LOCOMOTION IN ALLIGATOR-MISSISSIPPIENSIS - KINEMATIC EFFECTS OF SPEEDAND POSTURE AND THEIR RELEVANCE TO THE SPRAWLING-TO-ERECT PARADIGM

In terms of locomotory posture, amphibians and lizards are considered to be sprawlers, mammals and dinosaurs are considered to be erect, and extant crocodilians are considered to be intermediate because they us e the 'high walk', a semi-erect posture where the body is held half-wa y between the sprawling and erect grades during locomotion, In additio n, crocodilians occasionally use a sprawling posture. Extant crocodili ans, therefore, provide an interesting model in which to investigate t he sprawling-to-erect transition in vertebrate evolution, This study q uantifies the sprawl and high walk kinematics of the alligator. Alliga tor mississippiensis moving at different speeds on a treadmill and com pares them with kinematic data available for other vertebrates, These data allow us to examine the effects of speed on crocodilian postures and to examine how crocodilian locomotion relates to the sprawling-to- erect paradigm in vertebrate locomotion. Our results show that the cro codilian sprawl is not functionally equivalent to the primitive sprawl ing behaviors exhibited by salamanders and lizards. In fact, although the high walks and sprawls of alligators exhibit some kinematic differ ences, they are actually much more similar than expected and, essentia lly, the crocodilian sprawl is a lower version of a high walk and coul d be termed a 'low walk'. In terms of the sprawling-to-erect transitio n, the high walk has knee kinematics intermediate between those of bir ds and non-archosaurian tetrapods, but alligators increase speed in a way completely different from other terrestrial vertebrates (distal ra ther than proximal limb elements are used to increase speed). These ki nematic data viewed in the light of the fossil and phylogenetic eviden ce that modern crocodilians evolved from erect ancestors suggest that modern crocodilians have secondarily evolved a variable semi-erect pos ture and that they are problematic as an intermediate model for the ev olutionary transition from sprawling to erect postures in archosaurs.