Tetrapods are vertebrate animals with four legs or leg-like appendages, including amphibians, reptiles, dinosaurs, birds, and mammals. They evolved from lobe-finned fish and some evolved into legless forms like snakes. Over time, tetrapods diversified into amphibians and reptiles and eventually freed themselves from the need to lay eggs in water. Some impressive tetrapods, like therapsids, evolved and were followed by the age of dinosaurs.
Tetrapods are vertebrate animals with four feet, legs, or leg-like appendages. In Greek, “tetrapod” means “four legs”. Tetrapods include amphibians, reptiles, dinosaurs, birds, mammals, and some ancient forms intermediate between fish and amphibians. Some tetrapods evolved into legless forms, like snakes. In scientific classification, Tetrapoda is a superclass within the subphylum Vertebrata. While sharks, rays, rays and fish are representatives of vertebrates in the seas, tetrapods are the representatives of vertebrates on land.
Tetrapods evolved from lobe-finned fish (lungfish and coelacanths, also called sarcopterygians), which possessed lungs and gills. One of the earliest 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” due to its mix of tetrapods and fish-like characteristics. Tiktaalik had limb-like fins that could have carried him to the ground. Both Panderichthys and Tiktaalik were part of a lineage of Sarcopterygians that evolved to cope with oxygen-poor waters in shallow seas.
Some Sarcopterygians eventually evolved into intermediates between fish and tetrapods displaying recognizable limbs, including Acanthostega and Ichthyostega. These animals are stem-group tetrapods, classified in their own groups within the Tetrapoda, rather than being included in contemporary animal classes such as amphibians. These animals lived in shallow, weedy marshes and would initially have used their legs to climb over weeds rather than walk on dry land. Since evolution lacks foresight, it could not plan ahead to create an animal capable of walking on land, and the animals’ limbs would have had 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 freeing themselves from the need to lay their eggs in water. These tetrapods lived in the extremely dense forests of the Carboniferous era, 360 to 300 million years ago. By the Permian period, 300 to 250 million years ago, some large and impressive tetrapods had evolved, most notably the therapsids, mammal-like reptiles now extinct. This was followed shortly after by the age of the dinosaurs.
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