While clams may not be the first animal people think of, their ancient lineage and abundant fossil record provide key insights into Earth’s evolutionary history.
In 1998, Tennessee’s legislature officially designated the Late Cretaceous bivalve Pterotrigonia thoracica—commonly known as Ptero—as the state fossil, recognizing its importance in illuminating the region’s marine past.
Living approximately 70 million years ago, Ptero thrived in a warm, shallow sea that covered what is now western Tennessee during the Late Cretaceous. Its distinctive ribbed shell made it a prolific fossil, offering scientists a window into the era’s marine ecosystems.
During the Cretaceous, Tennessee was submerged under a shallow sea that extended from the modern Gulf of Mexico. This environment supported a diverse marine community that included mollusks, ammonites, and even marine reptiles.
Pterotrigonia thoracica was a burrowing suspension feeder, relying on particles in the water column for nourishment. Its robust, ribbed shell has left a well‑preserved fossil record, unlike its only living relative, Neotrigonia, which is now found off the coast of Australia.
Extinctions that ended the dinosaurs likely contributed to Ptero’s demise. Today, the species is extinct, but its fossils remain abundant in the Coon Creek Formation—an 73‑million‑year‑old, 240‑acre seafloor deposit located roughly 90 miles east of modern Memphis and containing nearly 700 preserved species.
Because of its prevalence and significance, the Tennessee legislature highlighted Ptero as a representative of the many extinct organisms whose remains are extensively preserved in the state’s rocks.
For those fascinated by extinction events, recent scientific efforts aim to resurrect the extinct Tasmanian tiger.
