Phylogenetic and Biogeographic Assessment of Ornithischian Diversity Throughout the Mesozoic: A Species-Level Analysis from Origin to Extinction

Ornithischian dinosaurs form a diverse, globally-distributed clade including the dominant large land herbivores in many Mesozoic faunas. However, because we lack a well-resolved species-level phylogeny including basal and derived members, our understanding of the initial ornithischian radiation in the Late Triassic is unclear, which further complicates our ability to test biogeographic scenarios.

A new species-level phylogenetic analysis, based on comprehensive empirical assessments of a wide sample of ornithischians with an emphasis on basal taxa, helps resolve the relationships of historically labile taxa. Heterodontosauridae is a monophyletic group outside of Genasauria, corroborating recent studies, though the current analysis did not recover distinct Laurasian and Gondwanan clades as has been previously reported. Lesothosaurus, traditionally considered as one of the most basal ornithischians, is here recovered as the basalmost neornithischian more closely related to cerapodans than to thyreophorans. The resultant phylogeny shows good resolution among basal taxa; however, most members of Cerapoda collapse into a polytomy recovering only a monophyletic Iguanodontia, Marginocephalia, and a few other derived clades. The phylogenetic placement of these taxa is critical for assessments of character evolution for more derived clades and ancestral state reconstructions for Ornithischia in general, as well as determining areas of origination for clades across the tree.

With a detailed species-level phylogeny, I was able to estimate the hypothetical ancestral body plan for all Ornithischia by reconstructing the character states in the present analysis. The results of this analysis suggest that the hypothetical ancestral ornithischian had similar skull morphology to Lesothosaurus, but that the postcranial skeleton was more similar to other basal ornithischians such as Heterodontosaurus and to basal saurischians such as Herrerasaurus and Eoraptor. Most analyses of derived clades of ornithischians rely on Lesothosaurus as a representative outgroup taxon; however, the postcranial skeleton of Lesothosaurus represents a fairly specialized, relatively abbreviated forelimb unlike what the ancestral ornithischian possessed.

Building on the species-level phylogeny focused on basal taxa, I performed a detailed event-based quantitative biogeographic analysis based on a comprehensive composite phylogeny including more derived taxa from previously published studies. A few patterns can be explained by vicariance but numerous dispersal events are required to explain overall ornithischian biogeography, particularly in the Late Jurassic and Early and Late Cretaceous.

Although the early record of ornithischians is sparse, it is clear that the clade originated from small, bipedal cursors in southern Gondwana and spread into Laurasia by the Early Jurassic and quickly diversified into the iconic groups that characterize Ornithischia. A well-resolved phylogenetic and biogeographic hypothesis of ornithischian diversity, particularly with basal taxa such as the heterodontosaurids and Lesothosaurus, provides a framework on which to test hypotheses of coevolution as well as the evolution of herbivory, ontogeny, physiology, and sexual dimorphism in the fossil record.