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What drove reversions to quadrupedality in ornithischian dinosaurs? Testing hypotheses using centre of mass modelling
Authors:Susannah C R Maidment  Donald M Henderson  Paul M Barrett
Institution:1. Department of Earth Sciences, The Natural History Museum, Cromwell Road, London, SW7 5BD, UK
2. Department of Earth Science and Engineering, South Kensington Campus, Imperial College, London, SW7 2AZ, UK
3. Royal Tyrrell Museum, Highway 838, Midland Provincial Park, Drumheller, AB, T0J 0Y0, Canada
Abstract:The exceptionally rare transition to quadrupedalism from bipedal ancestors occurred on three independent occasions in ornithischian dinosaurs. The possible driving forces behind these transitions remain elusive, but several hypotheses—including the development of dermal armour and the expansion of head size and cranial ornamentation—have been proposed to account for this major shift in stance. We modelled the position of the centre of mass (CoM) in several exemplar ornithischian taxa and demonstrate that the anterior shifts in CoM position associated with the development of an enlarged skull ornamented with horns and frills for display/defence may have been one of the drivers promoting ceratopsian quadrupedality. A posterior shift in CoM position coincident with the development of extensive dermal armour in thyreophorans demonstrates this cannot have been a primary causative mechanism for quadrupedality in this clade. Quadrupedalism developed in response to different selective pressures in each ornithischian lineage, indicating different evolutionary pathways to convergent quadrupedal morphology.
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