Zoology 10th Edition by Stephen Miller,John Harley

Exercise 3

The amniotic egg is the major synapomorphy that distinguishes reptiles, birds, and mammals from other vertebrates. This egg's extraembryonic membranes protect the embryo during development in terrestrial environments. Previous taxonomies have treated reptiles, birds, and mammals as separate classes. Modern cladistic analysis has revealed that Amniota is a monophyletic lineage. Cladistic methods have revealed that birds, turtles, alligators, lizards, and tuataras comprise the reptilian lineage. Reptiles have, or have evolved from ancestors with, diapsid skulls with upper and lower openings in the temporal region of the skull.
In addition to the amniotic egg, what characteristics unite the birds and other reptiles?