Question

1. which derived characters do mammals share with crocodiles?
2. which derived character makes mammals unique?
3. which derived character do crocodiles, birds, and snakes share? eggs with shells, fur, additional skull openings four limbs, amniotic eggs, additional skull openings four limbs, amniotic eggs, additional skull openings, fur

Explanation:

1. Mammals and crocodiles share amniotic eggs and additional skull openings. Amniotic eggs are an adaptation for life on land, and the additional skull - openings are related to muscle attachment and evolutionary development.
2. Fur is a unique derived character of mammals. It provides insulation and has other functions like camouflage and social signaling.
3. Crocodiles, birds, and snakes share amniotic eggs. Amniotic eggs have a protective shell and extra - embryonic membranes that allow for gas exchange and waste storage, enabling reproduction on land.

Answer:

1. Amniotic eggs, additional skull openings
2. Fur
3. Amniotic eggs