In the history of life, there are few evolutionary transformations to rival the impact of the development of land vertebrates from lobe-finned fishes. The fossil evidence for the evolutionary origins of tetrapods has been greatly enriched in recent years. The most important extinct representatives found during three overlapping stages of tetrapod evolution are as follows:
• Lobe-finned fishes (416 to 375 million years ago). These fully aquatic fishes exhibit some traits associated with early tetrapods. Lobe-finned fishes did not become extinct 375 million years ago, but although they were a diverse group in the Devonian Period, the coelacanth and lungfishes are the only lobe-finned fish still living. The lobe-finned fishes discussed here are those that are closely related to the first tetrapods.
• Near-terrestrial tetrapods (385 to 360 mya). These are specific taxa that exhibit some tetrapod traits but that probably lived mostly in the water. Their limbs were short and paddlelike.
• Early tetrapods (365 to 360 mya). These were increasingly terrestrial vertebrates with limbs and other traits adapted for land that did not exhibit clearly fishlike traits such as fins, gills, or paddlelike limbs. They were still primarily aquatic.
The earliest traces of basal tetrapods come in the form of partial-body fossils and trackways that date from the latter part of the Devonian, about 380 million years ago. Most of these partial specimens were so fragmentary that they did not readily reveal their tetrapod origins. Consisting mostly of lower jaw fragments and a few limb elements, fossils that included parts of Obruchevicthys (Latvia and Russia) and Elginerpeton (Scotland) were originally considered to be sarcopterygians, or lobe-finned fishes. These enigmatic specimens rested for many years in museum drawers in Europe until discoveries of additional remains of Late Devonian tetrapods encouraged scientists to take a closer look at the older specimens. Swedish paleontologist Per Ahlberg was instrumental in reinvestigating Obruchevicthys and Elginerpeton during the 1990s, establishing them as the two oldest tetrapods known from the fossil record. No feet or fins are known for these animals, but their jaws share many traits of tetrapods. The shape and length of their jaws are distinctive enough to suggest that Obruchevicthys and Elginerpeton were part of a short-lived branch of the early radiation of tetrapods. Ahlberg estimated that Elginerpeton was about five feet (1.5 m) long. Two additional Late Devonian specimens known only from fragments include Livoniana (Latvia) and Elpistostege (Canada), each of which hints at a transitional stage between the lobe-finned fishes and the tetrapods.
Several trackways from the Late Devonian have been referred to early tetrapods. The most notable come from the Genoa River area in Australia and include two sets of tracks. One set shows an animal that was dragging its tail. The other set includes no evidence of either a tail or belly drag; this suggests that the animal was walking. Unfortunately, it cannot be determined whether these tracks were left underwater or on dry ground, so it cannot be said with certainty that they were made by a walking or floating tetrapod.
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