Wooster’s Fossil of the Week: Encrusted strophomenid brachiopods from the Upper Ordovician of northern Kentucky (and the old concave-up or concave-down controversy)

After the delightful Joint North-Central and Southeastern Section Meeting of the Geological Society of America in Cincinnati this month, some of the Wooster Geologists visited a fossiliferous exposure of the Bellevue Formation (Upper Ordovician, Katian) along the Bullitsville Road in northern Kentucky (N 39.08121°, W 84.79230°; C/W-152). In between rainstorms we filled bags with lots of gorgeous fossils. (“Bellevue”, after all, means “beautiful view” in French, and we’re applying that to the view straight down to the ground.) The most common fossil we collected was the strophomenid brachiopod Rafinesquina alternata, an example of which is shown above. (Wooster senior Kate Runciman collected this specimen.)

We are looking above at the dorsal valve exterior of an articulated Rafinesquina alternata, meaning the valves are still joined as they were in life. The dorsal valve is concave, like the shape of your palm with your fingers slightly curled inwards. The hinge (H) of this brachiopod is the straight line at the top of the image. The curved line defining the shape of the brachiopod is the commissure (C), where the valves opened for filter-feeding. Most interesting for us is the encrusting trepostome bryozoan (B) at the bottom of the image. Most of it is on the other side of the brachiopod (see below), but note the clean boundary between its calcitic colony and the commissure of the brachiopod. Clearly the bryozoan grew on the living brachiopod, which continued to feed by opening and closing its valves. The bryozoan did not grow across this commissure, showing it and the brachiopod had a living relationship.

This is the other side of Kate’s brachiopod. We are looking at the ventral valve exterior which is convex (like looking at the back of your hand with fingers curved). The hinge (H) is again at the top. Note that the bryozoan (B) occupies about half of this ventral valve exterior. This shows us that the bryozoan was mostly living on the convex ventral valve, which must have been oriented upwards on the seafloor. This is a nice little fossil vignette supporting the hypothesis that these concave-convex brachiopods lived with the concave side down and the convex side up.

This concave up-or-down question was briefly a fun little controversy, with many debates in the early part of this century. I even covered it in this blog eight years ago. Certainly we can now say that the concave-down argument was won by the Dattilo et al. (2009) and Plotnick et al. (2013) teams.

There was another interesting encrusted Rafinesquina alternata specimen collected that day (above), this one found by Wenshuo Zhao (Fred, ’23). Here we are seeing the convex ventral valve with the hinge (H) at the top. Clustered along the anterior margin is a beautiful set of the encrusting inarticulate brachiopod Petrocrania scabiosa (P). They are irregular patches that have grown against each other like barnacles or oysters. These little brachiopods, which had no hinges between their valves, apparently attached along the margin of this larger brachiopod to take advantage of its feeding currents passing through its commissure. This is another indication of the host brachiopod being encrusted while it was alive, and the arrangement also supports the convex-up living orientation of Rafinesquina alternata.

Here is a closer view of the little Petrocrania scabiosa family.

There are always interesting specimens in bags of Upper Ordovician fossils from the Cincinnati region!

References:

Dattilo, B.F., Meyer, D.L., Dewing, K. and Gaynor, M.R. 2009. Escape traces associated with Rafinesquina alternata, an Upper Ordovician strophomenid brachiopod from the Cincinnati Arch Region. Palaios 24: 578-590.

Plotnick, R.E., Dattilo, B.F., Piquard, D., Bauer, J. and Corrie, J. 2013. The orientation of strophomenid brachiopods on soft substrates. Journal of Paleontology 87: 818-825.