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The Validity of Lagosuchus Talampayensis Romer, 1971 (Archosauria, Dinosauriformes), from the Late Triassic of Argentina

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The Validity of Lagosuchus Talampayensis Romer, 1971 (Archosauria, Dinosauriformes), from the Late Triassic of Argentina THE VALIDITY OF LAGOSUCHUS TALAMPAYENSIS ROMER, 1971 (ARCHOSAURIA, DINOSAURIFORMES), FROM THE LATE TRIASSIC OF ARGENTINA Authors: Federico L. Agnolin, and Martín D. Ezcurra Source: Breviora, 565(1) : 1-21 Published By: Museum of Comparative Zoology, Harvard University URL: https://doi.org/10.3099/0006-9698-565.1.1 BioOne Complete (complete.BioOne.org) is a full-text database of 200 subscribed and open-access titles in the biological, ecological, and environmental sciences published by nonprofit societies, associations, museums, institutions, and presses. Your use of this PDF, the BioOne Complete website, and all posted and associated content indicates your acceptance of BioOne’s Terms of Use, available at www.bioone.org/terms-o- use. Usage of BioOne Complete content is strictly limited to personal, educational, and non - commercial use. Commercial inquiries or rights and permissions requests should be directed to the individual publisher as copyright holder. Downloaded From: https://bioone.org/journals/Breviora on 28 Oct 2019 Terms of Use: https://bioone.org/terms-of-use Access provided by Utah Valley University US ISSN 0006-9698 CAMBRIDGE,MASS.N9SEPTEMBER 2019 UMBER 565 THE VALIDITY OF LAGOSUCHUS TALAMPAYENSIS ROMER, 1971 (ARCHOSAURIA, DINOSAURIFORMES), FROM THE LATE TRIASSIC OF ARGENTINA 1,2 3 FEDERICO L. AGNOLIN AND MARTIN´ D. EZCURRA ABSTRACT. The Chanares˜ Formation (latest Middle–earliest Late Triassic) of northwestern Argentina is part of the Ischigualasto–Villa Union´ Basin and preserves a rich fossil vertebrate record discovered by a crew of the Museum of Comparative Zoology (Harvard University) led by Alfred Romer. A diverse non-dinosaurian dinosauromorph assemblage has been described from this formation and includes the iconic, small-sized ‘‘proto-dinosaur’’ Lagosuchus talampayensis Romer, 1971. This species has been crucial to understand the body plan of dinosaur precursors and the higher-level phylogenetic relationships of dinosaurs. More recently, authors considered this genus and species as nomina dubia. The aim of this contribution is to reassess the taxonomic validity of L. talampayensis. New observations indicate that the holotype of L. talampayensis differs from other avemetatarsalians, but not from the hypodigm of Marasuchus lilloensis, in the presence of a globose femoral head (strongly convex proximal and medial surfaces) with a distinctly dorsal orientation and a sub-oval outline in proximal view as a result of incipient posterior and posteromedial tuberosities. Thus, Lagosuchus talampayensis is here considered a senior synonym of ‘‘Marasuchus lilloensis’’ (Romer, 1972). Our conclusions have implications in the knowledge about early dinosauromorph diversity and the taxonomic richness of the group in the Chanares˜ Formation. KEY WORDS: Triassic; La Rioja province; Argentina; Avemetatarsalia; Dinosauriformes; Marasuchus lilloensis 1 Laboratorio de Anatomıa´ Comparada y Evolucion´ INTRODUCTION de los Vertebrados, CONICETÀMuseo Argentino de Ciencias Naturales ‘‘Bernardino Rivadavia,’’ Av. The Ischigualasto–Villa Union´ Basin is an ´ Angel Gallardo 470 (C1405DJR), Buenos Aires, extensive succession of late Permian–Late Argentina e-mail: [email protected]. 2 Fundacion´ de Historia Natural ‘‘Felix´ de Azara,’’ Triassic sedimentary rocks that crops out in Universidad Maimonides,´ Hidalgo 775 (C1405BDB), northwestern Argentina (San Juan and La Buenos Aires, Argentina. Rioja provinces) and have yielded abundant 3 Seccion´ Paleontologıa´ de Vertebrados, CONI- fossil vertebrates (Reig, 1963; Romer, 1966; CETÀMuseo Argentino de Ciencias Naturales ‘‘Ber- Bonaparte, 1971, 1997). The information nardino Rivadavia,’’ Av. Angel´ Gallardo 470 (C1405DJR), Buenos Aires, Argentina; e-mail: provided by these fossils has become crucial [email protected]. to understanding the evolution of Triassic Ó The President and Fellows of Harvard College 2019. Downloaded From: https://bioone.org/journals/Breviora on 28 Oct 2019 Terms of Use: https://bioone.org/terms-of-use Access provided by Utah Valley University 2 BREVIORA No. 565 vertebrate assemblages in southern Pangaea. column, pectoral girdle and forelimb and The Chanares˜ Formation (latest Ladi- the pelvic bones associated with the right and nian?Àearly Carnian; Marsicano et al., left articulated hindlimbs of the same indi- 2015; Ezcurra et al., 2017) is one of the most vidual (Romer, 1971; Sereno and Arcucci, fossiliferous stratigraphic units of the Ischi- 1994). Romer (1971) proposed that this gualasto–Villa Union´ Basin, and its fossil taxon was bipedal and that it may be related content was discovered and originally exca- to the origin of ‘‘coelurosaurian’’ (cf. Coelo- vated by a crew of the Museum of Compar- physoidea) theropods based on tarsal char- ative Zoology (MCZ, Harvard University) acters. Subsequently, Romer (1972) referred led by Alfred S. Romer from October 1964 additional specimens to Lago. talampayensis until February 1965 (Romer, 1966; Romer and also erected the new species Lago. and Jensen, 1966). The fossils collected lilloensis. Regarding the holotype of this during this early field trip, most of them new species, Romer (1972: 5) stated that housed in the MCZ, represent one of the ‘‘except for larger size, the hind leg elements most informative Triassic vertebrate assem- closely resemble those of the type. I felt blages worldwide. Subsequently, JoseF.´ justified in concluding that the two were Bonaparte and collaborators increased the specifically identical,’’ but he concluded that sample of fossil vertebrates of this unit ‘‘we have three specimens of femur of during the late 1960s and 1970s. As a result holotype size, only one larger. It is highly of these explorations, an important diversity improbable that in a collection there would be of nondinosaurian dinosauromorphs (infor- three immature forms against only one adult. mally called dinosaur precursors or ‘‘proto- . I am therefore forced to the conclusion dinosaurs’’) was described, including Lager- that the present specimen represents a distinct peton chanarensis, Lagosuchus talampayensis, species of Lagosuchus.’’ Some years later, Lago. lilloensis (¼ Marasuchus lilloensis), Bonaparte (1975) described in detail all Pseudolagosuchus major,andLewisuchus available specimens of Lago. talampayensis admixtus (Romer, 1971, 1972a,b; Bonaparte, housed in the MCZ and the Instituto Miguel 1975; Arcucci, 1987; Sereno and Arcucci, Lillo (PVL) in Argentina and proposed that 1994a,b; Bittencourt et al., 2014).These taxa the size differences with Lago. lilloensis were have been and still are of key importance to not enough to support its taxonomic distinc- understanding the origin and early evolution tiveness. Thus, Bonaparte (1975) considered of Dinosauria (e.g., Romer, 1971, 1972; Lago. talampayensis aseniorsynonymof Bonaparte, 1975, 1995; Benton, 1990a,b; Lago. lilloensis, and this conclusion was Novas, 1992, 1996; Sereno and Arcucci, followed by subsequent authors in the 1980s 1994a,b; Ezcurra, 2006; Irmis et al., 2007; and early 1990s (Arcucci, 1987; Novas, 1989, Nesbitt et al., 2010, 2017; Bittencourt et al., 1993; Benton, 1990a,b). 2014; Cabreira et al., 2016). Sereno and Arcucci (1994) revised in detail Alfred Romer (1971) briefly described and both species and concluded that Lago. erected the new, long-limbed dinosauro- talampayensis was based on a nondiagnostic morph species Lago. talampayensis and holotype and considered this genus and Lage. chanarensis as part of a series of 20 species nomina dubia. As a result, Sereno contributions published in Breviora between and Arcucci (1994) erected the new genus 1966 and 1973 about the Chanares˜ reptile Marasuchus for the species ‘‘Lago.’’ lilloensis, fauna. Lagosuchus talampayensis is based on resulting in the new combination M. lilloen- an associated, articulated partial vertebrl sis (Romer, 1972). Most subsequent authors Downloaded From: https://bioone.org/journals/Breviora on 28 Oct 2019 Terms of Use: https://bioone.org/terms-of-use Access provided by Utah Valley University 2019 VALIDITY OF THE ‘‘PROTO-DINOSAUR’’ LAGOSUCHUS TALAMPAYENSIS 3 have followed this criterion (e.g., Novas, Marasuchus belong to a locality (northwest 1996, 2009; Ezcurra, 2006; Langer and pocket of Romer field notes 1964À1965 or Benton, 2006; Langer et al., 2013; Bitten- classic Los Chanares˜ locality of Rogers et al. court et al., 2014), but one exception was a [2001]) placed 3 km north of the Chanares˜ paper by Bonaparte (1995), in which he River and 5 km southwest of the Puerta de sustained the validity of Lago. talampayensis Talampaya (Romer, 1971, 1972; Bonaparte, and considered M. lilloensis its junior syno- 1975; Sereno and Arcucci, 1994). These nym. As a consequence, the aim of this dinosaur precursor fossils come from sub- contribution is to revaluate the taxonomic spherical concretions between 30 and 250 cm validity of Lago. talampayensis and its in diameter (Mancuso et al., 2013), placed synonymy with M. lilloensis. A full revision approximately 10À15 m above the base of of the alpha taxonomy of the other dino- the sedimentary column at the Chanares˜ type sauromorphs of the Chanares˜ Formation locality (Ezcurra et al., 2017: supplementary will be the subject of future contributions fig. 4). These concretions contain abundant based on recently collected specimens (e.g., vertebrate fossils that have been well known Novas et al., 2015; Ezcurra et al., 2018). since the 1960s and include abundant and usually articulated, fairly complete skeletons PALEONTOLOGICAL AND
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