Encrustation! Species Interaction in the Fossil Record

By: Brenda Hunda, Curator of Invertebrate Paleontology

You may think of barnacles attached to the bottoms of boats when you think of encrusting animals. These “epibionts” also often attach themselves to other marine organisms, such as crustaceans, whales and turtles and then rely on their hosts to lift them above the muddy substrates of the ocean floor. A secondary benefit is the mobility provided. Barnacles, oysters and algae are typical of the kinds of organisms that occupy this ecological role in marine communities.

Encrusted fossil specimens are important because they provide direct evidence of species interaction at a single point in geological time. With this information, paleontologists can reconstruct community composition, the ecological roles of various fossil organisms and the biological implications of such interactions.

Such is the case with Flexicalymene, a trilobite from our local Cincinnatian Series rocks (450 million years old). Encrusting bryozoans (tiny colonial organisms known to encrust kelp today) found on these specimens impact how paleontologists view trilobite lifestyles. Their exceptional preservation indicates that these trilobites were alive while being used as hosts by the bryozoans. While encrusted brachiopods and clams are routinely found in our local rocks, heavily encrusted trilobites are rare.

Conventional hypotheses regarding encrustation suggest that a detrimental impact on the living host animal by the encruster is rare. It does not make sense to have the host die due to its inability to forage for food, escape predation and so forth. This would also negatively impact the encruster. However, our trilobite specimens, with bryozoans covering their heads, suggest that the obstruction of vision, and perhaps other functions, may not have been such a big deal for them.

Rare specimens like these provide paleoecologists with unique information on how these fossil animals lived and when these types of species interactions first appeared in the fossil record. These specimens, and several others, were donated to the Invertebrate Paleontology collection by Jerry Rush, a member of our local amateur paleontological society, the Dry Dredgers.

Lateral view of Flexicalymene specimen with a bryozoan colony covering the left eye, and partially occluding the right eye, as well as covering most of the central region of the head. A branch of the colony can be seen extending off the front of the head. This pattern of encrustation would most certainly have had a direct impact on the mobility and visual acuity of these trilobites.

Dorsal (top) view of same specimen above showing the amount of encrustation on the cephalon (head) of the animal.

This specimen of Flexicalymene has a small bryozoan colony growing on its right eye.

Dorsal (top) view of the same specimen above showing complete coverage of the right eye.

Posted in Invertebrate paleontology.