A few million years after tetrapods came ashore, approximately 375 million years ago, an odd creature lived among the giant amphibians and represented the next great evolutionary leap. It may not have appeared so at the time. This creature was the initial common amniote that started the clade of tetrapods which includes all reptiles, birds, and mammals. This small protoreptile boasts its evolutionary position because it had an egg that could survive out of water. This required an amnion, and an impermeable shell or, later, internal protection in the form of a uterus. These adaptations constituted major evolutionary advances in the move from an aquatic existence to at least a partial terrestrial existence.

As the tetrapods expanded to fill empty terrestrial niches, they radiated into birds and mammals. Despite the evolution and radiation into other classes, fragments of the reptilian antecedents remain in all descendant groups. Even among mammals, echoes of a reptilian beginning persist in extant creatures.

The echidnas, Tachyglossus aculcatus (cover, upper) and Zaglossus bruijni (cover, lower) are primitive mammals representing a group …