Tetrapod Origins

Abstract

The origin of tetrapods, the transition from fish to amphibian, was a Devonian event that has been the focus of much research and discussion for over a century. New discoveries have augmented the fossil record of Devonian tetrapods, and there are now 16 named genera of Late Devonian tetrapods and three Devonian tetrapod tracksites of Middle-Late Devonian age. The closest relatives and likely ancestors of tetrapods are the tetrapodomorph fishes, of which Tiktaalik from Arctic Canada is the best known and closest to tetrapod ancestry. To evolve from living in the water as a fish to living on land, tetrapods faced many challenges that included support and locomotion, respiration, feeding, sensory processes, water balance and reproduction in the air. The first tetrapods did not solve all of these problems, as Late Devonian tetrapods were aquatic and likely stayed in and near water. The oldest tetrapod fossils are Middle Devonian (Givetian) footprints from Ireland. Devonian tetrapod footprints indicate an alternating pattern of limb supported locomotion (lateral sequence walking) with a larger hind foot than forefoot. The Devonian tetrapod track record thus indicates such locomotion was possible by some Devonian tetrapods, and their body fossils await discovery. No case for Late Devonian tetrapod mass extinctions can be made based on current data. The paleoenvironment of tetrapod origins has been debated—marine or nonmarine, dry or wet—for about a century, and there is no agreed scenario, with at least three published paleoenvironments of tetrapod origins advocated by different paleontologists: (1) seasonal droughts in a freshwater (nonmarine) setting; (2) wet terrestrial woodlands; or (3) marine or marginal marine environments.