POSTCRANIAL MORPHOLOGY AND THE LOCOMOTOR ADAPTATIONS OF EXTANT AND EXTINCT CROCODYLOMORPHS AND LEPIDOSAURS

Studies have shown that the morphology of the postcranial skeleton reflects the locomotor behavior of extant vertebrate taxa. Morphometric analyses can also be used to infer the locomotor modes of extinct taxa based on their morphological similarity to extant groups. Such studies have been conducted on many groups of mammals, however studies on reptiles are less common. As semi-aquatic, arboreal, and terrestrial locomotor specialists are seen in multiple groups of extant reptiles, among both crocodilians and lepidosaurs, the group provides the opportunity for examining potential convergent or parallel evolution within clades.

We have collected a series of linear measurements of the axial and appendicular skeletons of 55 extant crocodilian and lepidosaur taxa to determine if those engaging in similar locomotor behavior display similar morphology despite phylogenetic differences. A stepwise discriminant function analysis using 28 osteological indices reveals reptile locomotor mode can be accurately predicted (over 80% correct) based on morphology. Semi-aquatic taxa appear to be distinguished by a shorter proximal forelimb relative to scapula length, a longer ischium relative to pubis length, and a shorter distal hindlimb than terrestrial and arboreal taxa. These features potentially reflect adaptations to reduce drag while swimming and increase surface area for the attachment of muscles used in lateral undulations of the tail during aquatic locomotion. Semi-aquatic lizards from four families show parallel divergences from their terrestrial relatives, suggesting similar evolutionary responses in both lepidosaurs and crocodylomorphs. Arboreal taxa display a more elongate humerus and relatively smaller humeral proximal end, possibly to allow for a wider range of motion at the shoulder joint. This morphometric data can potentially be used to predict the locomotor behavior of a wide range of extinct reptile taxa, including both archosaurs and lepidosaurs. Within this study, five extinct crocodylomorph taxa were examined including Hyposaurus rogersii, Necrosuchus ionensis, Alligator sp. of the Gray Fossil Site, Crocodylus affinis, and Allognathosuchus mooki, all of which were inferred to be semi-aquatic by the discriminant function analysis.