This bivalve fossil were found in the Niobarra and Pierre formations of the Western Interior Seaway. The Ostrea glabra clam lived in the Cretaceous Period (Mesozoic Era). Some clams could grow over a meter in diameter.
|Cretaceous Period Map from Wikipedia.org|
This specimen (CM 53977) was on display in July 2011 at the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania. See my write up of that trip: CLICK HERE.