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A new hypothesis of dinosaur relationships and early dinosaur evolution


For 130 years, dinosaurs have been divided into two distinct clades—Ornithischia and Saurischia. Here we present a hypothesis for the phylogenetic relationships of the major dinosaurian groups that challenges the current consensus concerning early dinosaur evolution and highlights problematic aspects of current cladistic definitions. Our study has found a sister-group relationship between Ornithischia and Theropoda (united in the new clade Ornithoscelida), with Sauropodomorpha and Herrerasauridae (as the redefined Saurischia) forming its monophyletic outgroup. This new tree topology requires redefinition and rediagnosis of Dinosauria and the subsidiary dinosaurian clades. In addition, it forces re-evaluations of early dinosaur cladogenesis and character evolution, suggests that hypercarnivory was acquired independently in herrerasaurids and theropods, and offers an explanation for many of the anatomical features previously regarded as notable convergences between theropods and early ornithischians.

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Figure 1: Phylogenetic relationships of early dinosaurs.
Figure 2: Skeletal anatomy of ornithoscelidans.

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We thank S. Chapman (Natural History Museum, London, UK), R. Smith (Iziko South African Museum, Cape Town, South Africa), E. Butler (National Museum, Bloemfontein, South Africa) B. Zipfel (Bernard Price Institute for Palaeontological Research, Johannesburg, South Africa), J. Powell (Instituto Miguel Lillo, Tucumán, Argentina), R. Martinez (Museo de Ciencias Naturales, San Juan, Argentina) and D. Pol (Museo Paleontológico Egidio Feruglio, Trelew, Argentina) for access to specimens in their care, R. Butler, J. Choiniere, B. McPhee, C. VanBuren and K. Chapelle for helpful discussion, M. Williams for assisting with the production of figures, and C. Baron for helpful comments on the manuscript, and the Willi Hennig Society for making TNT 1.5-beta software freely available. Funding for M.G.B. was provided by a NERC/CASE Doctoral Studentship (NE/L501578/1).

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Authors and Affiliations



M.G.B., P.M.B. and D.B.N. designed this research project. M.G.B., D.B.N. and P.M.B. contributed data. M.G.B. conducted the phylogenetic analyses. M.G.B, D.B.N. and P.M.B. wrote the manuscript. M.G.B. and D.B.N. produced the figures.

Corresponding author

Correspondence to Matthew G. Baron.

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The authors declare no competing financial interests.

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Reviewer Information Nature thanks K. Padian, H.-D. Sues and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Figure 1 Reduced strict consensus tree of the main analysis showing bootstrap frequencies (above node) and Bremer support values (below node) that were calculated for each of the major nodes, after the exclusion of Saltopus elginensis, Agnosphitys cromhallensis, Eucoelophysis baldwini and Diodorus scytobrachion.

Ornithoscelida, Ornithischia, Theropoda, Herrerasauridae, Dinosauria and Silesauridae are all very well supported, with Bremer support values of 3 or more. Saurischia (new definition) and Sauropodomorpha are less well supported, with Bremer support values of 2. Bootstrap frequencies below 50 are not shown.

Extended Data Figure 2 Strict consensus tree produced when Dimorphodon macronyx was included in the dataset and chosen as the outgroup taxon (Euparkeria capensis and Postosucus kirkpatricki were not included).

The tree was produced from 79 MPTs (most parsimonious trees) each with a length of 1,627 steps. As in Extended Data Fig. 1, the clades Ornithoscelida and Sauropodomorpha plus Herrerasauridae (Saurischia, new definition) are both recovered. For further details on the additional analyses that were carried out as part of this study, see the Supplementary Information.

Extended Data Figure 3 Strict consensus tree produced when the non-dinosaurian silesaurid taxon Silesaurus opolensis was chosen as the outgroup taxon.

The tree was produced from 83 MPTs each with a length of 1,713 steps. For further details on the additional analyses that were carried out as part of this study, see the Supplementary Information.

Extended Data Figure 4 Strict consensus tree produced when no characters were treated as ordered.

Tree was produced from 83 MPTs each with a length of 1,690 steps. The clades Ornithoscelida and Saurischia (new definition, see Table 1) are both recovered in this analysis. For further details on the additional analyses that were carried out as part of this study, see the Supplementary Information.

Extended Data Figure 5 Strict consensus tree set against the geological timescale, showing the predicted Early Triassic divergence dates of Dinosauria (star) and of the major dinosaurian lineages when the potential ‘massospondylid’ sauropodomorph Nyasasaurus parringtoni is included in the analysis.

a, Origin of Dinosauria (new definition) when Nyasasaurus is considered. b, Origin of Saurischia (new definition) when Nyasasaurus is considered. c, Origin of Ornithoscelida when Nyasasaurus is considered. For further details on the additional analyses that were carried out as part of this study, see the Supplementary Information.

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Baron, M., Norman, D. & Barrett, P. A new hypothesis of dinosaur relationships and early dinosaur evolution. Nature 543, 501–506 (2017).

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