The relationship between body shape, body size and locomotor mode in extant lepidosaurs

D. B. Norman,L. R. Grinham

doi:10.1111/jzo.12771

Abstract

AbstractDespite historic work, the mechanisms and evolutionary drivers associated with the adoption of a facultatively bipedal locomotor mode in extant lepidosaurs are unclear. Recent work has provided insights into the biomechanical triggers of bipedal locomotion, but the associated anatomies are yet to be fully understood, particularly with regard to body size across Lepidosauria. Using a dataset derived from museum specimens, representing a range of lepidosaur body shapes, we highlight the differences between obligate quadrupeds and facultative bipeds within this group and demonstrate the value of non‐caudal skeletal material in identifying facultative bipeds using osteology alone. We use multiple statistical approaches to identify trends across locomotor modes relative to body size. Body size has a significant effect upon body proportions across the two locomotor modes, especially in the hindlimbs. Forelimb lengths do not differ significantly across locomotor modes for animals of similar body size, but distal hindlimbs are significantly longer in facultative bipeds. Interestingly, femoral length does not differ across locomotor modes of a similar body size. Our findings contrast with historical tropes and are significant for future work attempting to identify the factors driving the evolution of a facultatively bipedal locomotor mode in Lepidosauria.

Highlights

The subclass Lepidosauria, comprising Squamata and Rhynchocephalia (Sphenodon punctatus – the tuatara – being the only living representative), is one of the most diverse and widespread extant terrestrial vertebrate groups
The mechanisms and evolutionary drivers associated with the adoption of a high-performance, bipedal and locomotor mode remain unclear, despite facultative bipedality having evolved at least 110 million years ago in this group (Lee et al, 2018)
Using a diverse dataset that accounts for phylogenetic relatedness across a wide range of lepidosaur species, we find that differences in the hindlimb relative to snout–vent length (SVL) are most significant for differentiating facultative bipeds from obligate quadrupeds

Summary

Introduction

The subclass Lepidosauria, comprising Squamata and Rhynchocephalia (Sphenodon punctatus – the tuatara – being the only living representative), is one of the most diverse and widespread extant terrestrial vertebrate groups. An distinctive locomotor behaviour that is well-represented in squamates is facultative bipedality, defined as the ability of ordinarily quadrupedal animals to adopt a bipedal gait for movement. This locomotor style has been observed in several squamate species (e.g. basilisks, scrub lizards and others (Hsieh, 2003; Kinsey & McBrayer, 2018)) and in other vertebrates, such as rodents and primates (Alexander, 2004). The mechanisms and evolutionary drivers associated with the adoption of a high-performance, bipedal and locomotor mode remain unclear, despite facultative bipedality having evolved at least 110 million years ago in this group (Lee et al, 2018). Bipedal locomotion appears to be neither faster nor more energetically efficient in lizards expressing this behaviour, raising more profound questions focused upon why this behaviour has evolved (Clemente et al, 2008)

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