The evolution of quadrupedalism in ornithischian dinosaurs
Dr Susannah C. R. Maidment,
Department of Earth Science and Engineering,
Thursday 6th September
Neil Chalmers Seminar Room, DC2, 15:00
The most primitive ornithischian (bird-hipped) dinosaurs were bipedal (two-legged), but they radiated into a diversity of quadrupedal (four-legged) and bipedal forms. Quadrupedalism evolved in three major ornithischian lineages independently. Outside of Ornithischia, the reversion to quadrupedalism from bipedal ancestors has only occurred on two other occasions in the whole of tetrapod evolution (in the silesaurid dinosauriforms and the sauropodomorph saurischian dinosaurs); thus examination of the convergent acquisition of this stance in ornithischians is warranted. We use a diversity of techniques to investigate how and why multiple clades of ornithischian dinosaurs evolved quadrupedal locomotion. Muscle reconstruction suggests that quadrupedal ornithischians adopted a variety of different stances. Disparity in limb scaling between clades could be due to clade-specific behaviours. Anatomical features related to the evolution of quadrupedalism do not appear to have been acquired in the same order in all lineages, and mosaic character evolution in ornithopods suggests multiple independent acquisitions of quadrupedalism in the clade. Moment arm modelling suggests a more columnar hind limb in stance phase and loss of femoral rotation as a form of lateral limb support in quadrupedal ornithischians, and a wide-gauged stance in thyreophorans and ceratopsids. Centre of mass modelling indicates that ceratopsids may have evolved quadrupedalism due to the development of large heads, frills and horns as display structures. However, thyreophorans did not become quadrupedal as a result of development of hypertrophied dermal armour. Overall, quadrupedal ornithischians display a previously unrealised diversity in stance and locomotor mode, and do not appear to have been significantly constrained in their style of locomotion by their bipedal ancestry.
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