The phylogeny of the ornithischian dinosaurs

Synopsis Ornithischia is a familiar and diverse clade of dinosaurs whose global phylogeny has remained largely unaltered since early cladistic analyses in the mid 1980s. Current understanding of ornithischian evolution is hampered by a paucity of explicitly numerical phylogenetic analyses that consider the entire clade. As a result, it is difficult to assess the robustness of current phylogenetic hypotheses for Ornithischia and the effect that the addition of new taxa or characters is likely to have on the overall topology of the clade. The new phylogenetic analysis presented here incorporates a range of new basal taxa and characters in an attempt to rigorously test global ornithischian phylogeny. Parsimony analysis is carried out with 46 taxa and 221 characters. Although the strict component consensus tree shows poor resolution in a number of areas, application of reduced consensus methods provides a well?resolved picture of ornithischian interrelationships. Surprisingly, Heterodontosauridae is placed as the most basal group of all well?known ornithischians, phylogenetically distant from a stem?defined Ornithopoda, creating a topology that is more congruent with the known ornithischian stratigraphical record. There is no evidence for a monophyletic ?Fabrosauridae?, and Lesothosaurus (the best?known ?fabrosaur') occupies an unusual position as the most basal member of Thyreophora. Other relationships within Thyreophora remain largely stable. The primitive thyreophoran Scelidosaurus is the sister taxon of Eurypoda (stegosaurs and ankylosaurs), rather than a basal ankylosaur as implied by some previous studies. The taxonomic content of Ornithopoda differs significantly from previous analyses and basal relationships within the clade are weakly supported, requiring further investigation. ?Hypsilopho?dontidae? is paraphyletic, with some taxa (Agilisaurus, Hexinlusaurus, Othnielia) placed outside of Ornithopoda as non?cerapodans. Ceratopsia and Pachycephalosauria are monophyletic and are united as Marginocephalia; however, the stability of these clades is reduced by a number of poorly preserved basal taxa. This analysis reaffirms much of the currently accepted ornithischian topology. Nevertheless, instability in the position and content of several clades (notably Heterodontosauridae and Ornithopoda) indicates that considerable future work on ornithischian phylogeny is required and causes problems for several current phylogenetic definitions.