Pterotrigonia fossil; photo by Internet Archive Book Images on Flickr (no known copyright restrictions).
Official State Fossil of Tennessee
Tennessee designated Pterotrigonia (Scabrotrigonia) thoracica as the official state fossil in 1998. "Ptero" (thankfully it has a nickname) was a bivalve of the Cretaceous Age (a modern-day bivalve creates the state gem - Tennessee river pearl). All State Fossils
Fossils of this extinct prehistoric creature are found in the Coon Creek Formation of West Tennessee. Pterotrigonia was a shallow-burrowing suspension feeder that inhabited the ocean floor of West Tennessee 70 million years ago. The fossil shells of Pterotrigonia are abundant and easily recognized by collectors.