These skull characteristics can be examined in the many extinct fossil groups. The Linnean classification of tetrapods places them into four classes. With a few exceptions, the amphibians lay their eggs in water, where their aquatic larval stages develop. The larva breathes with gills. The terrestrial adults either breathe with lungs or directly through the integument.
Amphibians have smooth, moist skin without scales.
They are usually therefore associated with water or they live in moist environments. Reptiles can reproduce on dry land, because they have amniotic eggs and internal fertillization. They also have external scales that inhibit dessication. They have colonized even the driest desert environments. Class Aves- Birds were derived from a clearly reptilian ancestor.