The rise of the Tetrapod classes and the movement from water to land represents one of the major evolutionary events in the history of vertebrates. New structural designs were required to make the transition to land in order to cope with increased oxygen levels, decreased water supply, more fluctuating ambient temperature, and slight changes in the way sensory information is obtained. Group is ectothermic, and must regulate body temperature by moving to different microclimates within its environment. In some cases, animal may freeze and still survive, in part due to high concentrations of glycerol in cells that protect the cells from breaking apart during freezing. Therefore, reproduction is completely dependent on the presence of water or moist habitat.
Embryos lack an amnion, but eggs are laid in a jelly-like protective coating.
Young are hatched in water and develop as a larva before metamorphosing into an adult form. The class Reptilia began to appear approximately 320 mya during the Carboniferous period. Includes the turtles, lizards, and snakes. Ectothermic, sometimes called heliotherms because they can regulate body temperature by using solar radiation. With the Class Aves, we move to tetrapods that are endothermic rather than ectothermic.