The systematic relationships, physiology and life habits of the ‘hypsilophodontid’ dinosaurs

Brown, Emily Elizabeth (2024). The systematic relationships, physiology and life habits of the ‘hypsilophodontid’ dinosaurs. University of Birmingham. Ph.D.

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Abstract

The ‘hypsilophodontids’ are an informal group of ornithischian dinosaurs that share a similar bauplan: being bipedal, small to medium-bodied (~1–2 m), and relatively conservative in their morphology. These dinosaurs are known from Antarctica, Asia, Australasia, Europe, North America and South America, and have a temporal range from the Middle Jurassic to the latest Cretaceous. The phylogenetic relationships of the ‘hypsilophodontids’ remains one of the key questions in ornithischian dinosaur research, with recent phylogenetic studies recovering these taxa in one of two highly conflicting positions: (1) primarily within Cerapoda (Ornithopoda+Marginocephalia), as a paraphyletic assemblage of early ornithopods; and (2) primarily outside of Cerapoda, within the clade Thescelosauridae. This uncertainty has impeded our understanding of ornithischian evolutionary history and consequently, our interpretation of some of the major transitions in these groups, such as the acquisitions of obligate high-fibre herbivory and quadrupedality.

In this thesis, I use multiple palaeontological approaches to improve our understanding of non-cerapodan neornithischians and early-diverging ornithopods (‘hypsilophodontids’). Three recent phylogenetic studies that have recovered contrasting placements for the ‘hypsilophodontids’ were subject to in-depth character and taxon manipulation, with the aim of identifying the cause of these conflicting topologies. Results find that matrices which originally recovered the ‘hypsilophodontids’ within Cerapoda were comparably more stable in response to this manipulation, perhaps
suggesting these relationships are more robust. These three data matrices were further used to evaluate the morphological disparity within ‘hypsilophodontids’, finding that this group represents a diverse assortment of taxa. Disparity-through-time results show that the pelvic girdle and hind limb morphologies of ornithischians were comparatively diverse in the Middle Jurassic, indicating that important locomotory adaptations were evolving at this time. The long bone histology of the namesake ‘hypsilophodontid’, Hypsilophodon foxii, was examined to give insight into the growth patterns of these dinosaurs. Five histological ontogenetic stages were distinguished; however no unambiguous lines of arrested growth (LAGs) were recovered, suggesting Hypsilophodon foxii maintained uninterrupted growth even during periods of seasonal stress. The presence or absence of regular developing LAGs in other ‘hypsilophodontids’ implies that two distinct growth strategies were adopted by these taxa. Lastly, digital techniques were used to investigate the neuroanatomy and tooth replacement cycle of a new genus of ornithopod from Antarctica. Endocranial reconstructions reveal a large floccular lobe that extends considerably through the anterior canal of the endosseous labyrinth, suggesting this animal had enhanced gaze stabilisation and was capable of
rapid eye, head and neck movements. Digital segmentation of the dentary provides an in-depth assessment of an alternative ornithopod dental condition, compared to the more widely researched dental batteries of hadrosaurian ornithopods.

This thesis provides a significant contribution to our understanding of the'hypsilophodontids', showing that these dinosaurs exhibited disparate morphologies, neuroanatomies and even distinct patterns of growth. The phylogenetic affinities of the ‘hypsilophodontids’ remain contentious, but I hope the recommendations suggested in this thesis will aid future studies looking to resolve these relationships.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):