Trigonia Clam

The trigonia clam (Pterotrigonia thoracica) is Tennessee's official state fossil, designated by the General Assembly in 1998. It is an extinct marine clam that lived in the shallow sea covering western Tennessee during the Late Cretaceous period, roughly 75 million years ago. The species belongs to the trigoniid family, a group known for heavily ornamented shells that were once common worldwide but are today represented by just a handful of living species in Australian waters.

Tennessee's best specimens come from the Coon Creek Formation in McNairy County, a site so productive and well-preserved that scientists have studied it since the 1890s. Pterotrigonia thoracica shells from Coon Creek often retain their three-dimensional shape and sometimes their original shell material — an exceptional level of preservation for a fossil nearly 75 million years old.

Pterotrigonia thoracica was a compact, heavily built clam up to 4 inches across. Its shell was covered in bold, raised ribs that radiated from the hinge, giving it a texture unlike most modern clams. The rear of the shell flared into a flattened, wing-like extension — the feature that gives the genus its name (ptero is Greek for wing). The interior hinge was made of interlocking teeth that locked the two shell halves together precisely.

The animal lived buried in shallow marine sediment, filtering plankton and organic particles from the water. The Cretaceous sea that once covered western Tennessee was warm and shallow — similar in depth to today's Gulf of Mexico — and supported dense communities of bivalves, snails, sharks, and marine reptiles. Pterotrigonia thoracica went extinct 66 million years ago at the end of the Cretaceous, wiped out in the same mass extinction that ended the dinosaurs.

Tennessee designated Pterotrigonia thoracica as its state fossil in 1998. The choice reflected the species' deep ties to Tennessee geology: the Coon Creek Formation in McNairy County, where the finest Tennessee specimens are found, has been a landmark fossil site since paleontologist Charles Schuchert studied it in the 1890s. No other Cretaceous marine formation in the state has matched Coon Creek's combination of abundance and preservation quality.

The designation gave official recognition to a fossil already well known to Tennessee collectors and museum curators. The Memphis Pink Palace Museum, which oversees the Coon Creek Science Center, had been collecting and displaying Pterotrigonia thoracica for decades before the legislature acted in 1998.

The Coon Creek Formation in McNairy County is the most important Pterotrigonia thoracica site in the state and one of the most significant Cretaceous fossil deposits in North America. The formation is a Late Cretaceous marine marl — a soft, clay-rich limestone — that was deposited when a shallow sea covered the area. Fossils are preserved in exceptional detail: shells keep their three-dimensional shape, and some specimens retain traces of original shell coloration and soft anatomy.

The Coon Creek Science Center, managed by the Memphis Pink Palace Museum, protects the site and allows supervised collecting. Cretaceous marine deposits also crop out across western Tennessee in Hardeman, Hardin, and Fayette counties, where road cuts and creek beds produce Pterotrigonia thoracica along with oysters, shark teeth, and mosasaur bones.