Pseudoperna congesta (Conrad in Nicollet, 1843) - fossil oysters encrusting a Platyceramus platinus inoceramid bivalve shell in the Cretaceous of Kansas, USA.

The light-colored, irregularly-shaped structures shown above are Pseudoperna fossil oysters that are encrusting a portion of a large inoceramid bivalve shell that is eroding from Cretaceous-aged chalk in western Kansas. Inoceramids included the largest clams in Earth history - they are known to have reached about 10 feet in size (for a complete specimen, see: en.wikipedia.org/wiki/Platyceramus#/media/File:Platyceram...). Inoceramids are often described as having a turkey platter morphology - they had large, relatively thin shells that spread out their weight on soft, fine-grained, nonlithified chalk seafloors. Hard-substrate encrusting organisms such as oysters could not occupy such soft seafloors so they preferentially attached to the mineralized biologic surfaces of inoceramids.

Classification of encrusting Pseudoperna oysters: Animalia, Mollusca, Bivalvia, Ostreoida, Ostreidae

Classification of host inoceramid bivalve: Animalia, Mollusca, Bivalvia, Pteriomorphia, Praecardioida, Inoceramidae

Stratigraphy: Smoky Hill Chalk Member (a.k.a. Smoky Hills Member), Niobrara Formation, Upper Cretaceous