<h2>Tetrapoda</h2> 

<p>
The superclass Tetrapoda (Ancient Greek τετραπόδηs tetrapodēs, "four-footed"), or the tetrapods /ˈtɛtrəpɒd/, 
comprises the first four-limbed vertebrates and their descendants, including the living and extinct amphibians, 
reptiles, birds, and mammals. The tetrapods evolved from the lobe-finned fishes about 395 million years ago in the 
Devonian Period. The specific aquatic ancestors of the tetrapods, and the process by which land colonization 
occurred, remain unclear, and are areas of active research and debate among palaeontologists at present.
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<p>
While most species today are terrestrial, little evidence supports the idea that any of the earliest tetrapods could 
move about on land, as their limbs could not have held their midsections off the ground and the known trackways do not 
indicate that they dragged their bellies around. Presumably, the tracks were made by animals walking along the bottom of shallow bodies of water. 
Amphibians today generally remain semiaquatic, living the first stage of their lives as fish-like tadpoles. Amniotes evolved about 
300 million years ago, and their descendants drove most amphibians to extinction. One population of amniotes diverged into lizards, 
dinosaurs, birds and their relatives, while another diverged into mammals and their extinct relatives. Several groups of 
tetrapods, such as the snakes and cetaceans, have lost some or all of their limbs. In addition, many tetrapods have 
returned to partially aquatic or fully aquatic lives throughout the history of the group (modern examples of fully aquatic tetrapods 
include cetaceans and sirenians). The first returns to an aquatic lifestyle may have occurred as early as the Carboniferous Period, 
whereas other returns occurred as recently as the Cenozoic, as in cetaceans, pinnipeds, and several modern amphibians.
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<p>
The change from a body plan for breathing and navigating in water to a body plan enabling the animal to move on land is one of the most profound 
evolutionary changes known. It is also one of the best understood, largely thanks to a number of significant transitional fossil finds in the 
late 20th century combined with improved phylogenetic analysis.
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