Final answer:
Mammals and living birds share the characteristics of endothermy, descending from a common amniotic ancestor, having a dorsal, hollow nerve cord, and laying amniotic eggs. However, they do not share a hole behind the eye socket on each side of the skull.
Step-by-step explanation:
Mammals and living birds share the following characteristics:
- Endothermy: Both mammals and birds are endothermic, meaning they generate and maintain their internal body temperature.
- Descend from a common amniotic ancestor: Mammals and birds, along with reptiles, descended from a common amniotic ancestor, which is defined by the presence of an amniotic egg and its membranes.
- A dorsal, hollow nerve cord: Both mammals and birds have a dorsal, hollow nerve cord, which runs along their back and serves as the main communication pathway between the brain and the rest of the body.
- An amniotic egg: Both mammals and birds are amniotes, meaning they lay amniotic eggs that are adapted for survival on land.
However, mammals and living birds do not share a hole behind the eye socket on each side of the skull. This characteristic is specific to reptiles, which have a temporal fenestra, a bony opening behind the eye socket.