A comprehensive phylogenetic analysis on early ornithischian evolution

Resolving the evolutionary relationships of early diverging (‘basal’) ornithischian dinosaurs is a challenging topic in palaeontology, with multiple competing hypotheses on the phylogenetic relationships of heterodontosaurids, ‘hypsilophodontids’, and other early-diverging forms. These hypotheses cannot be directly compared because they are derived from differently constructed datasets (i.e. distinct samples of taxa and characters). This study aims to address these issues by revising and combining the distinct datasets into a single analysis in order to create the most comprehensive dataset for the investigation of the phylogenetic relationships of early-diverging ornithischians. A diphyletic model of <i>Dinosauria</i> is supported, with silesaurs nesting as members of <i>Ornithischia. Heterodontosauridae</i> is resolved as a clade of non-genasaurian ornithischians, rejecting a potential relationship with <i>Marginocephalia</i>. ‘Hypsilophodontid’ taxa span the neornithischian and ornithopod stem, with <i>Thescelosauridae</i> as the sister taxon to Cerapoda. A more restricted <i>Ornithopoda</i> is composed of five main clades: <i>Hypsilophodontidae</i>, <i>Rhabdodontomorpha</i>, <i>Elasmaria</i>, <i>Dryosauridae</i> and <i>Ankylopollexia. Hypsilophodontidae</i> is a valid clade, reduced to two European Barremian taxa. <i>Rhabdodontomorpha</i> does not contain <i>Muttaburrasaurus</i> as originally proposed, but instead expands to include a North American clade formed by <i>Convolosaurus</i>, <i>Iani</i> and <i>Tenontosaurus. Elasmaria</i> contains all non-dryomorph Gondwanan ornithopods, with its members possessing multiple distinct body plans. New results and comparison with previous studies suggest that some members of <i>Dryosauridae</i> are not ‘true dryosaurids’ but various early euiguanodontians that may be more closely related to either <i>Elasmaria</i> or <i>Ankylopollexia</i>. Results group most ‘hypsilophodontids’ in larger clades, significantly reducing the number and extension of ghost lineages throughout all of <i>Neornithischia</i>. These clades also show a degree of endemism, with different lineages present at different continents in the Late Cretaceous. This new phylogenetic analysis unifying previous works will provide a framework for future studies on origins and relations of early diverging ornithischians, and attempting to find stability among the different competing hypotheses.