The evolution of ornithischian quadrupedality

Abstract

Ornithischian dinosaurs were primitively bipedal, but reverted to quadrupedality on at least three (and potentially several more) occasions: in Ceratopsia, Thyreophora and Hadrosauriformes. Each of these reversals was accompanied by anatomical changes to the whole skeleton that enabled the forelimb to function in weight bearing and that also resulted in numerous changes to the hip and hind limb musculature. Each quadrupedal clade acquired a suite of similar biomechanical characters, although they varied in terms of function and in how these character complexes were assembled. Some similar changes occurred in parallel among sauropodomorph dinosaurs as they transitioned from bipedality to quadrupedality. It is unclear why bipedal ornithischians reverted to quadrupedalism, but neither increases in body size nor the acquisition of dermal armour seem to have played a significant role. Increased head size might have influenced the position of the centre of mass and stance in ceratopsians and it is plausible that the evolution of herbivory played an important role in both ornithischians and sauropods, but the latter hypothesis is difficult to test.