Encrustation provides clues about ancient seas

David L. Rodland, a Ph.D. student in Virginia Tech’s Department of Geological Sciences, has been studying encrustation, or the colonization of seashells by other marine organisms that live permanently attached to hard surfaces.

Examples of encrusting organisms (or epibionts) include serpulid and spirorbid worms, bryozoans, barnacles, and algae. Many epibionts produce their own calcareous tubes, shells, or skeletons, which are attached to that surface and may become fossilized along with it. “The encrustation of seashells by epibionts provides a great deal of ecological data or, for fossils, paleoecological, data,” Rodland said. “You can count the number and diversity of epibionts on a shell, for example, and see how it changes as a function of shell size. Or you can examine how encrustation varies between different kinds of shells or between the shells collected at different places and under different environmental conditions. Some workers have even suggested that they could be used to estimate the amount of nutrients and plankton available in ancient seas.”

At the meeting of the Southeastern Sections of the Geological Society of America (GSA) in Memphis March 12-14, Rodland presented a comparison of the encrustation of a bivalve mollusk (Macoma) with the encrustation of an articulate brachiopod (Bouchardia) from the coast of Brazil. “This is the only tropical / subtropical site where both bivalves and brachiopods occur in abundance in the present day, or at least, the only one we know,” Rodland said. “Brachiopods were a common element in Paleozoic fossil beds (>250 million years ago) and so this is the first opportunity we really have to compare brachiopods and bivalves in the modern world.”

“As it turns out,” he said, “epibionts appear to preferentially colonize the brachiopod Bouchardia, and occur less frequently on the bivalve Macoma. There are a large number of different measures one can use to evaluate the degree of encrustation on a shell, but Bouchardia is always preferred. This may be in part because Macoma lives in the sand, while Bouchardia sits on the surface; but because storms periodically rework everything, some shells get brought back to the surface while others get buried, so they both get encrusted eventually. The composition of the shells may also make a difference to the organisms colonizing them; Macoma is aragonitic while Bouchardia is calcitic.”

What does this mean? Bivalves are very common today, while brachiopods were much more common hundreds of millions of years ago, Rodland said. Therefore, differences in the encrustation of each may have implications for the evolution of the organisms that encrust them. “But no one really knows,” he said, “because there has been next to no study of brachiopod encrustation, and no one has really compared brachiopods and bivalves in this way before. If you’re trying to measure changes in ocean nutrients through the fossil record of epibionts, this means you have to account for differences between the shells that are getting encrusted in the first place.”