The origin and early evolution of the ornithischian bauplan: evaluation and implications for the Dinosauria

The origin of the dinosaurian clade Ornithischia is a topic that has attracted little attention. There are currently three competing hypotheses concerning the relationship between the Ornithischia and the two other principal clades of Dinosauria: Theropoda and Sauropodomorpha. The two latter clades have fossil representatives that are well-documented in the Carnian (Late Triassic). However, each of the phylogenetic hypotheses support tree topologies within Dinosauria that imply the existence of a ghost-lineage for Ornithischia that extends through a substantial portion (~25 Ma) of Triassic time because Ornithischia make their first unambiguous appearance in the Hettangian (Early Jurassic). Equally, little attention has been given to recent analyses that controversially have placed some Triassic dinosauromorph taxa (stem-lineage Dinosauria) within the clade Ornithischia. One large-scale phylogenetic analysis (Müller & Garcia, 2020a) recovered a preferred topology that featured an array of taxa (commonly referred to as silesaurids) as a paraphyletic assemblage of (dinosaurian) taxa placed along the branch leading to the clade Ornithischia. This latter hypothesis of relationships accounts for the apparent absence of Triassic ornithischians, because stem-lineage ornithischians (silesaurians in this article) have an exclusively Mid-Late Triassic stratigraphic distribution. The latter analysis used a dataset that did not include the diversity of known early representatives of Ornithischia (sensu lato) and did not incorporate all the anatomical characters that have been suggested to unite Ornithischia with the other dinosaurian clades (Theropoda and Sauropodomorpha). Nor did the initial study go on to expand upon some important taxonomic, palaeobiological and evolutionary implications of a topology that links a paraphyletic array of silesaurians to the clade Ornithischia. This article addresses these latter issues by evaluating the published expansion and re-analysis of the original dataset (Norman et al., 2022). The results supported the hypothesis that silesaurians comprise a paraphyletic grouping of taxa on the stem of Ornithischia and suggest that successive silesaur taxa acquire anatomical characters sequentially (potentially, anagenetically) in a process that culminates in the assembly of what may be described as the 'traditional' ornithischian bauplan. There are taxonomic consequences that arise if this new topology were to be accepted. For nomenclatural stability in this area of the tree, and to preserve the most widely recognised and relevant taxonomic names, we proposed a revised taxonomic framework for ornithischians that is consistent with this new topology. The name Ornithischia is retained for the cladistic 'total-group' (traditional Ornithischia of Seeley, plus its stem-lineage), and we resuscitated a name originally proposed by Richard Owen (Prionodontia = "coarse edged tooth") for the clade containing only what might be regarded as the 'traditional' ornithischian (objectively "bird-hipped") dinosaurs. The revised taxonomic framework provided a measure of phylogenetic clarity as well as a degree of stability with respect to the clades Ornithischia and Dinosauria. Consideration of the pattern of acquisition of pelvic and hind limb anatomical characters within the expanded clade Ornithischia has led to the suggestion that several of the supposedly 'key' anatomical traits that have been regarded as synapomorphies defining membership of the Dinosauria may alternatively have been acquired independently during the earliest phase of dinosaur diversification.