NEUROMUSCULAR DIVERSITY IN ARCHOSAUR DEEP DORSAL THIGH MUSCLES

The living members of the clade Archosauria, crocodilians and birds, d iffer markedly in the morphology of their deep dorsal thigh muscles. T o investigate whether this diversity is accompanied by differences in motor pattern and muscle function ? the hindlimbs of representative ar chosaurs were studied by electromyography and cineradiography during t errestrial locomotion. In a crocodilian, Alligator, the iliofemoralis and pubo-ischio-femoralis internus part 2 are both active during the s wing phase of the stride cycle. This appears to be the primitive motor pattern for archosaurs. There are four avian homologues of these musc les in the helmeted guineafowl, Numida. These are primarily active in the propulsive phase (iliotrochantericus caudalis and iliotrochanteric us medius), the swing phase (iliotrochantericus cranialis) and a speed -dependent combination of the propulsive and/or swing phases (iliofemo ralis externus). Differences between Alligator and Numida in the numbe r and attachment of deep dorsal muscles are associated with dissimilar motor patterns and functions. Evolutionary modifications of neuromusc ular control must be recognized when evaluating avian locomotor histor y, but are rarely considered by paleontologists. Even within the deep dorsal thigh muscles of Numida, developmentally and anatomically simil ar muscles are active out-of-phase. Therefore, although the actions of two adjacent muscles appear equivalent, their functions may differ dr amatically. The diversity of deep dorsal thigh muscles in modern birds may be a good model for studying the relationship between activity pa ttern and peripheral morphology.