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Tetrapods are vertebrate animals with four feet, legs or leglike appendages. In Greek, "tetrapod" means "four-legged". Tetrapods include amphibians, reptiles, dinosaurs, birds, mammals, and a few ancient forms intermediate between fish and amphibians. A few tetrapods have evolved into legless forms, such as snakes. In scientific classification, Tetrapoda is a superclass within the subphylum Vertebrata. Whereas sharks, rays, skates, and fish are representatives of vertebrates in the seas, tetrapods are the representatives of vertebrates on land.
Tetrapods evolved from the lobe-finned fishes (lungfish and coelacanths, also called Sarcopterygians), which possess lungs as well as gills. One of the first evolutionary steps towards tetrapods and away from fish was Panderichthys (dated to 380 million years ago), a fish with a large, tetrapod-like head, large strong fins on its underside, which probably lived in muddy shallows. Next was Tiktaalik (dated to 375 million years ago), sometimes called a "fishapod" because of its mix of tetrapod and fish-like characteristics. Tiktaalik had limb-like fins that could have taken it onto land. Both Panderichthys and Tiktaalik were part of a lineage of Sarcopterygians evolving to cope with oxygen-poor water in shallow seas.
Some Sarcopterygians eventually evolved into intermediates between fish and tetrapods that display recognizable limbs, including Acanthostega and Ichthyostega. These animals are stem-group tetrapods, classified in their own groups within Tetrapoda, rather than being included in contemporary animal classes such as amphibians. These animals lived in shallow, weed-covered swamps, and would have initially used their legs to climb over weeds rather than walk on land. As evolution has no foresight, it could not plan in advance to create an animal capable of walking on land, and the animals' limbs would have needed to evolve to meet the adaptive challenge of its initially aquatic lifestyle.
Over the next 100 million years, tetrapods diversified into amphibians and reptiles, eventually becoming free from the need to lay their eggs in water. These tetrapods lived among the extremely thick forests of the Carboniferous era, about 360 to 300 million years ago. By the Permian period, 300 to 250 million years ago, some large and very impressive tetrapods had evolved, especially the therapsids, mammal-like reptiles now extinct. This was followed shortly thereafter by the Age of Dinosaurs.
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