The Pinnidae is a family of large, semi-infaunal marine bivalves known since the Devonian. Modern pen shells are mostly found in back-barrier seagrass beds, where the grasses baffle wave and current action. On barren sand substrates, pen shells occur in much smaller densities. The seagrass leads to not only higher densities of pen shells, but greater ecological diversity overall by creating a food source that can support both herbivores and detritivores, as well as their predators.

An assemblage of Late Cretaceous pen shells occurs within the Tombigbee Sand Member caprock of the Eutaw Formation in Montgomery County, Alabama. Here articulated individuals, primarily in life position, have been mapped within a 10 m² area using a 0.25 m² grid, and orientation of the hinge and plane of commissure were recorded where possible. The average density over the whole assemblage is 17.2/m², while the highest observed density per grid (24) corresponds to 92/m². In addition, the associated fauna was assessed from bulk samples as well as outcrop examination, yielding a very low-diversity assemblage primarily composed of other molluscs and one cassiduloid echinoid.

A modern assemblage of the pinnid Atrina rigida in a grassbed in St. Joseph Bay, Florida, was studied as an actualistic analog. As before, individuals were mapped, and associated organisms were observed and sampled. The Holocene pen shells showed a much lower average density of only 1.6/m² over a 16 m² area, with the highest density being 5/m². However, other workers have found densities in other portions of the bay as high as 11/m². The associated fossilizable fauna was much more diverse than the Cretaceous community.

In the absence of marine grasses in Mississippian near-shore environments, arborescent bryozoans found along with articulated pen shells may have played a similar role (Radenbaugh, T. A., and McKinney, F.K., 1998, Palaios, 13:52-69). No obvious proxy for marine grasses has been found in the Tombigbee Sand occurrence. Although it is possible that marine grasses were present and left no fossil evidence, the unusually high density of in situ pen shells suggests that baffling may be attributed to the pen shells themselves. Unlike seagrass, the molluscan framework would not have provided the large amount of organic matter necessary to support a diverse benthic community.