BECK, A. L.*; BLOB, R. W.; University of Chicago; Clemson University: Form and Function in Tetrapod Locomotion: Implications for the Origin of Mammalian Locomotion

Although quadrupedal tetrapods have been historically divided into two groups, sprawling and erect, these distinctions actually represent endpoints of a continuum of observed postures. The ability to distinguish morphological features that correlate with the varying areas of this continuum would provide a rigorous framework for interpreting locomotor function from the skeletons of extinct tetrapods. We use quantitative and qualitative data collected from a range of extant lizards, crocodilians, and mammals and employ multivariate statistical methods to identify suites of morphologic characters that correlate with segments of the limb posture continuum. For example, sprawling taxa have, in addition to shorter epipodials than propodials, more proximally-placed, deeper deltopectoral crests and trochanters on the humeri and femora, respectively. More upright taxa tend to have longer, wider scapulae, longer humeral epicondyles and shorter ischia and pubes (relative to ilium length). These types of patterns generally hold, regardless of taxonomic affinity, with few exceptions. Taxa such as chameleons, which are upright reptiles, fill their own morphospace because they show a mosaic of reptilian and mammalian features, for example, a long posterior process on the ilium, and a large area for attachment of femoral adductor muscles, respectively. These morphological correlates can guide interpretations of the likely locomotor regimes of many non-mammalian synapsids, the extinct ancestors of mammals. Through an understanding of such morphological aspects of locomotor evolution among non-mammalian synapsids, it should be possible to gain insight into the origins of many correlated mammalian characteristics, such as endothermy and sustained aerobic capacity.