Femoral specializations to locomotor habits in early archosauriforms

Romain, Pintore; Alexandra, Houssaye; Nesbitt, Sterling J; Hutchinson, John R

Abstract

The evolutionary history of archosaurs and their closest relatives is characterized by a wide diversity of locomotor modes, which has even been suggested as a pivotal aspect underlying the evolutionary success of dinosaurs vs. pseudosuchians across the Triassic-Jurassic transition. This locomotor diversity (e.g. more sprawling/erect; crouched/upright; quadrupedal/bipedal) led to several morphofunctional specializations of archosauriform limb bones that have been studied qualitatively as well as quantitatively through various linear morphometric studies. However, differences in locomotor habits have never been studied across the Triassic-Jurassic transition using 3D geometric morphometrics, which can relate how morphological features vary according to biological factors such as locomotor habit and body mass. Herein, we investigate morphological variation across a dataset of 72 femora from 36 different species of archosauriforms. Firstly, we identify femoral head rotation, distal slope of the fourth trochanter, femoral curvature, and the angle between the lateral condyle and crista tibiofibularis as the main features varying between bipedal and quadrupedal taxa, all of these traits having a stronger locomotor signal than the lesser trochanter's proximal extent. We show that the locomotor signal was stronger than the phylogenetic one despite famous trends in locomotor mode across the evolutionary history of early archosauriforms. This enables us to predict locomotor modes of some of the more ambiguous early archosauriforms with a success rate of 93%, without relying on the relationships between hindlimb and forelimb linear bone dimensions as in prior studies. Second, we highlight that the most important morphological variation is linked to the increase of body size, which impacts the width of the epiphyses and the roundness and proximodistal position of the fourth trochanter. Furthermore, we show that bipedal and quadrupedal archosauriforms have different allometric trajectories along the morphological variation in relate to body size. Finally, we demonstrate a covariation between locomotor mode and body size, with variations in femoral bowing (anteroposterior curvature) being more distinct among robust femora than gracile ones. We also identify a decoupling in fourth trochanter variation between locomotor mode (symmetrical to semi-pendant) and body size (sharp to rounded). Our results indicate a similar level of morphological disparity linked to a clear convergence in femoral robusticity between the two clades of archosauriforms (Pseudosuchia and Avemetatarsalia), emphasizing the importance of accounting for body size when studying their evolutionary history, as well as for studying the functional morphology of appendicular features. Determining how early archosauriform skeletal features were impacted by locomotor habits and body size also enables us to discuss the potential homoplasy of some phylogenetic characters used in cladistic analyses as well as when bipedalism evolved in the avemetatarsalians lineage, adding to our understanding of the early radiation of dinosaurs.

Citation

Romain, P., Alexandra, H., Nesbitt, S. J., & Hutchinson, J. R. (2021). Femoral specializations to locomotor habits in early archosauriforms. Journal of Anatomy, https://doi.org/10.1111/joa.13598

• Journal Article Type: Article
• Acceptance Date: Nov 27, 2021
• Publication Date: Nov 28, 2021
• Deposit Date: May 17, 2021
• Publicly Available Date: Nov 29, 2021
• Print ISSN: 0021-8782
• Publisher: Wiley
• Peer Reviewed: Peer Reviewed
• DOI: https://doi.org/10.1111/joa.13598
• Keywords: appendicular skeleton; functional morphology; palaeontology; Archosauria; Triassic; 43 geometric morphometrics 44
• Public URL: https://rvc-repository.worktribe.com/output/1548876