Pterotrigonia (Scabrotrigonia) thoracica is the official state fossil, as designated by House Joint Resolution 552 of the 100th General Assembly in 1998. Tennessee was the thirty-eighth state to designate a state fossil.
Pterotrigonia (Scabrotrigonia) thoracica (nicknamed Ptero) was a Cretaceous bivalve found in the Coon Creek Formation of West Tennessee. It was a wedge-shaped, shallow-burrowing suspension feeder that inhabited the marine clayey-sand ocean floor that was West Tennessee seventy million years ago. Shells of Ptero are preserved unaltered in great abundance and are easily recognized by collectors. The associated ocean floor inhabitants were diverse and included other bivalves, snails, squid-like animals, worms, sponges, corals, crustaceans, sharks, fish, turtles, and marine reptiles. Ptero is now extinct. In fact, the extinction event that was responsible for the demise of the dinosaurs sixty-five million years ago may have contributed to the demise of Ptero. Only the genus Neotrigonia, with five species, has survived to the present and is found only in the Pacific Ocean, most commonly near New Zealand.