Wooster’s Fossils of the Week: Large Miocene barnacles with bioimmurations from Maryland

Barnacle side viewThese two beautiful barnacles are from the Calvert Formation (Middle Miocene) exposed near Parker Creek in Maryland. They are likely of the genus Chesaconcavus. Barnacles are most unlikely crustacean arthropods, cousins of shrimp, crabs and lobsters. Most, like these above, cement themselves head-downwards on a hard substrate like a rock or shell (or boat hull), build a carapace around themselves of calcitic plates, and then filter-feed by kicking their filamentous legs in the water above to catch suspended food. They are entirely marine and usually live in shallow water.
Chesaconcavus top view 585This is a top view of the barnacle pair. We can look straight into the carapace because the opercular plates, which form a kind of door system, have been removed. For barnacles, these are a healthy large size.
Barnacle baseNow we’ve turned the barnacles upside-down to see their attachment surface. The substrate to which they were glued is gone, so we can see the details of the basal plates. The barnacles may have just sloughed off a shell or rock, or maybe they were attached to an aragonitic shell that dissolved away. What is cool here is that we can see other organisms that were on the substrate the barnacles encrusted, including two smaller barnacles completely absorbed within the larger skeletons. This is again an example of bioimmuration. The smaller barnacles look like upside-down cones in this perspective. Note that in the apex of each you can see preserved opercular plates — the insides of the “doors” that are opened for feeding. In the fine-grained skeleton of the larger attachment surface you can see growth lines made by the large barnacles as they occupied the substrate. There are even some small serpentine impressions that may represent soft-bodied organisms that were bioimmured.
Chesaconcavus base detail 585Here’s a closer view of the above basal features. I love the frilly edge of the bioimmured barnacle in the top left.

References:

Kidwell, S.M. 1989. Stratigraphic condensation of marine transgressive records: Origin of major shell deposits in the Miocene of Maryland. Journal of Geology 97: 1-24.

Zullo, V.A. 1992. Revision of the balanid barnacle genus Concavus Newman, 1982, with the description of a new subfamily, two new genera, and eight new species. Paleontological Society Memoir 27: 1-46.

About Mark Wilson

Mark Wilson is a Professor of Geology at The College of Wooster. He specializes in invertebrate paleontology, carbonate sedimentology, and stratigraphy. He also is an expert on pseudoscience, especially creationism.
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5 Responses to Wooster’s Fossils of the Week: Large Miocene barnacles with bioimmurations from Maryland

  1. Paul Taylor says:

    Bioimmurations of epibionts (as opposed to the encrusted substrate) are none too common in acorn barnacles, so it is nice to see this example. The story goes that such barnacles are able to prise fellow epibionts off the substrate by wedging beneath them during growth.

  2. Bob Saunders says:

    I have a piece that a marine biologist told me has barnacles. May I email you images of this interesting piece? I am an armature collector.
    Bob Saunders

  3. Mark Wilson says:

    Hi Bob. Sure, you can email me photos. Mark Wilson

  4. Bob Saunders says:

    Thank you however I do not see a email address or link to. I am not a student. Can I register as a non student?

  5. Mark Wilson says:

    Hi Bob: mwilson at wooster dot edu

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