Which feature is characteristic of both reptiles and birds?

The characteristic feature that is shared by both reptiles and birds is the presence of an amniotic egg. Amniotic eggs are uniquely adapted to permit reproduction in terrestrial environments by providing a protective environment for the developing embryo. This adaptation allows for the retention of water and creates a self-contained ecosystem that includes fluid-filled membranes to protect the embryo from desiccation and physical shock.

Reptiles were among the first groups to evolve this type of egg, which allowed their ancestors to lay eggs far from water sources, leading to their successful colonization of dry land. Birds, being descended from theropod dinosaurs, inherited this trait, effectively using it throughout their evolutionary history.

In contrast, endothermic metabolism is a characteristic primarily associated with birds, allowing them to regulate their body temperature internally. Feathers, while a defining characteristic of birds, are not found in reptiles. Live birth is also not commonly associated with either group since both primarily reproduce through egg-laying, though some reptiles do exhibit live birth as an adaptation in specific environments.