Fossils of fish from the Ordovician period (500 million to 435 million years ago) can he found all over the world.
They were primarily small fish, no bigger than a minnow, with plates that protected their heads, thin scales, and no jaws or teeth. These primitive fish probably lived near the bottom of shallow sea waters, living off food particles found in the mud.
They had slits—not gills—on both sides of their throats, and their spines were more like flexible cartilage than bones. (Cartilage is rubbery bonelike material.)
The coelacanth is one type of fish that may be an ancestor of land animals. Scientists thought coelacanths were extinct until one was caught in 1938.