AMPHIBIAN LOCOMOTION IN EVOLUTIONARY TIME

Members of the three extant orders of the Amphibia exhibit markedly di fferent locomotor morphologies: gymnophiones are elongate, limbless, a nd most lack a tail; urodeles typically retain the morphology presumed ancestral for amphibians-limbs and a tail, with relatively short bodi es; anurans have elongate limbs, short bodies, and lack tails. The ear liest known gymnophione is from the Early Jurassic Kayenta formation, as is the earliest frog. The earliest salamander is from the Middle Ju rassic. Recurrent themes in the evolution of amphibian body form and, presumably, locomotion in several extinct and extant lineages, include body elongation, limb reduction and loss, and tail modification and l oss, though frogs reversed those trends early in their evolution. Comp arison of the fossil records of extinct and extant lineages of amphibi ans with phylogenetic hypotheses that invoke a ''molecular clock'' in order to date the probable times of divergences of taxa are instructiv e. In some cases, molecular/biochemical data give dates consistent wit h events in the geologic record; in other cases, the dates are incongr uent. At the same time, there is no evidence that morphological change , cladogenesis, or extinction are correlated with historical geologica l boundaries within and among amphibian lineages.