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Ecologically distinct dinosaurian sister group shows early diversification of Ornithodira

Abstract

The early evolutionary history of Ornithodira (avian-line archosaurs) has hitherto been documented by incomplete (Lagerpeton1) or unusually specialized forms (pterosaurs and Silesaurus2). Recently, a variety of Silesaurus-like taxa have been reported from the Triassic period of both Gondwana and Laurasia, but their relationships to each other and to dinosaurs remain a subject of debate3,4,5. Here we report on a new avian-line archosaur from the early Middle Triassic (Anisian) of Tanzania. Phylogenetic analysis places Asilisaurus kongwe gen. et sp. nov. as an avian-line archosaur and a member of the Silesauridae, which is here considered the sister taxon to Dinosauria. Silesaurids were diverse and had a wide distribution by the Late Triassic, with a novel ornithodiran bauplan including leaf-shaped teeth, a beak-like lower jaw, long, gracile limbs, and a quadrupedal stance. Our analysis suggests that the dentition and diet of silesaurids, ornithischians and sauropodomorphs evolved independently from a plesiomorphic carnivorous form. As the oldest avian-line archosaur, Asilisaurus demonstrates the antiquity of both Ornithodira and the dinosaurian lineage. The initial diversification of Archosauria, previously documented by crocodilian-line archosaurs in the Anisian6, can now be shown to include a contemporaneous avian-line radiation. The unparalleled taxonomic diversity of the Manda archosaur assemblage indicates that archosaur diversification was well underway by the Middle Triassic or earlier.

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Figure 1: New silesaurid from Tanzania, Asilisaurus kongwe (NMT RB9).
Figure 2: Skeletal anatomy of Asilisaurus kongwe.
Figure 3: Phylogenetic relationships of Asilisaurus kongwe within Archosauria and silesaurid biogeography.

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Acknowledgements

We thank L. Herzog for preparation of the material and P. Barrett, P. Makovicky, A. Turner and N. Smith for discussions. We acknowledge A. Tibaijuka and C. Saanane for their help with permits and fieldwork logistics. Funding was provided by a National Geographic Society Research and Exploration grant (7587-05 to C.A.S.), the Evolving Earth Foundation (to S.J.N.), The Grainger Foundation (to K.D.A.), an NSF Postdoctoral Research Fellowship (DBI-0306158 to K.D.A.), and NSF Graduate Research Fellowships (to S.J.N. and R.B.I.).

Author Contributions S.J.N. and R.B.I. designed the research project; C.A.S. and K.D.A. designed the field project; S.J.N., C.A.S., K.D.A., R.M.H.S. and L.A.T. conducted fieldwork; S.J.N. described the material; R.M.H.S. interpreted the sedimentology and geology; S.J.N. and R.B.I. conducted the phylogenetic analysis; K.D.A. and R.B.I. conducted the comparative phylogenetic analyses; and S.J.N., R.B.I., C.A.S., R.M.H.S., L.A.T. and K.D.A. wrote the manuscript.

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Correspondence to Sterling J. Nesbitt.

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Supplementary Information

This file contains Supplementary Information: 1) Differential diagnosis for Asilisaurus kongwe, 2) Paratype and referred material of Asilisaurus kongwe, 3) Synonymy of Lewisuchus admixtus and Pseudolagosuchus major, 4) Details of the phylogenetic analysis, 5) Geology and taphonomy of the type locality, 6) Age of the Manda Beds, 7) Additional occurrences of the Silesauridae and 8) Details of Comparative Phylogenetic Tests. It also contains Supplementary Figures S1-S3 with Legends, Supplementary Tables S1- S2, and Supplementary References. (PDF 748 kb)

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Nesbitt, S., Sidor, C., Irmis, R. et al. Ecologically distinct dinosaurian sister group shows early diversification of Ornithodira. Nature 464, 95–98 (2010). https://doi.org/10.1038/nature08718

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