Early tetrapods have been found in swampy habitats like shallow wetlands , coastal lagoons , brackish river deltas , and even shallow marine sediments . There is much to suggest that these are the kind of environments in which the tetrapods evolved . Swampy habitats like shallow wetlands , coastal lagoons and large brackish river deltas also existed at this time , and there is much to suggest that this is the kind of environment in which the tetrapods evolved .