What characteristic do reptiles and mammals have in common?

Reptiles and mammals both have amniotic eggs. This means that their eggs have a membrane called the amnion that surrounds the embryo, providing a protective, fluid-filled environment.

Here's why this is significant:

* Evolutionary adaptation: The amniotic egg was a major evolutionary breakthrough, allowing vertebrates to reproduce on land.

* Protection and development: The amnion protects the developing embryo from drying out and provides a cushion against shocks.

While there are many differences between reptiles and mammals, the amniotic egg is a shared characteristic that reflects their common evolutionary ancestor.