THE ORIGIN OF THE TETRAPODS

The search for the origins of the tetrapods, both in the sense of ancestry and the general biological context of the transition between fish and amphibian, has long been a central concern of vertebrate paleobiology. Recent discoveries have done much to improve our information about the interrelationships of lobe-finned fishes and tetrapods and to redefine the patterns of morphological change in key structures such as the skull, stapes, limb, and gills. The osteolepiform fishes remain the most likely ancestors. The first tetrapods were polydactylous and retained partial gills. The age of the first tetrapods is steadily being pushed back, and their geographical range widened. The transition probably occurred in a coastal wetland environment, no later than the end of the Middle Devonian and (depending on an inconclusive trackway) even earlier. Many unresolved questions remain, especially concerning the evolutionary mechanisms that underly the transition.