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A new herrerasaurid (Dinosauria, Saurischia) from the Upper Triassic Ischigualasto Formation of northwestern Argentina.

ABSTRACT: Herrerasauridae comprises a basal clade of dinosaurs best known from the Upper Triassic of Argentina and Brazil, which have yielded remains of Herrerasaurus ischigualastensis and Staurikosaurus pricei, respectively. Systematic opinion regarding the position of Herrerasauridae at the base of Dinosauria has varied. Here we describe a new herrerasaurid, Sanjuansaurus gordilloi gen. n., sp. n., based on a partial skeleton from Carnian-age strata of the the Upper Triassic Ischigualasto Formation of northwestern Argentina. The new taxon is diagnosed by numerous features, including long, band-shaped and posterolaterally oriented transverse process on the posterior cervical vertebrae; neural spines of the sixth to eighth dorsal vertebrae, at least, bearing acute anterior and posterior processes; scapula and coracoid with everted lateral margins of the glenoid; and short pubis (63% of the femoral length). Phylogenetic analysis placed Sanjuansaurus within a monophyletic Herrerasauridae, at the base of Theropoda and including Herrerasaurus and Staurikosaurus. The presence of Sanjuansaurus at the base of the Ischigualasto Formation, along with other dinosaurs such as Herrerasaurus, Eoraptor, Panphagia, and Chromogisaurus suggests that saurischian dinosaurs in southwestern Pangea were already widely diversified by the late Carnian rather than increasing in diversity across the Carnian-Norian boundary.

SUBMITTER: Alcober OA

PROVIDER: S-EPMC3088398 | biostudies-literature | 2010 Oct

REPOSITORIES: biostudies-literature

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A new herrerasaurid (Dinosauria, Saurischia) from the Upper Triassic Ischigualasto Formation of northwestern Argentina.

Alcober Oscar A OA Martinez Ricardo N RN

ZooKeys 20101019 63

Herrerasauridae comprises a basal clade of dinosaurs best known from the Upper Triassic of Argentina and Brazil, which have yielded remains of Herrerasaurus ischigualastensis and Staurikosaurus pricei, respectively. Systematic opinion regarding the position of Herrerasauridae at the base of Dinosauria has varied. Here we describe a new herrerasaurid, Sanjuansaurus gordilloi gen. n., sp. n., based on a partial skeleton from Carnian-age strata of the the Upper Triassic Ischigualasto Formation of n ...[more]

PMID: 21594020

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Project description:BackgroundArgentinean basal sauropodomorphs are known by several specimens from different basins; Ischigualasto, El Tranquilo, and Mogna. The Argentinean record is diverse and includes some of the most primitive known sauropodomorphs such as Panphagia and Chromogisaurus, as well as more derived forms, including several massospondylids. Until now, the Massospondylidae were the group of basal sauropodomorphs most widely spread around Pangea with a record in almost all continents, mostly from the southern hemisphere, including the only record from Antarctica.Methodology/principal findingWe describe here a new basal sauropodomorph, Leyesaurus marayensis gen. et sp. nov., from the Quebrada del Barro Formation, an Upper Triassic-Lower Jurassic unit that crops out in northwestern Argentina. The new taxon is represented by a partial articulated skeleton that includes the skull, vertebral column, scapular and pelvic girdles, and hindlimb. Leyesaurus is diagnosed by a set of unique features, such as a sharply acute angle (50 degrees) formed by the ascending process of the maxilla and the alveolar margin, a straight ascending process of the maxilla with a longitudinal ridge on its lateral surface, noticeably bulging labial side of the maxillary teeth, greatly elongated cervical vertebrae, and proximal articular surface of metatarsal III that is shelf-like and medially deflected. Phylogenetic analysis recovers Leyesaurus as a basal sauropodomorph, sister taxon of Adeopapposaurus within the Massospondylidae. Moreover, the results suggest that massospondylids achieved a higher diversity than previously thought.Conclusions/significanceOur phylogenetic results differ with respect to previous analyses by rejecting the massospondylid affinities of some taxa from the northern hemisphere (e.g., Seitaad, Sarahsaurus). As a result, the new taxon Leyesaurus, coupled with other recent discoveries, suggests that the diversity of massospondylids in the southern hemisphere was higher than in other regions of Pangea. Finally, the close affinities of Leyesaurus with the Lower Jurassic Massospondylus suggest a younger age for the Quebrada del Barro Formation than previously postulated.

Project description:We describe a new probainognathian cynodont, Pseudotherium argentinus, from the early Late Triassic Ischigualasto Formation of Argentina. Pseudotherium adds to a growing assemblage of small Triassic cynodonts that offers new insight into events leading up to the origin of crown Mammalia and the successively more inclusive Mammaliaformes and Mammaliamorpha. Using high-resolution X-ray computed tomography, we illustrate and describe the holotype and only known specimen, which consists of a well-preserved isolated skull. It preserves apomorphic features of the orbit and braincase. Prefrontal and vestigial postorbital bones are present, despite the absence of an ossified postorbital bar. As in Brasilitherium riograndensis, thin turbinal-like bones are present in the nasopharyngeal passage, and we discuss impediments to establishing their identity and function. Compared to more basal cynodonts, the cochlea is elongated but uncoiled and in this and other features it resembles basal mammaliamorphs. Our analysis found weak support for Pseudotherium as the sister taxon of Tritylodontidae. However, a broader assessment of its relationships in light of additional character data from the literature and unpublished computed tomography data suggest that it may be more realistic to view the relationships of Pseudotherium as an unresolved polytomy with tritylodontids, and the taxa referred to as tritheledontids and brasilodontids (groups of variable membership and questionable monophyly). Thus, Pseudotherium may lie just inside or just outside of Mammaliamorpha, and there is also weak character support for its sister taxon relationship with Brasilitherium. Our results amplify previous conclusions that phylogenetic relationships in this adaptive radiation of small cynodonts will remain somewhat uncertain until more complete specimens are recovered, and until high-resolution CT scans of existing specimens become available to the larger community. Toward that goal, we make the CT dataset for the holotype of Pseudotherium argentinus publically available under a Creative Commons license at www.DigiMorph.org.

Dataset Information

A new herrerasaurid (Dinosauria, Saurischia) from the Upper Triassic Ischigualasto Formation of northwestern Argentina.

Publications

A new herrerasaurid (Dinosauria, Saurischia) from the Upper Triassic Ischigualasto Formation of northwestern Argentina.

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