....."As it was for the first land plants and arthropods, several key adaptations make the vertebrates' transition to life on land successful. Amphibians evolve skeletal and muscular features that support body weight, enable walking, and keep their heads up off the ground. But they are not the first vertebrates to venture onto land; scientists think their ancestors, the rhipidistian fish, used fleshy, lobed fins to shuffle ashore.
Many unmistakable physical similarities exist between rhipidistians, which resemble modern lungfish, and the first true land vertebrates, the amphibians. From fossil evidence, skulls, teeth, and vertebrae are nearly identical between some "lobefins" like the rhipidistians and the earliest amphibians. It's also clear that fleshy ventral fins, attached directly to lobefin skeletons near the tail-end of the fishes, have moveable bones and muscles. Remarkably, the bones within these fins are matched, one to one, with those in the legs of early amphibians. Another shared trait is the ability to breathe air. Rhipidistians, along with several other groups of early fishes, had nostrils and lungs.
It's important to understand that these physical adaptations, while later recruited for use on land, originally evolved in the water. Lobefins would have used ventral fins to improve swimming and steering, and to scuttle through very shallow waters and stir up the silty bottom as they searched for food. Rhipidistians probably evolved the capacity to breathe air to cope with environments where oxygen levels in water were seasonally low, like in shallow lagoons..."
http://www.pbs.org/wgbh/evolution/change...
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