The first reptiles appeared in the Upper Carboniferous Period some 300 million years ago as an offshoot of the labyrinthodont amphibians, which they closely resembled. It is in fact difficult to distinguish reptile from amphibian at this stage in their evolution, so that there is no certainty as to the precise time when the transition from aquatic to land life was complete. In the rocks of the succeeding Permian Period, however, there is abundant fossil evidence of the existence of numerous reptiles. They follow at least four major lines of evolutionary divergence, distinguished among other things by the structure of the temporal region of the skull.
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