These 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.
This 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.
Now 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.
Here’s a closer view of the above basal features. I love the frilly edge of the bioimmured barnacle in the top left.
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.