Abstract

“Our understanding of the pattern of dinosaurian relationships has been challenged recently (Baron et al. 2017 Nature). Continuing this period of reassessment, a large-scale phylogenetic analysis (Müller & Garcia, 2020) recovered taxa, often referred to as silesaurids, on the branch leading to the clade Ornithischia (= “bird-hipped dinosaurs”). However, the analysis that produced this novel topology used a dataset that, in its original form, did not include early representatives of Ornithischia, and did not incorporate all the anatomical characters that have been suggested to unite Ornithischia with the other principal dinosaurian clades. These latter issues have been addressed by expanding taxon representation and adding anatomical characters to the original dataset. Our analysis of the revised dataset (Norman et al. 2022 ZJLS ) supports the hypothesis that ‘silesaurids’ comprise a paraphyletic grouping (more correctly referred to by the informal terms silesaurs or silesaurians) on the stem of Ornithischia. It can be demonstrated that silesaurs acquired anatomical characters anagenetically, culminating in the generation of recognisably ornithischian dinosaurs. This, in turn, creates a taxonomic conundrum. To resolve the latter, we retain the name Ornithischia for an inclusive clade that includes its stem-lineage (silesaurs + traditional ornithischians) and revive Richard Owen’s long-forgotten taxon Prionodontia (= “coarse edged teeth”) for the more exclusive clade containing only definitively “bird-hipped” dinosaurs. This revised taxonomic framework offers stability between the clades Ornithischia, Theropoda and Sauropodomorpha, while further analyses will continue to refine and re-shape the dinosaurian tree. If a consensus were to support our new hypothesis then paradoxically, but perhaps logically, the earliest ornithischians are saurischian!”