DIFFERENTIAL LIMB SCALING IN THE AMERICAN ALLIGATOR (ALLIGATOR MISSISSIPPIENSIS) AND ITS IMPLICATIONS FOR FOSSIL ARCHOSAUR LOCOMOTOR EVOLUTION

Bipedalism evolved multiple times within various archosaur clades, and relatively shorter forelimbs characterize both crocodyliforms and non-avian dinosaurs. For example, an embryonic to adult ontogenetic sequence of specimens of the sauropodomorph Massospondylus has shown that bipedal limb proportions result from negative forelimb allometry. We ask, is negative forelimb allometry a pattern basal to archosaurs, amplified in certain taxa to produce bipedalism? Given the phylogenetic position of extant crocodylians and their relatively shorter forelimb, we tested the hypothesis that prevalent negative forelimb allometry is present in Alligator mississippiensis from a sample of wild specimens from embryonic to adult sizes. Long bone lengths (humerus, radius, ulna, femur, tibia, fibula, third metapodials) were measured with their epiphyseal cartilage intact at all sizes. Our results show an overall isometric pattern for most elements regressed on femur length, humerus length, or total limb length. However, negative allometry was prevalent for the ulna, and the third metapodials scale with positive allometry embryonically. These data suggest that the general forelimb proportions in relation to the hindlimb do not change significantly with increasing size in A. mississippiensis. The negative allometry of the ulna and embryonically positive allometry of the third metapodials appears to be related to maintaining the functional integrity of the limbs. We show that this pattern is different from that of the sauropodomorph Massospondylus, and we suggest that if bipedalism in fossil archosaurs was tied, in part, to negative forearm allometry, it was either secondarily lost through isometric scaling, or never developed in the ancestor of A. mississippiensis.