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A new hypothesis of dinosaur relationships and early dinosaur evolution | Nature
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A new hypothesis of dinosaur relationships and early dinosaur evolution

Nature volume 543, pages 501–506 (2017)Cite this article

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Abstract

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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Acknowledgements

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).

Author information

Authors and Affiliations

  1. Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, UK

    Matthew G. Baron & David B. Norman

  2. Department of Earth Sciences, Natural History Museum, Cromwell Road, London, SW7 5BD, UK

    Matthew G. Baron & Paul M. Barrett

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  1. Matthew G. Baron
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Contributions

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.

Additional information

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). https://doi.org/10.1038/nature21700

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  1. Gregory Paul

    A New Name for a New Potential Dinosaur Clade in a New Era of Phylogenetic-Taxonomic Instability within Dinosauria

    Baron et al. (Nature 543, 501-506; 2017) and Holtz (Nature 545, 30; 2017) suggest radical redesignations of the old dinosaur group titles Saurischia and Pachypodosauria. They do so in order to deal with the plausible albeit not certain phylogentic results from the former?s major reordering of dinosaur relationships. Concerning the well-known Saurischia the Baron et al. redefinition will result in major confusion among the public. This is a pertinent issue because the International Commission of Zoological Nomenclature has ruled in favor of adjusting the generic titles of dinosaurs and other forms in a manner that helps preserve popular familiarity with the name, sometimes in ways that are a technical stretch1. It may also prove highly unstable because the scarcity and generalized anatomy of basal dinosaur fossils mean that sorting out the links between major dinosaur clades is likely to remain unsettled indefinitely, which risks the contents of Saurischia frequently flipping back and forth in contending studies. So while I concur with Holtz that Saurischia should not be redefined as per Baron et al., instability may also afflict his redefined Pachypodosauria, the original meaning of which is in any case much different from the Holtz proposal ? it originally included sauropodomorphs and large but not small theropods. It is therefore proposed that the dinosaur clade containing Diplodocus carnegii but not Triceratops horridus be assigned its own particular title, Paxdinosauria, in recognition of the remarkably long reign of the group, an existence only terminated by the accident of an extraterrestrial impact. In this arrangement when the evidence indicates paxdinosaurs are a real clade then saurischians and pachypodosaurs disappear, and sauropodomorphs are a subset of paxdinosaurs rather than saurischians.

    Predentata2 is usually considered a junior synonym of Ornithischia. But no designation specifically covers all dinosaurs in the clade that includes Triceratops horridus and possessed the predentate bone at the tip of the lower jaws so distinctive to the group, so it is proposed that this apomorphy plus clade group be assigned the title Predentata. In this scheme Ornithischia, as defined by Baron et al., remains a distinct node-based clade.

    The future possibility of differing results concerning the relationships of basal dinosaurs means that the taxonomic positioning of herrerasaurs, Eoraptor, Eodromaeus, Daemonosaurus and Tawa are subject to frequent, major alterations. Although technically unavoidable if this occurs it will be inconvenient, so for more general purposes it may often be useful to informally refer to these early forms as basodinosaurs, which may become a formal clade if some or all of these forms prove to be their own clade distinct from Saurischia, Ornithischia, Ornithoscelida, Paxdinosauria, Phytodinosauria.

    The Baron et al. results somewhat undermine the taxonomic utility of Theropoda, a node based clade that although real may or may not include some or all tetradactyl basodinosaurs, with the types of the latter in Theropoda being additionally uncertain. There is a need for a label that unambiguously includes all bird footed tridactyl theropods that either possessed a pes in which metatarsal 1 did not contact the distal tarsals, or descended from such theropods, and belong to the clade that includes Neotheropoda. Apomorphy plus clade Avepoda3-5 accurately describes and contains the distinctively avian configuration of the feet of the only dinosaur group with living examples. Because Neotheropoda6 is a node based clade based on the tridactyl taxa known when it was redefined7, it does not and cannot include all tridactyl theropods that are within the clade that includes avians, and is therefore a derived subset of Avepoda that must exclude the most basal three toed theropods, rather than being the senior synonym of the latter. Some major divisions of Dinosauria such as Theropoda, Avepoda, Neotheropoda, Sauropodomorpha, Sauropoda, Ornithischia, and Predentata are well substantiated monophyletic clades, and are likely to remain so. Other, higher rank divisions such as Saurischia, Ornithoscelida, Paxdinosauria and Phytodinosauria are markedly more speculative at this time, and are likely to remain that way for a considerable period, and perhaps forever. Baron et al. claim that the standard split of dinosaurs into Saurischia and Ornithschia has been ?universally accepted? in recent decades, but the reality of Saurischia has been challanged or outright rejected by some researchers3-5,-6,8.

    1. Paul, G. S. A revised taxonomy of the iguanodont dinosaur genera and species. Cretac. Res. 29, 192-216 (2008).

    2. Marsh, O. C. The typical Ornithopoda of the American Jurassic. Amer. J. Sci. 48, 85-90 (1894).

    3. Paul, G. S. Dinosaurs of the Air.
    (The Johns Hopkins University Press, 2002).
    4. Paul, G. S. The Princeton Field Guide to Dinosaurs. (Princeton University Press, 2010).

    5. Paul, G. S. The Princeton Field Guide to Dinosaurs, 2nd Ed. (Princeton University Press, 2016).

    6. Bakker, R. T. Dinosaur Heresies. (William Morrow and Company, 1986).

    7. Sereno, P. A rationale for phylogenetic definitions, with application to the higher-level taxonomy of Dinosauria. Neues Jahrb. Geol. Paläont. Abh. 210, 41-83 (1998).

    8. Paul, G. S. Predatory Dinosaurs of the World. (Simon and Schuster, 1988).

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