A quantitative method for inferring locomotory shifts in amniotes during ontogeny, its application to dinosaurs, and its bearing on the evolution of posture

KIMBERLEY E. J. CHAPELLE; ROGER B. J. BENSON; JOSEF STIEGLER; ALEJANDRO OTERO; QI ZHAO; JONAH N. CHOINIERE

PALAEONTOLOGY

WILEY-BLACKWELL PUBLISHING, INC

Lugar: Londres; Año: 2019 p. 1 - 14

0031-0239

Macroevolutionary transitions between quadrupedal and bipedal postures played a central role in the diversification of amniotes on land, including in our own lineage (Hominini). Heterochrony has been suggested as a macroevolutionary mechanism for postural transitions, but understanding the evolution of posture in deep time has been hindered by a lack of broad comparative methods. Dinosaurs are an excellent natural laboratory for understanding postural transitions, because their lineage contains at least four separate instances of quadrupedality evolving from bipedality, and because heterochronic processes have been put forward as an explanatory model for these transitions. Here, we extend a quantitative method for reliably classifying posture to the study of ontogenetic postural transitions, using measurements of proportional limb circumference ratios. We apply this method to ontogenetic series of living and extinct amniotes, with a focus on dinosaurs. Our method identifies a general pattern of ontogenetic conservation of quadrupedal and bipedal postures in many living amniote species and in some dinosaurs, and it correctly predicts the human ontogenetic postural shift from quadrupedal crawling to bipedal walking. We also predict a transition from early ontogenetic quadrupedality to late-ontogenetic bipedality in the transitional sauropodomorph dinosaur Mussaurus patagonicus and possibly in the early branching ceratopsian Psittacosaurus lujiatunensis. The phylogenetic positions of these ontogenetic shifts suggest that heterochrony may indeed play a role in the macroevolution of posture, at least in dinosaurs. This method has substantial potential for testing evolutionary transitions between locomotor modes, especially in elucidating the role of evolutionary mechanisms such as heterochrony.