
ceratosaurs, p. 429 Marginocephalia, p. 442 septum (of cephalopod), p. 423
chloroplast, p. 456 monotreme, p. 452 shocked quartz, p. 460
coccolith, p. 456 multituberculate, p. 452 stishovite, p. 461
coelurosaurs, p. 429 Nautiloidea, p. 423 suture (of cephalopod), p. 423
coevolution, p. 459 Neoceratopsia, p. 442 symmetrodont, p. 452
convergence (evolutionary), p. 427 nodosaurine, p. 441 tektites, p. 461
crustacean, p. 425 nothosaur, pp. 448 temnospondyl, p. 426
diatom, p. 457 Ornithischia, pp. 429 tempestites, p. 462
dinosaur, p. 429 ornithopod, p. 438 therian, p. 455
Dinosauria, p. 429 pachycephalosaur, p. 442 theropod, p. 429
docodont, p. 452 phytoplankton, p. 456 Theropoda, p. 429
ectothermic, p. 444 phytosaur, pp. 427 thyreophoran, p. 439
endothermic, p. 444 placodont, p. 448 triconodont, p. 452

QUESTIONS FOR REVIEW AND DISCUSSION

1. During what geologic period of the Mesozoic is chalk 6. Discuss the differences between the marine invertebrate
particularly abundant? What group of organisms provides faunas of the Mesozoic and those of the Paleozoic. What
skeletal remains for chalk deposition? Why are chalk for- Paleozoic groups are not seen in the Mesozoic?
mations rare in Paleozoic rocks?
7. Discuss several lines of evidence that indicate that
2. What are diatoms? How do they differ in composition certain Mesozoic reptile groups were endothermic. Why
and morphology from coccolithophorids? How might the would endothermy be less important for the giant sauri-
distribution of diatoms and coccolithophorids be related to schians than for small dinosaurs?
the acidity or alkalinity of ocean water?
8. What attributes of Cretaceous mammals may have
3. How did the terrestrial plant flora of the Jurassic differ contributed to their survival during the biologic crisis at the
from that of the Cretaceous? What environmental condi- end of the Mesozoic?
tions might have driven the change in floras?
9. Discuss the differences between world geography at the
4. What are ammonoid cephalopods? What attributes of end of the Permian as compared to the end of the
ammonoids result in their having special value as guide or Cretaceous.
index fossils?
10. What two classes of vertebrates appear for the first time
5. What are foraminifera? Why are they of particular value during the Mesozoic Era?
to petroleum geologists involved in the correlation of sub-
surface strata? 11. Cite two examples of evolutionary convergence among
animals living during the Mesozoic Era and today.

466 c Chapter 14. Life of the Mesozoic 19. Which of the below are not theropods?
___a. Ceratopsians
12. What is the particular evolutionary importance of the ___b. Ceratosaurs
following: (a) basal archosaurs, (b) Archaeopteryx, and (c) ___c. Carnosaurs
angiosperms? ___d. Coelurosaurs

13. What functions do feathers serve in addition to their 20. Which of the below are not ornithischians?
role in flight? ___a. Heterodontidae
___b. Hyspsilophodontidae
14. What is convergent evolution and how does it relate to ___c. Iguanodontidae
the pelvic structure in ornithischians and in birds? ___d. Hadrosauridae
___e. Sauropoda (Sauropodomorphs)
15. Prepare a list of the reptilian groups that survived
the mass extinction at the end of the Cretaceous 21. Which of the following is typically not a mammalian
Period. characteristic?

16. Oceans cover about 71% of Earth’s surface, yet ___a. Well-differentiated teeth
evidence of impact craters on the ocean floor is rarely seen. ___b. Single ear bone, the stapes
Why? ___c. Single bone in lower jaw, the dentary
___d. Two large openings behind the eye orhits in
17. What evidence at the boundary between the
Cretaceous and Paleogene systems at many localities the skull
favors the bolide impact hypothesis for the extinction of ___e. Whisker pits visible along front of upper jaw
the dinosaurs and many other animal groups? What
arguments can be advanced against this popular bones
hypothesis?

18. When were the Deccan Traps extruded? Discuss the
possible role of this and other synchronous volcanism as a
cause for mass extinctions.



15

The Cascade Range in Washington state. The volcanic
peaks of the Cascades were built on the western side of the
Columbia lava plateau during the Neogene. They are
related to subduction of the Pacific plate along the western
margin of North America. (# Marli Bryant Miller)

CHAPTER 15

Cenozoic Events

Many an aeon moulded earth before Key Chapter Concepts
her highest, man, was born,
Many an aeon too may pass when The Cenozoic Era spans from 65.5 million years
earth is manless and forlorn. ago to present. We divide it into the Paleogene,
Earth so huge and yet so bounded— Neogene, and Quaternay Periods. Major features
pools of salt and plots of land of our modern Earth formed during the Cenozoic.
Shallow skin of green and azure—
chains of mountains, grains of sand! The ridges and valleys of the present Appalachian
Mountains are the result of repeated cycles of
—Alfred, Lord Tennyson, “Locksley Hall: uplift and erosion.
Sixty Years After,” 1866
Eight marine transgressions and regressions
OUTLINE caused by fluctuating sea levels are recorded in
c THE TECTONICS–CLIMATE CONNECTION the Cenozoic sedimentary sequence of the
c STABILITY AND EROSION ALONG THE Atlantic and Gulf Coastal Plains.

NORTH AMERICAN EASTERN MARGIN Most geologic structures (folds, faults) of the Rocky
c GULF COAST: TRANSGRESSING AND Mountains formed during orogenic events from
Late Cretaceous to early Paleogene. Detritus from
REGRESSING SEA erosion of the ranges first filled intermontane
c THE MIGHTY CORDILLERA basins. Then broad regional uplift inaugurated a
c BOX 15–1 ENRICHMENT: OIL SHALE new erosional cycle that spread sediment eastward
c CREATING THE BASIN AND RANGE and gave the Rocky Mountains their present form
and relief.
PROVINCE
c BOX 15–2 GEOLOGY OF NATIONAL PARKS The Basin and Range Province was formed by
normal faulting from tensional forces during the
AND MONUMENTS: BADLANDS NATIONAL middle Cenozoic. Upfaulted blocks along a
PARK, SOUTH DAKOTA north–south trend formed elongate mountain
c BOX 15–3 ENRICHMENT: HELLISH CONDI- ranges that supplied erosional detritus to
TONS IN THE BASIN AND RANGE PROVINCE downfaulted basins between the upfaulted blocks.
c COLORADO PLATEAU UPLIFT
c COLUMBIA PLATEAU AND CASCADES The Colorado Plateau was uplifted, but not
VOLCANISM folded, during the Cenozoic. Its rejuvenated
c SIERRA NEVADA AND CALIFORNIA streams eroded the Grand Canyon and other
c THE NEW WEST COAST TECTONICS deep canyons.
c MEANWHILE, DRAMA OVERSEAS . . .
c BIG FREEZE: THE PLEISTOCENE ICE AGE The Columbia Plateau was created by Cenozoic
c WHAT CAUSED THE ICE AGE? basaltic lava flows. It was bordered on the west by
c CENOZOIC CLIMATES: GLOBAL WARMING Cascade Range volcanoes.
THEN COOLING
c SUMMARY The tectonic style changed during the Cenozoic.
c KEY TERMS The earlier compressional style of collision–
c QUESTIONS FOR REVIEW AND DISCUSSION subduction–thrust-faulting was replaced by
lateral movements of the American and Pacific
Plates grinding past one another.

The Tethys Sea closed and the Alpine–Himalayan
mountain systems formed. Both were caused by

469

470 c Chapter 15. Cenozoic Events TABLE 15-1 Geochronologic Terminology for the
Divisions of the Cenozoic Era. (From the International
the northward movement of Africa and the drifting Stratigraphic Chart of the International Commission on
of India toward Asia. Stratigraphy)

African rift valleys and associated volcanoes and Erathem AgeSeries
lakes were produced in the Late Cenozoic from Era m.y.a.Epoch
tensional forces along the east side of the African
continent. System Holocene
Period 0.01
During the Cenozoic, the Andes continued to
grow adjacent to the subuction zone that plunged Quaternary Pleistocene
beneath the western margin of South America.
Volcanoes and igneous intrusions above the Pliocene 1.8
subduction zone added great volumes of 5.3
igneous rock to the Andean crust.
Cenozoic Miocene
Continental glaciers covered a third of the Neogene
Northern Hemisphere during the Pleistocene
Epoch. The repeated advance and retreat of Oligocene 23.0
continental glaciers during the Pleistocene was 33.9
related to Milankovitch cycles, involving periodic
changes in Earth’s rotational and orbital Paleogene Eocene
movement.
58.8
cTHE TECTONICS–CLIMATE Paleocene
CONNECTION
65.5
The Cenozoic (“recent life”) is our own era of geologic
history. It is the era during which continents and harbors in Norway (at the same latitude as glacial
landscapes acquired today’s form, sea level reached Greenland) are testament to the powerful warm-
its present position, and today’s plants and animals ing influence of the Gulf Stream.
evolved. We divide the Cenozoic Era into three peri-
ods, the Paleogene, Neogene, and Quaternary. The isthmus provided a pathway for plant, ani-
Each is subdivided into various epochs (Table 15-1). mal, and human migration between the Americas.

The Cenozoic was busy with continued plate motion The isthmus created a barrier for efficient human
and seafloor spreading. In fact, half of the present seafloor travel between the Atlantic and Pacific that was
formed during the past 65.5 million years. Much of this new finally breached by the 51-mile-long Panama
ocean floor, extruded along the midoceanic ridges, was Canal a hundred years ago.
emplaced in the fast-expanding Atlantic and Indian
Oceans. As this widening progressed, the Americas There were two important continental breakups dur-
moved westward. The area that is now California ing the Cenozoic. First, the North Atlantic rift
came into contact with the northward-moving Pacific extended toward the North Pole, separating Green-
plate, thereby producing the San Andreas fault system. land from Scandinavia and severing the land connec-
South America moved against the Andean trench, actu- tion between Europe and North America. And second,
ally bending and displacing it. Australia separated from Antarctica, beginning its
journey to today’s location. Prior to this separation,
Throughout this discussion, please refer to the Antarctica was warmed by ocean currents flowing
highlighted areas in Figure 15-1. Down the western toward it from warmer regions to the north.
backbone of the Americas, vigorous orogenic and
volcanic activity formed the Isthmus of Panama, which However, by Oligocene time (about 30 million
today links North and South America. This little years ago), a frigid northward-flowing current devel-
Panamanian land bridge had remarkable implications: oped in the widening rift between Antarctica and
Australia. This current deflected warmer waters that
It blocked the westward movement of the North had given Antarctica a milder climate. Around the
Equatorial Current, forcing it to swing and flow
northeastward as the Gulf Stream. This famous
stream of warm water flows up the U.S. East
Coast, across the Atlantic, to England and then to
northern Europe (Fig. 15-2). Today’s remarkably
mild climate of the British Isles (which are at the
same latitude as Newfoundland) and ice-free

The Tectonics–Climate Connection b 471

FIGURE 15-1 Eocene global
paleogeography. (A) Major
landmasses during the Eocene,
about 50–45 million years ago.
Note that Antarctica and
Australia were still connected,
and the Americas were not.
(B) The world today. Compare
the highlighted areas on both
globes to note major tectonic
changes. (Drewry, G., et al.,
1974, Climatically Controlled
Sediments, the Geomagnetic
Field, and Trade Wind Belts in
Phanerozoic Time. Jour. Geol.,
82:556–558.) Although
Antarctica lay astride the South
Pole, it did not experience extreme
cold. How do we know this, and
what would have caused its mild
Eocene climate? (Answers to
questions appearing within figure
legends can be found in the Student
Study Guide.)

472 c Chapter 15. Cenozoic Events NorwaySweden 60°

Greenland
60°

North America Current United
Kingdom
ATLANTIC
45° Atlantic OCEAN Europe 45° FIGURE 15-2
30° Present trend of the
North Gulf Stream and
associated currents.
30° 0 1000 km Africa These warm surface
0 1000 mi currents bring
moderate climates to
northern Europe.

now-isolated Antarctica, cold circumpolar currents Streams transported terrigenous clastic sediments
were set in motion, and soon the continent began to from the highlands and deposited them on the plains.
assume its famous frigidity. Ocean water made dense On occasion, these were reworked by waves and cur-
by the extreme cold sank to the bottom and began rents during marine transgressions. Because these seas
drifting northward along the ocean floor, exterminat- were shallower along the western margins, the Ceno-
ing benthic invertebrates that were adapted to warmer zoic section of marine rocks is thinner near the Appa-
conditions. This transfer of frigid waters northward lachian source areas and becomes thicker and less
likely contributed to cooler temperatures during the clastic offshore (Fig. 15-4).
Late Eocene, Oligocene, and Miocene.
Around Florida, which lacked both erodible high-
The most dramatic Cenozoic tectonic event was the lands and major rivers, terrigenous clastics were sparse.
collision of Africa and India with Eurasia. This titanic
smashup transformed much of the Tethys Sea into lofty FIGURE 15-3 The Red Sea. The seaway opened about
mountain ranges, among which are the present Alps and 30 million years ago when the Arabian Peninsula rifted away
Himalayas. Also during the Cenozoic, a branch of the from Africa, and it continues to widen today. This view
Indian Ocean opened between Arabia and Africa, cre- from the Gemini spacecraft is toward the south. The land
ating the Gulf of Aden and the Red Sea (Fig. 15-3). wedge at the lower left is the Sinai Peninsula, bordered on
the right by the Gulf of Suez and Egypt, and on the left by
Continental interiors stood relatively high above sea the Gulf of Aqaba and Saudi Arabia. (NASA)
level during the Cenozoic. As a result, marine trans-
gressions were limited. Warm climates characterized
the early Cenozoic. We know this from the distribution
of fossil tropical and subtropical plants. By mid-
Cenozoic, however, temperate floras were spreading
across the continents. Plants requiring warmer condi-
tions retreated toward the equator. A reflection of this
change was the development of extensive grasslands.
Cooling continued throughout the Cenozoic,
culminating in Pleistocene glaciation.

cSTABILITY AND EROSION
ALONG THE NORTH AMERICAN
EASTERN MARGIN

Erosion continued to rule in the Appalachians, with
little orogenic activity occurring along North America’s
eastern margin. Periodically, as the uplands were bev-
eled by erosion, broad isostatic uplifts occurred. This
revitalized streams, which sculpted a new generation of
well-defined ridges and valleys. The most recent uplifts
in the Appalachian belt were accompanied by gentle
tilting of the Atlantic coastal plain and adjacent parts of
the continental shelf.

The Mighty Cordillera b 473

cGULF COAST: TRANSGRESSING
AND REGRESSING SEA

FIGURE 15-4 Cenozoic strata across the New Jersey The best record of Cenozoic strata in North America
coastal plain. The Cenozoic section of marine rocks is is found in the Gulf of Mexico coastal plain. Eight
thinner near the Appalachian source areas and becomes major transgressions and regressions are recorded in
thicker and less clastic offshore. (Cook, T.D; this region (Fig. 15-6). The Paleocene transgression
STRATIGRAPHIC ATLAS OF NORTH AND brought marine waters as far north as southern Illinois.
CENTRAL AMERICA # 1975 by SHELL OIL Co., 1977 Frequently, during marine regressions, nearshore del-
Reprinted by Princeton University Press by permission of taic sands were deposited above offshore shales.
Princeton University Press.)
The resulting interfingering of permeable sands and
Carbonate sediments accumulated to exceed 2500 impermeable clays provided ideal conditions for the
meters thickness along subsiding, elongate coralline eventual entrapment of oil and gas. Much of the oil was
platforms that resembled today’s Bahama Banks trapped in structures around salt domes (see Fig. 13-9).
(Fig. 15-5). In the early Neogene, uplift along the Wells drilled in the Gulf Coast have penetrated more
northern end of this tract raised the land area of Florida than 200 salt domes, and many more exist beneath the
above the waves, where it remains—so far. (Had this continental shelf. Gulf Coast Cenozoic rocks are
uplift not occurred, Walt Disney would have had to find famous for the petroleum they once yielded in great
a different location for his famous theme park.) quantities. Although recovery continues in the Gulf,
activity is much reduced today because of depletion.

“A wedge of sediments that thickens seaward” is a
good description for the Cenozoic formations of the
Gulf Coast. Geophysical measurements suggest that
the thickness of Cenozoic sediments in the Gulf may
exceed 12,000 meters. The area must have been sub-
siding rapidly in order to provide space for this great
thickness of sediment.

cTHE MIGHTY CORDILLERA
The most dramatic landscapes of North America lie
within a vast and geologically diverse region of moun-
tains, plateaus, basins, and volcanic peaks known as the

FIGURE 15-5 Cenozoic strata across trend A–A0 from southern Georgia to the Islands of
the Bahamas. Carbonate sediments accumulated more than 2500 meters thick. (Cook, T.D;
STRATIGRAPHIC ATLAS OF NORTH AND CENTRAL AMERICA # 1975 by
SHELL OIL Co., 1977 Reprinted by Princeton University Press by permission of Princeton
University Press.)

474 c Chapter 15. Cenozoic Events

FIGURE 15-6 Cenozoic strata across the Gulf coastal plain and Gulf of Mexico. “A wedge of
sediments that thickens seaward” is a good description for the Cenozoic formations of the
Gulf Coast. (Cook, T.D; STRATIGRAPHIC ATLAS OF NORTH AND CENTRAL
AMERICA # 1975 by SHELL OIL Co., 1977 Reprinted by Princeton University Press by
permission of Princeton University Press.) Marine invertebrate fossils indicate that most
formations shown here accumulated at shallow-to-moderate water depths. How could 27,000 feet of
sediment accumulate in water depths that rarely exceeded 1000 feet?

North American Cordillera. The Cordillera extends It remained for erosion, acting on the tilted and folded
southward from Alaska to Guatemala, and westward strata, to shape the landscapes we see today. Initially, the
along the 40th parallel from the eastern front of the fluvial sands and muds eroded from upland areas were
Rocky Mountains to the Pacific Ocean. Geologic trapped in basins between mountain ranges. These
features across this region include volcanic highlands, sediments hold the record of early Paleogene conditions.
canyons cut deeply into great thicknesses of layered, Later uplifts and erosion caused the sediment in basins,
colorful sedimentary rocks, basins with interior drain- as well as detritus from the uplift of the Rocky Moun-
age, fossil beds rich in the remains of terrestrial plants tains, to spread over the plains to the east. The result was
and animals, and an array of north–south trending a vast apron of nonmarine, Oligocene-through-Pliocene
fault-block mountains. The largest of these is the sands, shales, and lignites that covered the western high
spectacular Sierra Nevada, which rises 3000 feet above plains. Beds of volcanic ash are interspersed among the
the floor of the adjacent Owens Valley. sediments. They provide useful isotopic dates.

Geological variety is a characteristic of the Cordil- Sediment and Mineral Wealth
lera. It has may distinctive components. We can exam-
ine these components sequentially, beginning in the The Lower Paleogene sedimentary rocks deposited in
Rocky Mountains and moving westward. the intermountain basins included gray siltstones and
sandstones, carbonaceous shales, lignites, and coal.
The Rocky Mountains During the Cenozoic These rocks are well represented in the Fort Union
Formation. The Fort Union is approximately 1800
Most of Cordillera was emergent at the beginning of the meters (over a mile) thick. It contains huge tonnages of
Cenozoic (Fig. 15-7). An exception was the relatively low sulfur coal. This makes it very desirable, because
small and short-lived Paleocene Cannonball Sea, a lower sulfur means less atmospheric pollution. The
remnant of the earlier Cretaceous epeiric sea. Late Fort Union coal beds record widespread swampy
Cretaceous and Paleogene deformation had created conditions in this region during the Paleocene.
the major uplifts, faults, and folds of the Cordillera.

The Mighty Cordillera b 475

FIGURE 15-7 Generalized paleogeographic map of North America during the Paleogene
(65–23.5 million years ago). Terrestrial deposition prevailed in the Rocky Mountain region, except
for a solitary marine incursion called the Cannonball Sea. It probably was a vestige of the Late
Cretaceous epicontinental sea. Paleocene strata reveal the Cannonball Sea’s presence in dark shales of
western North Dakota, with more than 150 marine invertebrate species.

Many of these basins had no outlet during the early The laminations are varves, each of which consists of
Cenozoic, so water filled them to create large lakes. One a thin, dark winter layer and a lighter-colored summer
formed in the Green River basin of southwestern Wyo- layer. Counting varves reveals that more than 6.5 mil-
ming (Fig. 15-8). Sediments deposited in this basin lion years were required to deposit the Green River
comprise the Eocene Green River Formation (Fig. sediments. Fossils are abundant in Green River sedi-
15-9). Deposited here were more than 600 meters of mentary rocks, including insects, plant fragments, and
freshwater limestones and fine, laminated shales. well-preserved fishes (Fig. 15-10).

476 c Chapter 15. Cenozoic Events

FIGURE 15-9 The Green River Formation in a roadcut
near Soldier Summit, Utah. The exposure here consists of
dark shales and thin, lighter-colored limestones. (Field of
view 200 m.) (Harold Levin)

underlies and interfingers laterally with Green River
beds. The source rock for the hydrocarbons was prob-
ably the Green River Shale. There is also plenty of
colorful landscape geology in the region (Fig. 15-11).

To the south, in northeastern Utah, the Uinta basin
is notable as the structurally deepest in the Colorado
Plateau (see Fig. 15-8). It received a particularly thick
succession of Paleogene sediments (Fig. 15-12).

FIGURE 15-8 Cenozoic basins containing important Remarkable Fossils
oil shale deposits in Colorado, Utah, and Wyoming.
The base map outlines the drainage basin of the Upper During the late Eocene and Oligocene, explosive
Colorado River. (After D. A. Ulman et al. [eds], 1979, volcanic activity blanketed with volcanic ash the region
Synthetic Fields Development, U.S. Geol. Survey that today includes the San Juan Mountains and Yel-
publication) lowstone National Park (Fig. 15-13). For paleontolo-
gists, the more interesting rocks of this epoch are
In addition, the shales are rich in waxy hydrocar- floodplain deposits of the White River Formation.
bons. For this reason, they are called oil shales and can
be processed to yield petroleum (see Enrichment Box, This famous formation contains entire skeletons of
“Oil Shale”). Several Wyoming fields pump oil from Oligocene mammals in extraordinary number, variety,
the Claron Formation, an Eocene stream deposit that and excellence of preservation. The animals were vic-
tims of floods. The clays, silts, and ash beds of the White
River Formation are the sediments from which the
Badlands of South Dakota have been sculpted (see
Fig. A in “Badlands National Park, South Dakota”).

FIGURE 15-10 Eocene
freshwater fish
(Diplomystis) from the
Green River Formation,
Wyoming. Fossils are
abundant in Green River
sedimentary rocks, including
insects, plant fragments, and
well-preserved fishes like this
specimen, which is 8 cm
long—approximately the size
of a minnow or sardine.
(Harold Levin)

The Mighty Cordillera b 477

ENRICHMENT

Oil Shale giant pressure cooker that fuels itself with the very gases
generated during heating.
The Eocene oil shale (Fig. A) that occurs in parts of Wyoming,
Utah, and Colorado is rich in an oil-yielding organic compound Many oil shales yield between one-half and three-quarters
known as kerogen. When heated to 480 C, the kerogen in oil of a barrel of oil (42 U.S. gallons) per ton of rock. If you mined
shale vaporizes. This vapor can be condensed to form a thick only the known shale layers that are thicker than 10 meters,
oil. When enriched with hydrogen, it can be refined into they would yield an impressive 540 billion barrels of oil. If this
gasoline and other products in much the same way as ordinary oil were produced during the next decade, it would appreciably
crude oil. The shale is heated in a retort, which resembles a reduce the U.S. dependency on foreign crude. Production of
oil from oil shale, however, requires vast amounts of water in a
FIGURE A A block of oil shale from the Eocene Green River Formation region of North America where water is in short supply.
in Colorado and a glass beaker of crude oil obtained from the same
formation. (U.S. Department of Energy/Photo Researchers, Inc.) Two additional problems have prevented full-scale mining
and retorting of oil shales. The first is the waste-disposal prob-
lem. In the process of crushing and retorting, the shale expands
to occupy about 30% more volume than was present in the
original rock. Geologists call this the “popcorn effect.” Where
could this great volume of light, dusty material be placed?

The second problem is air quality. Processing huge ton-
nages of oil shale is likely to release large amounts of dust into
the atmosphere. Unfortunately, the shales occur in arid
regions, and there is little water available to suppress the
dust and revegetate the land. Until these problems are solved,
producing oil from oil shale will be slow. Perhaps this works to
our advantage, for we may need this oil for processed goods a
century or two from now, after the internal combustion engine
has experienced its own mass extinction.

Other interesting Oligocene beds are exposed in nous ash that settled to the bottom of a neighboring
central Colorado, in the Florissant Fossil Beds National lake, burying countless insects (Fig. 15-14), leaves, some
Monument. Volcanoes in this area produced volumi- fish, and even a few birds. Plant remains—including
tree trunks in their original position, leaves, spores, and
pollen—indicate subhumid conditions for the region
and elevations between 300 and 900 meters.

FIGURE 15-11 Eocene rocks at Cedar Breaks National Miocene Crustal Unrest
Monument, Utah. Here the Eocene Cedar Breaks
Formation has been eroded into steep ravines, pinnacles, and The Miocene was a time of continued stream and lake
razor-sharp divides. Early explorers used the term “breaks” sedimentation in intermountain basins as well as on
to describe the change in topography where an elevated level the plains east of the mountains. By Miocene time,
area “breaks down” by eroding to a lower elevation. The climates had cooled. Expanding grasslands supported
formation is eroded into a magnificent badlands topography. Miocene camels, horses, rhinoceroses, deer, and other
(Charlie Ott/Photo Researchers, Inc.) grazing mammals. In the central and southern Rock-
ies, Miocene formations include beds of volcanic ash
and lava flows, attesting to vigorous volcanic activity.
The well-known gold deposits at Cripple Creek,
Colorado, are mined from veins associated with a
Miocene volcano.

Regional uplift of the Rockies also began in the
Miocene. The uplift increased stream gradients and
rates of erosion. Great volumes of sediment filled
low-lying areas and then spread eastward, helping to
construct the Great Plains.

Today’s Rocky Mountain topography is the result
of uplift and erosion that began in the Miocene.
Sediment eroded from the mountains covered the

478 c Chapter 15. Cenozoic Events

FIGURE 15-12 A north–south view through the Uinta basin. The basin received a thick

succession of Paleogene sediments, including the Claron, Green River, Uinta, and Duchesne

River formations. Paleogene units are above the Cretaceous Mesaverde Group. These

formations can still be observed in exposures in the interior of the basin, where they have been

affected only slightly by post-Eocene erosion.

cWhite River deposits and equivalent beds over exten- CREATING THE BASIN AND RANGE

sive areas of South Dakota and Nebraska. Remains of PROVINCE

plants and animals in some of the overlying Pliocene The Basin and Range Province is a large area of, as its
strata indicate the presence of cooler and dryer condi- name says, alternating basins and mountain ranges
tions, a harbinger of an impending ice age. (Fig. 15-16). The province extends across a region
from Nevada and western Utah southward into central
Crustal movement continued throughout the Mexico. To the west, the province includes the Sierra
remaining epochs of the Cenozoic, raising some of Nevada, a range that owes its spectacular elevation to a
the highest peaks of the Rockies. Normal faulting and normal fault zone along its eastern margin.
volcanism accompanied many of these events, provid-
ing spectacular scenery like that of the famous Grand Most broadly, the basin-and-range pattern results
Tetons of Wyoming (Fig. 15-15). from stretching of Earth’s crust:

1. The region was up-arched during the Mesozoic
Era.

2. Beginning in the Miocene, the arch subsided
between great normal faults.

3. The uplifted blocks formed linear mountain
ranges. These became sources of sediment,
which filled adjacent downdropped basins.

4. Faulting opened fissures for the escape of molten
rock from magmatic bodies below. The resulting
volcanoes along the western side of the province
produced extensive lava flows, while in the east-
ern region, the landscape was buried in ash and
pumice.

5. Vigorous erosion of the upthrown fault blocks
followed the volcanism. Coarse terrigenous clas-
tics eroded from the mountains filled the down-
faulted basins, clogged rivers, and caused lakes to
develop behind debris dams.

6. Gypsum and salt layers formed when these lakes
evaporated.

FIGURE 15-13 Yellowstone Falls and canyon, The cause of the crustal stretching—the tensional
Yellowstone National Park, Wyoming. Rocks in the faulting—that produced the Basin and Range Province
canyon walls are lava flows and volcanic ash, often altered to is much debated. Here are current hypotheses:
bright colors by hydrothermal activity. (L. E. Davis)
When westward-moving North America over-
rode a spreading center (the East Pacific rise),

GEOLOGY OF NATIONAL PARKS AND MONUMENTS

Badlands National Park, South Dakota

When you visit Badlands National Park, FIGURE A Badlands National Park, South Dakota. This rugged terrain has been water-carved
you enter an alien land of steep ravines from flat-lying Eocene and Oligocene shales, mudstones, and ash beds. What bedrock
and ethereal spires. You are amidst the and climatic conditions favor the formation of badlands? (R. F. Dymek)
world’s finest example of badlands
topography (Fig. A). Badlands are nearly its name from the whitish color forming an early badlands topography
devoid of vegetation, where erosion dis- imparted to the water by particles of that subsequently eroded away. As trib-
sects the land into a labyrinth of chasms, bentonitic clays. utary streams continued to extend their
steep ridges, and pinnacles. Bedrock of channels northward into the upland
impermeable clays and shales, the Formation of the badlands topogra- area, the badlands we view today
absence of a protective plant cover, phy began with an episode of regional began their development.
and infrequent but heavy rains are the uplift late in the Pliocene Epoch. As a
conditions under which badlands form. result of the uplift, major streams reju- Badlands National Park has an inter-
venated and entrenched themselves esting geologic history. Its oldest forma-
The rocks here are horizontal layers into the underlying poorly resistant tion is the Pierre Shale, deposited in the
of clay, shale, and volcanic ash. Much beds: The White River became one of shallow, extensive sea that covered
of the ash has been weathered to a these entrenched streams. The process much of western North America during
clayey sedimentary rock called benton- provided steeper gradients to tributary the Late Cretaceous. The Cretaceous
ite, which erodes easily. Beds of river streams that flowed southward into the ended with regional uplift, and as the
sandstones are more difficult to White River. The southern perimeter of sea receded, erosion produced the
erode, and they often form a caprock the uplifted area that paralleled the unconformity on which the principal
atop buttes and provide an overhanging White River Valley to the north became beds of the badlands were deposited.
ledge, forming erosional features intricately gullied and dissected, These are sediments of the Oligocene
called “mushroom rocks.”
White River Group—primarily
Running water has been the most deposits of slow-moving streams
important agent in forming the bad- that flowed across a landscape char-
lands. Rain falling suddenly during acterized by wide floodplains,
cloudbursts cannot infiltrate the marshlands, lakes, and ponds.
impermeable clays and shales.
Instead, the rain runs off and becomes At this time, abundant plants
channeled into numerous small rivu- nourished a rich fauna of turtles,
lets that have eroded a dense system of lizards, alligators, huge titanotheres,
ever-enlarging, coalescing gullies and aquatic rhinoceroses, three-toed
ravines. The impact of raindrops as horses, early camels, entelodonts,
they pelt the soft surfaces dislodges oreodonts, and tapirs. Many of these
particles of rock, speeding the ero- herbivores were prey for predatory
sional processes. members of the dog and cat families.
Figure 16-45 provides a panoramic
Bentonite in the beds also facilitates view of the environment and its
erosion. The clay in bentonite swells inhabitants. The rich fossil discover-
enormously when it becomes wet. ies in Badlands National Park
On drying, bentonite crumbles and prompted some geologists to call
disintegrates, forming masses of easily the Oligocene “The Golden Age of
eroded sediment. The White River takes Mammals.”

FIGURE B Map of the park.

479

480 c Chapter 15. Cenozoic Events

FIGURE 15-14 Fossil spider and insects from Oligocene beds near Florissant, Colorado.
Volcanoes in this area produced voluminous ash that settled to the bottom of a lake,
burying countless insects and leaves. They are preserved in shale-like tuff, which is consolidated
volcanic ash. (Harold Levin)

FIGURE 15-15 Wyoming’s
lofty Teton Range. The Teton
Range was elevated along great
normal faults, with displacement
reaching nearly 6000 meters
(3.7 miles). The magnificent
eastern face is a fault scarp rising
2500 meters to an elevation
exceeding 4000 meters.
Following uplift, the range has
been cut by water and ice
erosion. (Flickr Open/Getty
Images)

Creating the Basin and Range Province b 481

A

B
FIGURE 15-16 (A) Shaded relief map of the Basin and Range Province. As indicated in
(B), normal faults separate the ranges and intervening basins. Major C. E. Dutton (1841–1912),
who made major contributions to our understanding of the geology of the American West,
described the appearance of the north–south trending ranges on a map as looking like “an army
of caterpillars crawling northward out of Mexico.” (RSI/NASA/N. W. Short)

482 c Chapter 15. Cenozoic Events

ENRICHMENT

Hellish Conditions in the Basin and Range Province

If the great Italian poet Dante Alighieri (1265–1321), author up to 100 miles per hour. The volume of ash expelled was
of The Inferno, had strolled the Basin and Range Province truly incredible. A single ash fall covered more than 10,000
during the Oligocene and Early Miocene, his vision of the square km (roughly the area of Florida’s Everglades) to a
lower world might have included turbulent clouds of white- depth exceeding 180 meters.
hot ash, jets of searing gases, and fiery explosions of molten
rock. Such was the scene during the Mid-Cenozoic. Along Commonly, high-silica magmas cool slowly at depth and
the trails of mountain ranges in the province, evidence in the form batholiths of granite and granodiorite. However, the
form of hardened ash falls and solidified lava beds is nearly magma of the Basin and Range Province breached the crust
everywhere underfoot. Volcanic rocks also lie between the and produced the intense episode of volcanism. What might
mountain ranges, hidden by sediment layers. have been the cause?

Volcanic rocks of the Basin and Range Province are During the Mid-Cenozoic, the North American plate was
derived from high-silica magmas, which are highly viscous moving rapidly westward at about 15 cm per year. It overrode
(thick). Such magmas produce explosive volcanism in which the Pacific plate so quickly that the plate was unable to
hot gases, steam, ash, and pyroclastics of all sizes erupt with descend into the mantle. Rather, the Pacific plate slid
enormous force. Fiery turbulent clouds of hot gas and ash horizontally under North America. As the hot seafloor plate
called nue e ardentes accompany such eruptions and dev- moved along beneath what is now the Basin and Range
astate the lands as they rush swiftly down slopes at speeds of Province, it probably provided much of the heat responsible
for volcanism.

the subducted spreading center caused uplift and The plateau was repeatedly raised during early-to-
stretching of the crust. middle Pliocene time (about 10 to 5 million years ago).
Steep faults developed on the plateau during its rise,
Normal faulting in the Basin and Range is simply providing avenues for extrusion of lavas and volcanic
how the crust adjusted to the change along the ash. The San Francisco Peaks near Flagstaff, Arizona,
California coast when oblique shearing of the edge are a group of recent Colorado Plateau volcanoes and
of the continent during the Miocene replaced cinder cones (Fig. 15-18).
the earlier subduction zone.
The best-known feature resulting from the linked
This is extension and uplift of the crust from the processes of uplift and erosion on the Colorado Pla-
remnants of an oceanic plate that was carried teau is the Grand Canyon of the Colorado River (Fig.
beneath the Basin and Range region by an earlier 15-19). This awesome monument to the forces of
subduction episode. When subduction ceased, erosion exceeds a depth of 2600 meters (1.6 miles)
the oceanic slab may have formed a partially in places. The river has eroded through a half-billion
molten buoyant mass that pressed upward against years of Phanerozoic strata into crystalline Pre-
the overlying crust, causing tensional faulting and cambrian basement rocks.
escape of lava along fault and fracture zones.
“Wow! How long did it take to make this?” That’s
The crustal extension and tensional faulting is the question people ask when they see the Grand
related to convectional movements beneath the Canyon for the first time. Investigations of water table
continental plate similar to those that cause the evidence preserved in caves along the walls of the
breaking apart of continents. canyon indicate canyon carving may have started in
the western part of the canyon about 17 million years
Each hypothesis is supported by evidence, and the ago. About 6 million years ago, the western drainage
truth could be a combination of these ideas. As with was captured by what was to become the Colorado
so many things in science, more research is needed River. From that time onward, cutting of the canyon
before we can conclusively explain the formation of the proceeded at the rapid rate of about 23 cm every
remarkable Basin and Range. thousand years. Erosion proceeds more slowly today
because of water removed from the Colorado River for
cCOLORADO PLATEAU UPLIFT human use.
The Colorado Plateau is a vast and magnificent region
of uplift (Fig. 15-17, A and B). Here Paleozoic and cCOLUMBIA PLATEAU AND CASCADES
Mesozoic rocks are relatively flat-lying, so we know VOLCANISM
the Plateau remained undeformed during Mesozoic
orogenies. The region formed a buttress around which The Columbia Plateau, named for the Columbia
folding and faulting produced highlands. River, lies in the U.S. northwest (Fig. 15-20). Unlike
the Colorado Plateau, which is constructed of layered

Columbia Plateau and Cascades Volcanism b 483

B

FIGURE 15-17 (A) National Geographic Map of the Colorado Plateau. (B) A Colorado
Plateau canyon formed by the Dolores River near Grand Junction, Colorado. ((A) NG Maps,
(B) Harold Levin.)

sedimentary rocks, the Columbia Plateau was built by spread out and buried more than 500,000 square km of
volcanic activity. existing topography beneath layer upon layer of lava
(Fig. 15-21). This is similar to the area covered by the
During the Miocene, about 15 million years ago, basaltic Deccan Traps in India.
basaltic lavas like those that pour from midoceanic
ridges erupted along deep fissures approximately par- West of the Columbia Plateau lies a belt that also was
allel to the Washington–Idaho border. The liquid rock the site of extensive volcanic activity. Here, however, the

484 c Chapter 15. Cenozoic Events

Pacific Ocean 116º
CASCADE MOUNTAINS0 100 km

BC
WA ID

Seattle
COLUMBIA
PLATEAU

FIGURE 15-18 Bird’s-eye view of a large cinder cone in OR
the San Francisco volcanic field, northern Arizona. The CA
solidified flow from the cone is 7 km long and more than
30 meters thick. (USGS) Did the lava flow from the volcano FIGURE 15-20 The Columbia Plateau and Cascade
before or after extrusion of the pyroclastics that built the cinder Range are located in the U.S. northwest.
cone?
Cascades are manifestations of an ongoing collision
outpourings of more viscous lavas generated the moun- between the American plate and the small Juan de
tains of the Cascade Range. Volcanism in this region Fuca plate of the eastern Pacific. As the Juan de Fuca
began about 4 million years ago and continues today, as plate plunges beneath Oregon and Washington, molten
was made dramatically obvious during the eruptions of rock rises to supply lava for the volcanoes (Fig. 15-23).
Mount St. Helens in 1980 and 2004 (Fig. 15-22).
Water played an important role in producing the
The recent activity of Mount St. Helens and the magmas that are the source of Cascade volcanism.
older eruptions that gave us the volcanic peaks of the Calculations indicate that the temperature of the

FIGURE 15-19 The Grand Canyon of the Colorado FIGURE 15-21 Basalt lava flows of the Columbia Plateau.
River. This great gorge is largely the result of uplift In some places, these low-viscosity lavas remained liquid and
accompanied by stream erosion during the Cenozoic Era. flowed 170 km (roughly 100 miles) from their source. Their
(R. F. Dymek) combined thickness exceeds 2800 meters (1.7 miles). The
Snake River is in the foreground. (# Marli Bryant Miller)

What is the origin of the closely spaced vertical cracks seen on the
cliff faces?