Journal of South American Earth Sciences 121 (2023) 104155 Available online 10 December 2022 0895-9811/© 2022 Elsevier Ltd. All rights reserved. New records of continental vertebrates from the Triassic of the Atacama Desert, northern Chile Rodrigo A. Otero a,b,c,* , David Rubilar-Rogers d , Sergio Soto-Acuna˜ b,e , Alexander Vargas M. b , Gonzalo Mella Rojasf , Raúl Ugalde e,f , Osvaldo Rojas a , Jennyfer Rojas a , Fernando E. Novas g a Museo de Historia Natural y Cultural del Desierto de Atacama, Interior Parque El Loa s/n, Calama, Chile b Laboratorio de Ontogenia y Filogenia, Departamento de Biología, Facultad de Ciencias, Universidad de Chile, Las Palmeras, 3425, Nu˜ noa, ˜ Santiago, Chile c Paleo Consultores Ltda, Huel´en 165, oficina C, Providencia, Santiago, Chile d Museo Nacional de Historia Natural. Interior Paque Quinta Normal, Santiago, Chile e KayTreng Consultores SpA, Jos´e Domingo Canas ˜ 1640, AP1502, Nu˜ noa, ˜ Santiago, Chile f Escuela de Geología, Facultad de Ciencias, Ingeniería y Tecnología, Universidad Mayor, Manuel Montt 367, Santiago, Chile g Laboratorio de Anatomía Comparada, Museo Argentino de Ciencias Naturales – Conicet, Av. Angel Gallardo 470, Buenos Aires, Argentina ARTICLE INFO Keywords: Fossil vertebrates Ichnology Triassic Atacama Basin Northern Chile ABSTRACT This contribution describes new vertebrate remains found in Triassic strata informally known as “Estratos El Bordo”, in the Atacama Desert, northern Chile. These include the first temnospondyl record in the El Bordo Basin, plus actinopterygians referable to the clade Pseudobeaconiidae and a second form with affinities to the incertae sedis ray-finned fish Guaymayenia paramillensis. Associated to these taxa, we describe two spiral coprolites. Their vertebrate producers are discussed. These new freshwater vertebrates add to previous archosaur records from “Estratos El Bordo”, represented by the putative aetosaur Chilenosuchus forttae, an indeterminate silesaurid and a fragmentary indeterminate Crocodylomorpha. Vertebrate remains here described show affinities to the known diversity from the Cuyana Basin of Argentina, but differ from those recorded in northern Argentina (Ischigualasto-Villa Union ´ Basin) and southern Brazil (Parana ´ Basin). This suggests preliminary paleoenvironmental insights for the continental vertebrates that inhabited the southwestern margin of South America during the Triassic. The new findings also support a Middle-to-Upper Triassic age for the “Estratos El Bordo” unit, as previously indicated by radioisotopic datating. 1. Introduction In contrast with the rich Triassic vertebrate record documented in Argentina (Romer, 1960; Bonaparte, 1963; Desojo et al., 2020 and references therein) the coeval vertebrate fauna described to date from the western side of the Andes Cordillera remains scarce. Records from northern Chile came from the informal unit called “Estratos El Bordo” (El Bordo strata) and are restricted to a single taxon referred to the Aetosauria, represented by the fragmentary skeleton of Chilenosuchus forttae Casamiquela (1980) (Desojo, 2003). An articulated partial postcranial skeleton referred to an indeterminate silesaurid was later reported from the same unit (Rubilar-Rogers et al., 2013), as well as limb remains of an indeterminate Crocodylomorpha, preliminarily described by Soto-Acuna ˜ et al. (2016). The present contribution documents on new vertebrate remains recovered from the Triassic “Estratos El Bordo” in the Atacama Desert, northern Chile. These include the first local record of a temnospondyl, plus two types of freshwater actinopterygians. Two coprolites produced by aquatic vertebrates are also described. The new records enrich the still scarce knowledge of the vertebrate continental fauna that inhabited the southwestern margin of South America during the Triassic. 2. Locality and geologic setting The studied material was recovered in the locality of Sierra del Quimal (Fig. 1), east San Pedro de Atacama, in northern Chile. The fossil-bearing levels are part of the informal unit “Estratos El Bordo” (Fortt, 1981). Its first description considered ca. 1200 m of a continental, well-stratified, volcano-sedimentary sequence exposed on the noreastern part of Cerro Quimal (Fig. 1) with their strata in a subvertical dip and * Corresponding author. Museo de Historia Natural y Cultural del Desierto de Atacama, Interior Parque El Loa s/n, Calama, Chile. E-mail address: [email protected] (R.A. Otero). Contents lists available at ScienceDirect Journal of South American Earth Sciences journal homepage: www.elsevier.com/locate/jsames https://doi.org/10.1016/j.jsames.2022.104155 Received 20 May 2022; Received in revised form 4 December 2022; Accepted 6 December 2022
Journal of South American Earth Sciences 121 (2023) 104155 2 slightly variable N–S strike. Later, Basso and Mpodozis (2012) extended the thickness of the unit to 4800 m. Its base is unknown, being obscured by an angular discordance with lavas and volcanic brecchias of the upper Carboniferous-lower Permian Agua Dulce Formation (García, 1967), and by the distinctive reddish sandstones and conglomerates of the Late Cretaceous Tonel Formation (Ramírez and Gardeweg, 1982; Breitkreuz et al., 1992; Troncoso et al., 2002). The visible base of the sequence is conformed by tuffs and grey-to-black, finely stratified sandstones which include poorly preserved algae (Rivano, pers. comm. in Ramírez and Gardeweg, 1982) and plant imprints (Ramírez and Gardeweg, 1982). The middle levels of the section described by Ramírez and Gardeweg (1982) are characterized by the presence of well-stratified shale lenses under 20 cm of thickness, with dark grey color, followed by levels of ostracod coquines intercalated with vitreous tuffs, shales with silicified nodules, and calcareous sandstones. The latter also include plant remains and continental vertebrates (Casamiquela, 1980; Covacevich, 1981; Ramírez and Gardeweg, 1982). Particularly, the ostracod-rich coquinaceous levels host very frequent disarticulated vertebrate remains and occasionally associated skeletal portions. These sandstones are covered by volcanic rocks, mostly andesites and tuffs. The upper levels are dominated by greenish, coarse sandstones and coarse, green to brownish tuffs. These sandstones include concretionary levels with occasional vertebrate remains (SSA and RAO, pers. comm., 2019) and coprolites (this study). The tuffs include poorly preserved plant remains. In contrast to the scheme of Ramírez and Gardeweg (1982), Basso and Mpodozis (2012) considered the ostracod- and vertebrate-rich levels as the upper part of the section (“member 4” of Basso and Mpodozis, 2012). The age of the “Estratos El Bordo” unit was disputed. Casamiquela (1980) suggested a ‘Neotriassic’ age based on the presence of aetosaurs (i.e., Chilenosuchus forttae). Covacevich (1981) suggested a Triassic-Jurassic age based on the paleobotanical assemblage including Taeniopteris sp., Pterophyllum sp., Dicroidium? sp., and Neocalamites sp. Later, Osorio (pers. comm. in Ramírez and Gardeweg, 1982) suggested a Carboniferous-Permian age based on ostracods. Covacevich (1981) identified conchostracans referring them to Cyzicus sp. The recognized assemblage suggests an upper Paleozoic-Triassic age. Breitkreuz et al. (1992) described a plant assemblage including Botriochiopsis, Cythaocarpus and Paracalamites, thus, assigning a Westphalian-Permian age for the ostracod-coquina levels, and contrasting with the age previously suggested on the basis of fossil plants and vertebrates. Later, the age of the underlying Agua Dulce Formation was restricted to the Carboniferous-Permian based on radioisotopic dates (Morand´e, 2014), while the basal levels of the “Estratos El Bordo” were radioisotopically dated between 238.7 ± 0.4 and 240.8 ± 0.3 My (Basso and Marinovic, 2003), being the second one recently recalculated to 240 ± 0.6 Ma (Irmis et al., 2022), thus, indicating a maximum Middle Triassic age for the base of the sequence. The age of the upper levels (point 1 of this study; see Material and Methods) should be constrained to the Middle-to-Upper Triassic (see below), although no datations are still available from the fossil-bearing levels neither for the available roof of the unit. For the studied unit, a ca. 500 m sedimentary section is here described (Fig. 2), excluding the lower volcanic section. The studied material was recovered from three different points (1, 3 and 4 in Fig. 1), representing three different levels in the stratigraphic section. Point 1 is placed in the upper third of the section, while point 3 represents the mid part of the section. The stratigraphically lower (and therefore, older) material is that from point 4, placed in the lower 80 m of the section. 3. Material and Methods Institutional Abbreviations–MUHNCAL, Museo de Historia Natural y Cultural del Desierto de Atacama, Calama, Chile. Fig. 1. Map of the studied locality and plot of the main geologic units exposed near Cerro Quimal, in the Region ´ de Antofagasta, northern Chile. Black circles indicate the points of provenance (1–4 as indicated in the text) for the studied material. Modified from Ramírez and Gardeweg (1982) and Basso and Mpodozis (2012). R.A. Otero et al.
Journal of South American Earth Sciences 121 (2023) 104155 3 Fig. 2. Stratigraphic composite log of ’Estratos El Bordo’ unit and position of the fossils here Described. Abbreviations: Treb: Triassic ’Estratos El Bordo’ unit, as in the formal geological maps of Sernageomin. Treb (a), (b), (c) and (d) are the equivalent members proposed by Basso and Mpodozis (2012). The U–Pb age at the bottom was recalculated following Irmis et al. (2022). The covered layers between members comprise lava successions, which are not represented. R.A. Otero et al.
Journal of South American Earth Sciences 121 (2023) 104155 4 The studied material was recovered in two field visits. A first fieldwork was carried on in 2016 in the context of a Chilean-Argentinean study project between the Museo de Historia Natural y Cultural del Desierto de Atacama (Museum of Natural and Cultural History of the Atacama Desert, Calama, Chile; MUHNCAL) and Museo Argentino de Ciencias Naturales, Buenos Aires (MACN), which allowed the recovering of several fragmentary specimens, including MUHNCAL.20240. During 2018, a new Chilean fieldwork supported by Proyecto Anillo ACT172099 and by the MUHNCAL re-discovered fossiliferous outcrops that might correlate with the localities of few historical local specimens (Casamiquela, 1980; Rubilar-Rogers et al., 2013). The latter fieldwork also found previously unreported fossiliferous levels with vertebrate remains. The specimens here studied came from three points within the Estratos El Bordo unit (points 1, 3 and 4 in Fig. 1). Point 1 is exposed in the eastern part of the unit (representing the youngest fossiliferous levels) and comprises strata with few concretionary levels including occasional vertebrate remains and coprolites. Both coprolites MUHNCAL.20241 and MUHNCAL.20242 were recovered at this site, being associated to transported, fragmentary bone remains and in situ skeletal remains (not prepared yet) embedded in discrete concretions restricted to a single level. Points 2 and 3 are part of the mid section exposed in sub-vertical orientation and secondarily faulted. This is separated 1.3 km southeast from point 1. MUHNCAL.20233 was recovered on point 2 from ostracod-coquina levels. The temnospondyl MUHNCAL.20238 was recovered on point 3, ex-situ but evidencing scarce transportation, based on the associated presence of the cast and its respective countermold. Finally, point 4 (upper levels, 2.9 km SW from point 3) corresponds with the historical site where Chilenosuchus forttae was recovered (Casamiquela, 1980). In this point we recovered numerous dermal scutes referable to the same latter taxon (Soto-Acuna ˜ et al., 2016), as well as fragmentary, indeterminate archosaur remains and the actinopterygian MUHNCAL.20233. These levels are part of the lower section of the column. Samples have been consolidated with cyanocrilate. Most samples were not prepared due to its preservation as a delicate bony layer. MUHNCAL.20238 and 20233 were cleaned by RAO with an air scribe 9100 on facilities of Red Paleontologica ´ U. Chile (Facultad de Ciencias, Universidad de Chile). 4. Systematic paleontology Temnospondyli sensu Milner (1990). Temnospondyli indet. Fig. 3. Temnospondyli indet. MUHNCAL.20,238, skull fragment. Cerro Quimal, northern Chile. Point 3, Estratos El Bordo, Middle-to-Upper Triassic. A) Detailed photograph of the cast in right dorsolateral view. B) Anatomical interpretation of the previous. C) Interpreted anatomical position of the skull fragment. D) Skull fragment and its mirrored outline with respect to the ventral view of the skull. Cranial outline based on Marsicano (1999: Fig. 4B). E) countermold of the same fragment in A and B. F) close-up of a preserved tooth in the countermold. Anatomical Abbreviations: apt, broken attachment of the pterygoid; ch, choana; d, dentary; ept þ pl, ectopterygoid + palatine; mx, maxilla; os, ostracods; rp, resorption pits; s?, ectopterygoid-palatine suture?; tso, tooth sockets. Scale bars equals 10 mm, except in F, 1 mm. R.A. Otero et al.
Journal of South American Earth Sciences 121 (2023) 104155 5 (Fig. 3) Material–MUHNCAL.20238, a fragment of the right side of the skull. Horizon and Age–"Estratos El Bordo” unit, northern Chile. Member 4 of Basso and Mpodozis (2012), 380 m up from the base of the sedimentary section. Middle-to-Upper Triassic. Description–The material is a right skull fragment, preserved as a cast and its respective countermold with bony parts of the occlusal margin. The cast mostly represents the inner (ventral) surface of what is interpreted as the right ectopterygoid, the right palatine and part of the maxilla. Clear sutures are not visible, although a clear separation of the ectopterygoid + palatine surface can be observed in an anatomically sound position, being tentatively interpreted as a real suture (labeled as ’s?’in Fig. 3B). The posterior part of the cast preserves the complete medial contour of the bone, which is medially concave and consistent with the margin of the right interpterygoid vacuity. The posterior end of the cast also has a smooth, unbroken contour. Between both preserved contours is the broken medial extension of the ectopterygoid. The anterior third of the cast has a second, small concave contour, which may correlate with the right palatal choana (Marsicano, 1999: Fig. 12B). Length of MUHNCAL.20238 skull is estimated in 9 cm, based on the preserved bone fragments and considered the complete skull of Pelorocephalus for comparison (Marsicano, 1999: Fig. 4B). The occlusal part is preserved as a groove (negative in the cast), thus, indicating this was a prominent element in the ventral margin of the skull. The presence of teeth indicates this belongs to the maxilla. More than thirty tooth sockets are preserved, being homogeneously distributed and very close from each other. Few resorption pits are present, being comparatively larger than the teeth sockets and consistently placed in a dorsal position with respect to the tooth row. The element here interpreted as the maxilla is confined to the ventral part of the skull, without any dorsal projection, thus, appearing as a bony strip with teeth. A small anterior occlusal fragment of the dentary is also preserved, with similar kind of dental sockets as those seen in the skull. A single tooth (3 mm high) is preserved in the countermold. This has a sharp and internally hollow crown, slightly recurved backwards. Remarks– The concretion containing MUHNCAL. 20238 naturally split through the bones, causing an irregular exposure of the anatomical elements, and leaving part of them in both concretion fragments. The largest bones are presented in Fig. 3A and B. In these, the maxilla appears as a strip of bone with teeth, which is attached to the ectopterygoid + palatine. This is a feature considered to be typical of stereospondyl temnospondyls (Pineiro ˜ et al., 2012: Fig. 1; Dias-Da-Silva et al., 2006; Schoch, 2013: Fig. 1). Osteichthyes Huxley, 1880. Actinopterygii Cope, 1887. Neopterygii Regan, 1923. Louwoichthyiformes Xu, 2021. Pseudobeaconiidae Lopez-Arbarello ´ and Zavattieri, 2006 Pseudobeaconiidae indet. (Fig. 4) Material–MUHNCAL.20233, partial impression of the body and small part of a dorsal fin. Horizon and Age–"Estratos El Bordo” unit, northern Chile. Member 4 of Basso and Mpodozis (2012), 200 m up from the base of the sedimentary section. Middle-to-Upper Triassic. Description–MUHNCAL.20233 is three-dimensionally preserved. The visible side is the left one (Fig. 4A and B), which is completely covered by rhomboidal ganoid scales with straight posterior border. The lateral line is visible in the lower third of the preserved body, and is conformed by centrally ridged scales. The dorsal margin of MUHNCAL.20233 has distinctively sharp, spine-like scales (under 5 mm). In its postero-dorsal preserved part, the anterior fragment of a fin is poorly preserved, with at least three basal fulcra that can be observed. The cross-section view (Fig. 4C) confirms the orientation of the specimen and its available left view, based on the presence of a coelom (abdominal cavity) and the typically narrower ventral musculature that encloses the trunk. This indicates that the available portion anatomically belongs to the trunk instead of the tail, and then, the small fin fragment preserved should belong to a dorsal fin instead to a caudal fin. A dorsal prominent ridge with spine-like scales is present on the anterior half of the preserved portion. Spine-like scales are also present anterior and posterior to the ridge. Remarks–MUHNCAL.20233 possesses a distinctive incomplete dorsal ridge with spine-like scales in the dorsal margin of the trunk and anterior to the dorsal fin. This feature is considered a synapomorphy of South American Triassic Pseudobeaconiidae (Lopez-Arbarello ´ and Zavattieri, 2006; Lopez-Arbarello ´ et al., 2010). Moreover, MUHNCAL.20233 also possesses few characters indicated for the diagnosis of Pseudobeaconiidae (Lopez-Arbarello ´ and Zavattieri, 2008), being these an estimated total length under 120 mm, and scales with straight posterior border. The presence of an elevated central region and marginal concentric ridges of ganoine (which is another synapomorphy of Pseudobeaconiidae) cannot be verified in the Chilean material due to its fragmentary preservation. Based on these features, MUHNCAL.20233 is here referred to the Pseudobeaconiidae, precluding a more precise determination due to the incompleteness of the material. The presence of ’Perleidiformes’ in northern Chile was previously documented by Arratia and Schultze (1999) based on material recovered from Triassic marine deposits at Quebrada San Pedrito, Region ´ de Atacama. The Pseudobeaconiidae is a clade comprised by freshwater forms, all of them recovered from Upper Triassic beds of the Cuyana Basin in Argentina (Lopez-Arbarello ´ and Zavattieri, 2006; Lopez-Arbarello ´ et al., 2010). The current record of a Fig. 4. Pseudobeaconiidae indet. MUHNCAL.20233,. dorsal articulated body fragment and small part of the dorsal fin. Point 3, Cerro Quimal, northern Chile. Estratos El Bordo, Middle-to-Upper Triassic. A) Left lateral view. B) Anatomical interpretation of the previous. C) anterior cross-section view. D) Preserved anatomical portion of the individual. Anatomical abbreviations: bfu, basal fulcra; co, coelom; df, anterior part of the dorsal fin; idr, incomplete dorsal ridge with spine-like scales; lat, lateral line. Scale bar equals 10 mm in A to C, D not to scale. R.A. Otero et al.
Journal of South American Earth Sciences 121 (2023) 104155 6 Pseudobeaconiidae in the “Estratos El Bordo” unit of northern Chile represents the northernmost known occurrence of this clade. Actinopterygii incertae sedis. Genus Guaymayenia Rusconi, 1946 aff. Guaymayenia paramillensis Rusconi (1946) (Fig. 5) Type and only known species–Guaymayenia paramillensis Rusconi (1946). Agua de la Zorra Formation, likely middle Triassic, Uspallata, Argentina. Material–MUHNCAL.20240, Partial postcranial impression of the body skin and fins, preserved as a cast and its respective countermold. Horizon and Age–"Estratos El Bordo” unit, northern Chile. Member 4 of Basso and Mpodozis (2012), 80 m from the base of the sedimentary section. Middle-to-Upper Triassic. Description–MUHNCAL.20240 preserves part of its body convexity, although this is laterally crushed. The recovered elements are visible in the cast and its countermold (Fig. 5A–C). These represent the posterior half of the body lacking the tail and the dorsal margin of the trunk. The two pelvic fins are preserved (Fig. 5D). These are slightly shifted into the right side. The right pelvic fin is the best preserved, showing a distally lobated shape and being composed by at least fifteen lepidotrichia. Its distal part is faded and reduced only to its outline, but further details cannot be observed. The body is covered by very small scales (under 1 mm) with quadrangular rhombic outline, with all the trunk scales being of equivalent size and shape, becoming axially larger on the posterior preserved part near the tail (Fig. 5E and F). The countermold preserves most of the anal fin (Fig. 5G), which is very elongate and larger than the pelvic fins. This is comprised by at least eighteen lepidotrichia which appear to be ridged, based on few more prominent rays preserved in the anterior part of the fin. Remarks–The very small, quadrangular scales of MUHNCAL.20240 represent a distinctive pattern nominated as micromeric squamation by Long (1988), in reference to the presence of more than one row of scales per somite. The possession of micromeric squamation, small quadrangular rhombic scales (of conserved size in the trunk and axially larger near the tail), and the ridged lepidotrichia of the anal fin in MUHNCAL.20240, are characters shared with Guaymayenia paramillensis holotype, which are part of its diagnosis (Lopez-Arbarello ´ et al., 2010). Additional characters cannot be verified due to the partial overlap between both specimens. In addition, the general shape of the anal fin is remarkably similar to that of the holotype of Guaymayenia paramillensis (see Lopez-Arbarello ´ et al., 2010: Fig. 1), and the estimated whole length of MUHNCAL.20240 (roughly 20–25 cm) is very similar to the holotype of the latter species. Due to the lack of other directly comparable anatomical elements (particularly, the caudal portion), we refer MUHNCAL.20240 as to aff. Guaymayenia paramillensis, which remains as Fig. 5. aff. Guaymayenia paramillensis Rusconi (1946). MUHNCAL.20240, partial postcranial impression of the body. Point 4, Cerro Quimal, northern Chile. Estratos El Bordo, Middle-to-Upper Triassic. A) Right lateral view of the cast. B) left view of the negative countermold. C) Preserved anatomical portion of the individual. D) Detail of the two pelvic fins. E, F) Detail of the micromeric squamation pattern. G) Detail of the anal fin. Scale bar equals 10 mm. C not to scale. R.A. Otero et al.
Journal of South American Earth Sciences 121 (2023) 104155 7 a incertae sedis ray-finned fish but enough distinctive for hold a specific diagnosis (Lopez-Arbarello ´ et al., 2010). 5. Systematic paleoichnology Coprolite Morphotype F1 (sensu Hunt and Lucas, 2012). (Fig. 6A-C) Material–MUHNCAL.20239, two complementary fragments of a partial scroll coprolite. Horizon and Age–Uppermost levels of the “Estratos El Bordo”, northern Chile, Middle-to-Upper Triassic. Description–MUHNCAL.20239 is a scroll coprolite of type F1 (sensu Hunt and Lucas, 2012) characterized by a general shape resembling a rolled sheet of paper (Hunt and Lucas, 2012: p. 153). The preserved part is covered by ostracods forming a hard coquina. Externally, the sample has a rounded contour. In transversal cross-section this is almost circular with a marked concentric coil that represents the cross-section of the scroll. Few indeterminate fragments of biologic source (likely, bony remains or fragmentary scales) can be observed in the section. Remarks–This type of coprolites are remarkably uncommon (Hunt and Lucas, 2012; Stringer and King, 2012), but they are present since the Silurian up to Recent (Gilmore, 1992; Kent, 1994; Rummy et al., 2021). Cenozoic and extant producers are referred to some (but not all) carcharhiniform sharks (Kent, 1994), being related to a singular type of valvular intestine, namely, the valvula voluta (Hunt and Lucas, 2012: Fig. 2). However, early Mesozoic producers are difficult to assess. Gilmore (1992) attributed scroll coprolites from the Silurian of Ireland to agnathan producers, but also suggested that these may be produced by early chondrichthyans. Moreover, this author discussed that the scroll valve could represent a first type of valvular intestine from which the transverse spiral valve evolved as a secondary modification. Considering the latter producer alternative, freshwater chondrichthyans have been recorded in Triassic deposits of south Pangea (Shafi et al., 2018) including hybodontids (Shafi et al., 2017). The latter group was also recorded in the Permian and the Lower Triassic of northern Chile (Breitkreuz, 1986; Suarez ´ et al., 1995), thus, representing a plausible aternative for the scroll coprolite producer of the ‘Estratos El Bordo’ unit. Coprolite Morphotype F2 (sensu Hunt and Lucas, 2012). (Fig. 6D) Material–MUHNCAL.20242, a complete spiral coprolite. Horizon and Age–Uppermost levels of the “Estratos El Bordo”, northern Chile, Middle-to-Upper Triassic. Description–MUHNCAL.20242 is a lens-shaped, amphipolar spiral coprolite consistent with the type F2 of Hunt and Lucas (2012). In lateral view this shows an oval contour, being laterally compressed (lens-shaped). The spiral shows four clear turns of similar thickness. Remarks–For spiral coprolites, Hunt and Lucas (2012) indicated stem actinopterygians and dipnoans as possible producers. Stem Teleostei can be discarded as producers due to the absence of spiral valve (Argyriou et al., 2016). Spiral coprolites have been described since the Ordovician up to Recent (Mancuso et al., 2004; Aldridge et al., 2006; Hunt and Lucas, 2012; Rummy et al., 2021, among others), being uncommon since the Late Triassic onwards, but becoming abundant in the Cenozoic again (Hunt and Lucas, 2012; Stringer and King, 2012). 6. Discussion 6.1. Middle-to-Upper Triassic temnospondyl records from South America While the South American temnospondyl record during the PermianLower Triassic is rich (Pineiro ˜ et al., 2007; Cisneros et al., 2015; Eltink et al., 2016) to date, there is a regional gap in the South American temnospondyl record during the Middle Triassic (Marsicano, 2005). The group is well-represented again from the Upper Triassic and onwards. Upper Triassic temnospondyls from Brazil include a tentative Mastodonsauroidea (Dias− da− Silva and Ramos Ilha, 2009), indeterminate stereospondyls remains (Dias− da− Silva et al., 2011) and chigutisaurids represented by Compsoceros cosgriffi (Dias− da− Silva et al., 2012). In Argentina, Upper Triassic temnospondyls include the mastodonsauroid Promastodontosaurus bellmanni (Bonaparte, 1963), a fragmentary indeterminate brachyopid (Marsicano, 2005), and the chigutisaurids Pelorocephalus mendonzensis, Pelorocephalus cacheutensis (Marsicano, 1999), and Pelorocephalus tenax (Marsicano, 1999). The current record from northern Chile, although indeterminate, it represents one the few materials known in South America during the Middle Triassic. It also extends the geographic distribution of the group into the western margin of south Gondwana, opening the chance for more informative material from the ‘Estratos El Bordo’ unit. 6.2. Chronostratigraphical value of the new actinopterygian records Guaymayenia paramillensis is so far known by a unique specimen (holotype) from the Middle Triassic beds of Agua de la Zorra Formation, Uspallata, Argentina. Previous to this research, no other Triassic actinopterygian from South America has shown a micromeric squamation Fig. 6. Aquatic vertebrate coprolites. MUHNCAL.20239, incomplete scroll coprolite preserved in two parts. A) lateral view. B, C) Respective cross-sections of each available part. MUHNCAL.20242, lense-shaped spiral coprolite. D) Lateral view. Both specimens from Point 1, Cerro Quimal, northern Chile. Estratos El Bordo, Middle-to-Upper Triassic. Scale bar equals 10 mm. R.A. Otero et al.
Journal of South American Earth Sciences 121 (2023) 104155 8 pattern, making this taxon especially unique in the regional fossil record. On the other hand, Pseudobeaconiidae were exclusively restricted to the Upper Triassic of the Cuyana Basin in western Argentina (Lopez-Arbarello ´ and Zavattieri, 2008; Lopez-Arbarello ´ et al., 2010). The stratigraphic position of MUHNCAL.20233 (here referred to as Pseudobeaconiidae indet.) occurs in the lower part of the ‘Estratos El Bordo’ sedimentary section, ca. 80 m from its base. On the other hand, MUHNCAL.20240 (here referred to as aff. Guaymallenia paramillensis) occurs in the middle section, ca. 200 m from its base, and 40 m above the level with radioisotopic results (240.8 ± 0.6 Ma). The presence of these two taxa is consistent with the Middle-to-Late Triassic age proposed by Basso and Mpodozis (2012) for the ‘Estratos El Bordo’ unit. Moreover, their relative stratigraphic occurrences reinforce a Middle Triassic age for the MUHNCAL.20240 bearing level, and a Late Triassic age for the MUHNCAL.20233 bearing level, respectively. 6.3. Paleobiogeography The presence of Pseudobeaconids in the lower section of the ‘Estratos El Bordo’ unit suggests affinities between the latter unit of northern Chile and the Cuyana Basin of Argentina (Cacheuta + Potrerillos + Santa Clara Abajo formations, upper Triassic) during the Upper Triassic. In addition, the presence of a taxon with affinities to Guaymayenia paramillensis in the ‘Estratos El Bordo’ reinforces this pattern and suggests that such interactions could start even earlier during the Middle Triassic, based on the age of the Agua de la Zorra Formation, which is the fossilbearing unit of Guaymayenia pramillensis and also part of the Cuyana Basin (see Lopez-Arbarello ´ et al., 2010). In contrast, coeval ray-finned fishes from the Ischigualasto-Villa Union ´ Basin are scarce, with taxa mostly identified as incertae sedis actinopterygians (i.e., Gualolepis carinaesquamosa, Rastrolepis spp.: see Lopez ´ Arbarello et al., 2006), plus members of the Clade Acrolepidae represented by Challaia elongata Cabrera (1944). The continental freshwater ray-finned diversity from the Triassic of Brazil mostly comprises fragmentary remains from the Santa María Formation (Lima et al., 1984; Lopez-Arbarello, ´ 2004), with the exception of Ptychoceratodus cf. philippsi, an endemic lungfish (Richter and Toledo, 2008). On the other hand, the presence of temnospondyls is common in southern Pangea, with records in the lower Triassic of Australia (Warren, 1981; Damiani and Warren, 1996), and the upper Triassic of Argentina (Rusconi, 1946, 1951; Marsicano, 1999), Brazil (Dias− da− Silva et al., 2012) and India (Sengupta, 1995). As previously pointed out by Lopez-Arbarello ´ et al. (2010), the eventual endemism of the Pseudobeaconiidae (and other actinopterygians of the Cuyana Basin) was difficult to assess because of the lack of Late Triassic fish faunas from south Pangea. In this sense, the two new records from northern Chile extends into the north the distribution of two taxa (Pseudobeaconiidae and aff. Guaymayenia paramillensis) previously restricted to the Cuyana Basin, thus, reinforcing their potential endemism to southwest Pangea. Environment of the ‘Estratos El Bordo’–Covacevich (1981) and Ramírez and Gardeweg (1982) reported the presence of plant remains referable to Neocalamites sp., Taeniopteris sp., Pterophyllum ap. and Dicroidium? sp. to which are added the taxa Sphenobaiera steinmanni, Pseudoctenis multilineata, Kutziana cacheutensis, Neocalamites carrerei, cf. Santaecruzia sp. and cf. Heidiphyllum elongatum (Troncoso, 2001 in Basso and Mpodozis, 2012; Troncoso et al., 2002), all recorded in middle levels of the unit (Basso and Mpodozis, 2012). This assemblage correlates with a highly moist environment having a mixture of herbaceous and dense scrubs associated with lacustrine conditions and/or flood plains (Leppe et al., 2003). Assemblages comprising similar taxa have been also found in Upper Triassic units of central Chile, with evidence of a coastal marine environment (Troncoso and Herbst, 2007). However, chigutisaurids are only known by freshwater forms (Rusconi, 1949; Warren, 1981; Sengupta, 1995; Marsicano, 1999). The same occurs with pseudobeaconiids, restricted to freshwater deposits of the Cuyana Basin (Lopez-Arbarello ´ et al., 2010). This is consistent with the depositional environment interpreted by Basso and Mpodozis (2012) as a lacustrine system under influence of volcanism. The presence of ostracod-rich coquinaceous levels (also including vertebrate remains) interbedded with volcanic deposits (tuffs), suggests local mortality events linked to non-favorable water conditions that may stressed the base of the trophic chain. 7. Conclusions This research presents the first records of temnospondyls, actinopterygians and aquatic vertebrate coprolites from the Triassic of the ‘Estratos el Bordo’ unit in the Atacama Desert, northern Chile. The recovered temonspondyl specimen represents a small morphotype with still unsolved taxonomic affinities. Its stratigraphic position indicates a Middle Triassic age, being one of the scarce South American temnospondyls recorded during this lapse. Regarding the new actinopterygian records, one of the specimens recovered from the lower part of the studied section (Middle Triassic) is here referred to as aff. Guaymayenia paramillensis Rusconi (1946), a taxon previously recorded in the Cuyana Basin of Argentina, with a probable Middle Triassic age (Lopez ´ Albarello et al., 2010). A second actinotpterygian specimen recovered from the mid part of the studied section is here referred to as an indeterminate Pseudobeaconiidae. So far, the latter clade seems to be endemic of the Cuyana Basin of Argentina. Both actinopterygian records from the ‘Estratos El Bordo’ of northern Chile are consistent with the available radioisotopic date of the unit, while their relative stratigraphic occurrences are also sound with a Middle-to-Upper Triassic age for this part of the unit. This vertebrate fauna is complemented by two coprolites produced by aquatic vertebrates. The presence of a scroll coprolite opens the possibility of a freshwater, early chondrichthyan as a producer, alternative which is also suggested by two previous records of hybodontids found in Permian (marine) and Triassic (freshwater) deposits of northern Chile (Breitkreuz, 1986; Su´ arez et al., 1995). The new finds complement the scarce knowledge of Middle-to-Upper Triassic continental vertebrates from Chile. CRediT authorship contribution statement Rodrigo A. Otero: Validation, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. David Rubilar-Rogers: Validation, Supervision, Methodology, Formal analysis. Sergio SotoAcuna: ˜ Validation, Supervision, Investigation, Formal analysis, Conceptualization. Alexander Vargas M.: Validation, Supervision, Methodology, Investigation, Conceptualization. Gonzalo Mella Rojas: Conceptualization, Investigation, Validation. Raúl Ugalde: Conceptualization, Data curation, Validation. Osvaldo Rojas: Validation, Methodology, Funding acquisition, Data curation. Jennyfer Rojas: Validation, Methodology, Data curation, Conceptualization. Fernando E. Novas: Validation, Supervision, Methodology, Investigation, Conceptualization. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Data availability Data used in this research is properly curated at the Museo de Historia Natural y Cultural del Desierto de Atacama, Calama, Chile R.A. Otero et al.
Journal of South American Earth Sciences 121 (2023) 104155 9 Acknowledgments RAO, ORM and JR were supported by Museo de Historia Natural y Cultural del Desierto de Atacama. SSA is supported by ANID (former Conicyt) Scholarship grant for PhD studies in Chile. This research was also supported by the project “New data sources on the fossil record and evolution of vertebrates”, Anillos de Ciencia y Tecnología‒ACT-172099, ANID‒Chile. AVM was also supported by Proyecto REDES 190190 (ANID‒Chile). Thanks to late Coleman Burke (New York) for supporting the joint exploration in 2016. Thanks also to Marcelo P. Isasi, Federico Agnolín, Federico Brisson, and Carlos Alsina for their participation in field exploration. References Aldridge, R.J., Gabbott, S.E., Siveter, L.J., Theron, J.N., 2006. Bromalites from the Soom Shale Lagerst¨ atte (upper Ordovician) of South Africa: palaeoecological and palaeobiological implications. Palaeontology 49, 857–871. 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