68.6 Friday, Jan. 6 The origin and early evolution of terrestrial locomotion SWARTZ, Brian; Univ. of California, Berkeley Brian.Darwin@Berkeley.edu
The origin of terrestrial vertebrates involved an integrated series of changes to the ancestral sarcopterygian bauplan. However, many traits often considered apomorphic for tetrapods have a much deeper origin in vertebrate history. Terrestrial locomotion integrates many such plesiomorphies that facilitated the diversification of vertebrate life on land. In a phylogenetic assessment of over 150 modern and fossil taxa, I incorporate data from osteological, myological, and locomotor records to test how gaits have evolved over gnathostome evolution, and how variation in the historical, constructional, and functional components of the axial and appendicular systems underpins these changes. I show that (a) the trot evolved at least three times in gnathostome evolution; (b) similarities in the trunk muscles of extant lungfishes and tetrapods suggest that the tetrapod myaxial condition evolved in water ~35 million years before the origin of amphibious sarcopterygians; (c) trackway data from modern and fossil records cannot verify whether the lateral sequence diagonal-couplet gait evolved by the end of the Devonian; and (d) the original function of the physical neck—i.e., a space between head and shoulder—was more likely related to the origin of terrestrial locomotion than to any requirement for neck mobility. A pluralistic approach to thinking about macroevolutionary changes—those that distinguish aptations and nonaptations in a continuum of historical, constructional, and functional influences—better elucidates evolutionary transformations than a functionalism that focuses on the cycling of adaptations and exaptations.