Between the western valley of the Tennessee River and the Mississippi River is an area of rolling hills and flat lowlands. This is the Gulf Coastal Plain. The land along the Mississippi River may be covered with water when the river is high. This narrow strip of swampy land is called the Mississippi Bottoms. Pterotrigonia is a bivalve that lived during the Cretaceous Period more than 70 million years ago, when dinosaurs still stalked the earth.
Bivalves are mollusks with two shells, including clams, oysters, scallops and brachiopods.
Tennessee's state fossil is found in the Coon Creek Formation of West Tennessee. The Appalachian Mountains towered above its home to the east. But shallow seas still covered much of the interior of North America. Pterotrigonia 's fossilized shells are abundant, easily recognized and often well preserved. Only one relative of Tennessee's state fossil still survives.