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Deep faunistic turnovers preceded the rise of dinosaurs in southwestern Pangaea


The Triassic period documents the origin and diversification of modern amniote lineages and the Late Triassic fossil record of South America has been crucial to shed light on these early evolutionary histories. However, the faunistic changes that led to the establishment of Late Triassic ecosystems are largely ignored because of the global scarcity of fossils from assemblages a few million years older. Here we contribute to fill this gap with the description of a new tetrapod assemblage from the lowermost levels of the Chañares Formation (uppermost Middle–lower Late Triassic epochs) of Argentina, which is older than the other vertebrate assemblages of the same basin. The new assemblage is composed of therapsids, rhynchosaurids and archosaurs, and clearly differs from that of the immediately overlying and well-known historical Chañares vertebrate assemblage. The new tetrapod association is part of a phase of relatively rapidly changing vertebrate assemblage compositions, in a time span shorter than 6 million years, before the diversification of dinosaurs and other common Late Triassic tetrapods in southwestern Pangaea.

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We thank the Secretaría de Cultura de La Rioja and the Administración de Parques Nacionales for granting permits to work in the Talampaya National Park and the field crews of 2011–2014 and 2016–2017; the rangers of the National Park for their help in the field; J. F. Bonaparte, S. J. Nesbitt, C. L. Schultz and M. B. Soares for discussions on the Chañares Formation; J. Kaluza, M. Cárdenas, G. Aguirrezabala, L. Acosta, F. Solari Orellana, A. Lecuona, R. Barros, M. Iberlucea, A. Bustamante and S. de la Vega for preparation of fossil material. Research was funded by the Agencia Nacional de Investigaciones Científicas y Técnicas (PICT 2012-0925 and PICT 2014-0609 to J.B.D.), The Jurassic Foundation (to M.D.E.), Secretaría de Gobierno, La Rioja (to L.E.F.), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq, to A.G.M.) and Universidad Nacional de San Juan (PROJOVI, UNSJ-SECITI to M.J.T.).

Author information

M.D.E., L.E.F. and J.B.D. designed the research project; all authors conducted fieldwork; M.D.E., L.E.F., A.G.M., M.B.v.B. and J.B.D. described the material; M.D.E., M.B.v.B. and J.B.D. conducted the phylogenetic analyses; S.R. and M.E. conducted the geologic studies; all the authors contributed to the text of the manuscript; M.D.E., L.E.F., A.G.M., S.R., M.B.v.B., J.R.A.T., E.M.H. and M.J.T. made the figures.

Competing interests

The authors declare no competing financial interests.

Correspondence to Martín D. Ezcurra.

Electronic supplementary material

Supplementary Information

Supplementary Figures, Supplementary Description, Supplementary Information of the Datings and Supplementary References

Supplementary Data 1

Skeletal and skull reconstructions of Tarjadia ruthaeSkeletal and skull reconstructions of Tarjadia ruthae

Supplementary Data 2

TNT data matrix used for the phylogenetic analysis

Supplementary Data 3

NEXUS data matrix used for the phylogenetic analysis

Supplementary Data 4

Detailed information of the dated zircons of the sample AM-260

Supplementary Data 5

Detailed information of the dated zircons of the sample AM-242

Supplementary Data 6

R script for the sampling simulations

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Further reading

Fig. 1: Chronostratigraphy and vertebrate assemblage zones of the Chañares Formation.
Fig. 2: Skeletal anatomy of the erpetosuchid pseudosuchian Tarjadia ruthae.
Fig. 3: Other tetrapods from the Tarjadia Assemblage Zone and phylogenetic relationships of the archosauromorphs from the Chañares Formation.