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