The evolution of the functional role of trunk muscles during locomotion inadult amphibians

The axial musculature of all vertebrates consists of two principal masses, the epaxial and hypaxial muscles, The primitive function of both axial musc le masses Is to generate lateral bending of the trunk during swimming, as i s seen in most fishes. Within amphibians we see multiple functional and mor phological elaborations of the axial musculature. These elaborations appear to be associated not only with movement into terrestrial habits (salamande rs), but also with subsequent locomotor specializations of two of the three major extant amphibian clades (frogs and caecilians), Salamanders use both epaxial and hypaxial muscles to produce lateral bending during swimming an d terrestrial, quadrupedal locomotion. However during terrestrial locomotio n the hypaxial muscles are thought to perform an added function, resisting long-axis torsion of the trunk. Relative to salamanders, frogs have elabora te epaxial muscles, which function to both stabilize and extend the iliosac ral and coccygeosacral joints, These actions are important in the effective use of the hindlimbs during terrestrial saltation and swimming. In contras t, caecilians have relatively elaborate hypaxial musculature that is linked to a helix of connective tissue embedded in the skin, The helix and associ ated hypaxial muscles form a hydrostatic skeleton around the viscera that i s continuously used to maintain body posture and also contributes to forwar d force production during burrowing,