Earlier this month, Luke Kosowatz, Matt Shearer and I went on a field trip through the Cincinnati region collecting Upper Ordovician (Katian) bryozoans and examples of bioerosion for their Independent Study projects and other investigations. I picked up the above slab and put it in our vehicle for future study not because of its beauty, but the preservational modes it displays. The black, rounded objects are bivalves, probably of the Order Modiomorphida. They are miserable fossils to identify because they originally had shells made of the mineral aragonite, which dissolved quickly after the animals died. What is left are a few scrappy molds and that black film. This is a common preservation of bivalves in the Cincinnatian.
As you see, not much detail in the bivalves other than an outline matching somewhat the modiomorphids. Those of you with sharp paleontological eyes will note a round gray patch with radiating lines. This is a bryozoan that was attached to the bivalve shell. When the shell dissolved, the bryozoan attachment surface became visible. In other words, this is an upside-down encrusting bryozoan, a condition we’ve seen several times in this blog.
Here’s another bivalve with an upside-down encrusting bryozoan. This time you can see that the black film was underneath the bryozoan and on the outside of the bivalve shell. In a 2004 paper, Tim Palmer and I wrote: “We have also long been curious about why some of the epifaunal aragonitic Ordovician genera in the Cincinnatian such as Modiolopsis are preserved with a thick black outer shell covering (e.g. Pojeta 1971, pl. 15, fig. 6). It now seems likely that this was a hypertrophied periostracum that conferred some protection against dissolution during life, similar to the situation seen in Recent unionids that are susceptible to dissolution in their fresh-water habitats” (p. 425). Maybe it’s time we followed up on these speculations? I’m sure other paleontologists have had similar ideas.