Paleozoic Seas
Symmorium, an ancestor of sharks, lived in the Pennsylvanian Period, 311-290 million years ago, when warm, shallow seas covered much of present day Arizona.

Mammal-Like Reptiles
Inostrancevia belongs to the group of mammal-like reptiles known as therapsids. This individual lived during the Permian Period, about 245 million years ago, in what is now Russia. Note how the teeth are of different shapes and sizes—a trait he shares with mammals.

Estemmenosuchus was a mammal-like reptile with horns that remind one of a moose. He was about the size of a modern rhinoceros and lived in the Late Permian Period of Russia, 245 million years ago.