Locomotor and postural diversity among reptiles: palaeobiological implications

Gônet, Jordan; Bardin, Jérémie; Girondot, Marc; Hutchinson, John; Laurin, Michel

Abstract

The conquest of land by the first tetrapod vertebrates represents a key stage in their evolution. Selection pressures exerted by this new environment on animals led to the emergence of new locomotor and postural strategies that favoured access to different ecological niches and contributed to their evolutionary success. Today, amniotes show great locomotor and postural diversity, particularly among Reptilia, whose extant representatives include erect bipeds (birds), “semi-erect” quadrupeds (crocodilians) and sprawling quadrupeds (squamates and turtles). But the different steps leading to such diversity remain enigmatic and the type of locomotion adopted by many extinct species raises questions. This is notably the case of certain Triassic taxa such as Euparkeria and Marasuchus. However, the exploration of the bone microanatomy in reptiles helps to overcome these uncertainties. Indeed, this locomotor and postural diversity is accompanied by great microanatomical disparity. On land, the bones of the appendicular skeleton support the weight of the body and are subject to multiple constraints that partly shape their external and internal morphology. Here we show how microanatomical parameters measured in cross-section, such as bone compactness or the position of the medullocortical transition, can be related to locomotion. Bipedal taxa had a larger medullary cavity than quadrupeds, but no significant difference in terms of compactness. We hypothesise that the different nature of the forces acting on the bone induced by the different postures may explain this result. Using statistical methods that take phylogeny into account (flexible phylogenetic discriminant analyses), we develop different models of locomotion from a sample of femur cross-sections from 51 reptile species. We use these models to infer locomotion and posture in 7 extinct reptile taxa for which they remain debated. Our models produced reliable inferences for taxa that preceded and followed the quadruped/biped and sprawling/erect transitions, notably within the Captorhinidae and Dinosauria. For taxa contemporary with these transitions, such as Terrestrisuchus and Marasuchus, the inferences are more questionable. We use linear models to investigate the effect of body mass and functional ecology on our inference models. We show that body mass seems to significantly impact our models in most cases, unlike the functional ecology. Finally, we illustrate how taphonomic processes can impact certain microanatomical parameters, especially the eccentricity of the section. Our study provides insight into the evolution of enigmatic locomotion in various early reptiles. Our models and methods could be used by palaeontologists to infer the locomotion and posture in other extinct reptile taxa, especially when considered in combination with other lines of evidence.

Citation

Gônet, J., Bardin, J., Girondot, M., Hutchinson, J., & Laurin, M. (2023). Locomotor and postural diversity among reptiles: palaeobiological implications. Journal of Anatomy, https://doi.org/10.1111/joa.13833

• Journal Article Type: Article
• Acceptance Date: Jan 1, 2023
• Online Publication Date: Feb 18, 2023
• Publication Date: Feb 18, 2023
• Deposit Date: Mar 21, 2022
• Publicly Available Date: Feb 20, 2023
• Print ISSN: 0021-8782
• Publisher: Wiley
• Peer Reviewed: Peer Reviewed
• DOI: https://doi.org/10.1111/joa.13833