Several key innovations for conserving water contributed to the ability of the amniotes to exploit a wide range of terrestrial habitats. The amniote egg (which gives the group its name) is relatively impermeable to water and allows the embryo to develop in a contained aqueous environment (Focus: Key Figure 32.19A). Its leathery or brittle, calcium-impregnated shell retards evaporation of the fluids inside but permits passage of oxygen and carbon dioxide. The amniote egg also stores large quantities of food in the form of yolk, allowing the embryo to attain a relatively advanced state of development before it hatches. Within the shell are extraembryonic membranes that protect the embryo from desiccation and assist it with gas exchange and excretion of nitrogenous waste products of metabolism.

As we described in the opening of this chapter, modifications of the amniote egg in several different groups of amniotes allowed the embryo to develop inside (and exchange nutrients and wastes with) its mother’s body (Focus: Key Figure 32.19B). For example, in most mammals the egg lost its shell entirely while the functions of the extraembryonic membranes were retained and expanded, giving rise to viviparity. Viviparity appears to have evolved many times among lizards and snakes. In Investigating Life: Have Shelled Eggs Re-Evolved among Viviparous Reptile Lineages?, we explore the possibility that shelled eggs could have re-evolved in some of these lineages.

699

Other innovations evolved in the organs of terrestrial adults. A tough, impermeable skin, covered with scales or modifications of scales such as hair and feathers, greatly reduced water loss. Adaptations of the kidneys (the vertebrate excretory organs), allowed amniotes to excrete concentrated urine, ridding the body of nitrogenous wastes without losing a large amount of water in the process (see Chapter 51).

During the Carboniferous, the amniotes split into two major groups: the reptiles and the lineage that eventually led to the mammals (Figure 32.20).