Amphibians

Frogs, salamanders, newts, toads and caecilians, all modern amphibians: They continue to evolve and diversify.

Reptiles

Turtles: Archelon, a large sea turtle, grows up to 12 feet (4 meters) in length. Snakes: Earliest known snakes appear.

Crocodiles: Many crocodiles become massive, including the Deinosuchus, a large terrestrial crocodile that reached 50 feet (15 meters) in length.

Lizards: True lizards continue to evolve and diversify.

Mammals

Triconodonts: Late Triassic to Late Cretaceous mammals; one of the oldest fossil mammals; three cusps of teeth in a straight row give them their name.

Symmetrodonts: Late Jurassic to Early Cretaceous mammals; they had upper and lower cheek teeth with many cusps in a triangular pattern.

Multituberculates: Late Jurassic to Late Eocene mammals; they had cheek teeth with many cusps in more than one row; they probably filled a “rodent” niche that had once been filled by cynodont therasids, and was later filled by true rodents.

Monotremes: First appearance, Early Cretaceous to the present; these animals eventually led to the true mammals (especially with hair and mammary glands). Today, there are three species of monotremes, which are mammals that lay eggs: the Australian duck-billed platypus and two kinds of echidnas (spiny anteaters), which live in New Zealand and Australia.

Early marsupials: First appear during the Middle Cretaceous and survive to the present; these pouched animals had distinct lower and upper cheek teeth.

Early placentals: Middle Cretaceous to the present; the mother nourished her developing fetus through a placenta; the cheek teeth were even more elaborate; the Late Cretaceous placental mammals include insectivores, or mammals that ate insects.