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,Jonah N Choiniere,Alejandro Otero,Qi Zhao,Josef Stiegler,Roger B J Benson,Laura Porro

doi:10.1111/pala.12451

Abstract

AbstractEvolutionary transitions between quadrupedal and bipedal postures are pivotal to the diversification of amniotes on land, including in our own lineage (Hominini). Heterochrony is suggested as a macroevolutionary mechanism for postural transitions but understanding postural evolution in deep time is hindered by a lack of methods for inferring posture in extinct species. Dinosaurs are an excellent natural laboratory for understanding postural transitions because they demonstrate at least four instances of quadrupedality evolving from bipedality, and heterochronic processes have been put forward as an explanatory model for these transitions. We extend a quantitative method for reliably inferring posture in tetrapods to the study of ontogenetic postural transitions using measurements of proportional limb robusticity. We apply this to ontogenetic series of living and extinct amniotes, focusing on dinosaurs. Our method correctly predicts the general pattern of ontogenetic conservation of quadrupedal and bipedal postures in many living amniote species and infers the same pattern in some dinosaurs. Furthermore, it correctly predicts the ontogenetic postural shift from quadrupedal crawling to bipedal walking in humans. We also infer 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 but not in the sauropodomorph Massospondylus carinatus. The phylogenetic positions of these ontogenetic shifts suggest that heterochrony may play a role in the macroevolution of posture, at least in dinosaurs. Our method has substantial potential for testing evolutionary transitions between locomotor modes, especially in elucidating the role of evolutionary mechanisms like heterochrony.

Highlights

Changes in posture, from quadrupedal to bipedal or vice-versa, occurred rarely in tetrapod evolution
Comparisons of humeral and femoral shaft circumference are effective in assessing posture in individual amniotes
A split between bipeds and quadrupeds is clearly observed when Log10(humeral circumference) is plotted against Log10(femoral circumference) (Fig 1A). This pattern is mostly validated by our discriminant function analysis (Fig 2 and S3 Table)

Summary

Introduction

From quadrupedal to bipedal or vice-versa, occurred rarely in tetrapod evolution. We understand the reviewer’s concern, and we hope that we can qualify our statements here in two ways: 1) to build classification models using DFA, we did assemble a dataset from measurements and from the literature comprising hundreds of specimens; 2) we worked for two years to assemble the largest ontogenetic dataset that we could, and the n=36 sample size represents our best attempts to collate these measurements for 22 quadrupeds and bipeds. This reflects the difficulty in obtaining measurement data on specimens.

Objectives

Methods

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Discussion

Conclusion