89.4  Sunday, Jan. 6  Assessing the evidence for the evolution of asymmetrical gaits in Crocodylomorpha HUTCHINSON, JR; The Royal Veterinary College, Structure and Motion Laboratory jrhutch@rvc.ac.uk

Some Crocodylia use asymmetrical gaits, including bounding and galloping, at near-maximal speeds. This ability is commonly assumed to have evolved in stem Crocodylomorpha, related to changes in limb/axial morphology. Body size is typically assumed to limit the capacity to use asymmetrical gaits, but it is unknown how size might mechanistically constrain such athleticism. I report on collaborative efforts to reconstruct these constraints and how they shaped locomotor evolution in the crocodile lineage.

Experimental data (50-100 Hz video; also limited forceplate data) were collected from 189 near-steady strides of 32 individuals from 15 species of Crocodylia across a broad speed range (0.15-4.4 m s-1). Locomotor parameters were quantified for each stride and compared statistically. These experimental data reveal the absence of asymmetrical gaits in Alligatoroidea, whereas Crocodyloidea >2-4m total length do not use asymmetrical gaits. Otherwise, many aspects of crocodylian locomotion vary more within species than among them.

Additionally, for 6 taxa, we conducted scaling analyses of anatomical data for the 78 limb muscles of 18 individuals (0.13-278 kg) (e.g. physiological areas calculated from muscle mass, pennation and fascicle length) to examine whether limb muscles showed structural changes for supporting faster gaits at larger sizes. Alligatoroidea and Crocodyloidea lack any strong evidence for allometry of limb muscle architecture. Similarly, while axial “bracing systems” could constrain/enable such gaits, their biomechanics remains only qualitatively explored. Better evidence for limb/axial function-form linkages in extant Crocodyloidea, as well as quantitative morphology of fossils, could resolve this mystery and reconstruct when, how and why asymmetrical gaits evolved in Crocodylomorpha.