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The oldest rhynchosaur from Argentina: a Middle Triassic rhynchosaurid from the Chañares Formation (Ischigualasto–Villa Unión Basin, La Rioja Province)

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Abstract

Rhynchosaurs first appear in the Early Triassic fossil record and flourished during the late Carnian as the dominant members of several assemblages worldwide. In Argentina, the rhynchosaur record is currently restricted to the Ischigualasto Formation of late Carnian–earliest Norian age. Recent fieldwork in the new locality of Brazo del Puma, in the lowermost levels of the Chañares Formation, yielded three rhynchosaur tooth-bearing bones, which were collected five metres above the contact with the underlying Tarjados Formation. The most complete specimen is the posterior end of the alveolar region of a left dentary. The dentary possesses densely packed tooth rows on the lingual surface and medial half of the occlusal surface, showing longitudinal Zahnreihen. The teeth of the occlusal surface are worn flat and those of the lingual surface are organized in multiple rows, supporting the referral of the specimen to Rhynchosauridae. In addition, the dentary teeth are conical to mesiodistally compressed, resembling the condition observed in hyperodapedontines. The rhynchosaur remains reported here are the oldest collected in Argentina and among the oldest in South America, together with an unnamed form from Brazil. The new rhynchosaur specimens come from levels in which dicynodonts are numerically dominant, whereas cynodonts are considerably less abundant. Accordingly, the specimens reported here bolster faunal differences within the Chañares Formation and add a new faunal component to this already diverse vertebrate assemblage.

Kurzfassung

Rhynchosaurier sind erstmals für die Untere Trias im Fossilbericht nachgewiesen und erlebten ihre Blüte im Oberen Karnium, als eine dominierende Gruppe in merheren Fossilgesellschaften weltweit. In Argentinien sind Rhynchosauier bislang nur für die Ischigualasto Formation (Oberes Karnium-Unteres Norium) belegt. Kürzliche Grabungen in einer neuen Lokalität in Brazo del Puma, im unteresten Niveau der Chañares Formation, ergaben drei zahntragende Knochen von Rhynchosaurierern. Die Knochen wurden ungefähr fünf Meter über unterliegenden Tarjados Formation gefunden. Der vollständigste Knochen repräsentiert das posteriore Ende der Zahnregion in der linken Dentale. Die Dentale besitzt dichtgepackte Zahnreihen auf der lingualen Seite und längslaufenden Zahnreihen auf der medialen Hälfte der okklusalen Seite. Die abgeflachten Zähne der okklusalen Seite sowie die mehrreihige Anordnung der Zähne auf der lingualen Seite erlaubt eine Zuordnung zu den Rhynchosauriden. Die Zähne der Dentalen sind zusätzlich konisch bis mesiodistad zusammengedrückt, ähnlich den Zähnen von Hyperodapedontinen. Zusammen mit unbeschriebenen Resten aus Brasilien, repräsentieren die hier vorgestellten Reste den ältesten Nachweis von Rhynchosauriern aus Argentinien beziehungsweise Südamerika. Die Rhynchosaurierknochen stammen aus einem Niveau, dass vor allem durch Dicynodonten dominiert ist, während Cynodonten seltener vorkommen. Die hier vorgelstellten Reste erweitern daher die bekannte Fauna der Chañares Formation um eine weitere Gruppe und belegen zusätzlich zeitliche Unterschiede in der Faunenzusammensetzung innerhalb der Formation.

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Abbreviations

CRILAR-Pv:

Centro Regional de Investigaciones y Transferencia Tecnológica de La Rioja, Paleontología de Vertebrados, Anillaco, La Rioja, Argentina

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Acknowledgments

We thank the Agencia de Cultura de La Rioja and the Administración de Parques Nacionales of Argentina for granting permits to work in the Talampaya National Park. We are also indebted to the rangers of the Talampaya National Park for their help in the field. We appreciate discussion with José F. Bonaparte about the fossiliferous levels of the Chañares Formation and information on the field trips conducted by him in the 1970s. We thank Richard Butler for his comments on the manuscript and we appreciate the comments of Max Langer, Cesar Schultz and Atila Da-Rosa regarding the identification of the specimens and the reviewers Michael Benton and Max Langer for their comments and suggestions that improved the quality of the manuscript. We thank the following curators and researchers who provided access to specimens under their care for the purpose of this research: Billy de Klerk (AM), Gabriela Sobral and Johannes Müller (MB), Sandra Chapman and Lorna Steel (NHMUK), and Sheena Kaal and Roger Smith (SAM). M.D.E. is supported by a grant of the DFG Emmy Noether Programme to Richard Butler (BU 2587/3-1). Fieldwork and research funded by the Agencia Nacional de Investigaciones Científicas y Técnicas (PICT 2010-0207 to J.B.D.) and Secretaría de Gobierno, La Rioja (to L.E.F.).

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Correspondence to Martín D. Ezcurra.

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Ezcurra, M.D., Trotteyn, M.J., Fiorelli, L.E. et al. The oldest rhynchosaur from Argentina: a Middle Triassic rhynchosaurid from the Chañares Formation (Ischigualasto–Villa Unión Basin, La Rioja Province). Paläontol Z 88, 453–460 (2014). https://doi.org/10.1007/s12542-013-0203-3

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