Big Ordovician brachiopods: Strophomena and friends

Starting out in the Ordovician rocks of Minnesota and surrounding states, you run into a few kinds of fossils right away. Depending on the rocks you're looking at, these might include nut-like small brachiopods, fragments of branching bryozoans, ring-like or gear-like crinoid columnals, snail shells, conical horn corals and so on. Among the most noticeable of the common fossils are larger D-shaped shells, up to a few cm across. We've encountered these large shells a few times before in the Platteville and Decorah; they are the shells of brachiopods in the Order Strophomenida, one of the most abundant and diverse groups of brachiopods. To avoid any confusion with Strophomenida, Strophomenoidea, Strophomenidae, etc., and because we're all friends here, we'll just call these large shells "strophs". There are also smaller members of Strophomenida, but we'll leave them be for the present.

A good-sized brachial valve of a stroph in exterior view, embedded in a slab. On the left (A) is the valve viewed from the top, while on the right (B) is the profile, showing how steeply convex it is. Because of mud, this was the only fossil I could see on the slab when I picked it up, but the high profile of the shell clued me in.

A bit of anatomy will help before we go much farther (check out this page for more details). As mentioned way back when, a brachiopod has two shells, or valves, making it a bivalve in the anatomical sense, if not in the phylogenetic sense. The two valves are dorsal and ventral (top and bottom). We could get into a dense discussion of the actual suitability of "dorsal and ventral" and which valves they should really be applied to, but for now it's enough to note that we could make things really complicated and confusing (especially for strophs, which are thought to have lived "upside-down" compared to the standard brachiopod). The technical name for the dorsal valve is the brachial valve; it's the valve that anchors the lophophores, which the brachiopod uses for feeding. The name for the ventral valve is the pedicle valve, because it supports a structure called the pedicle, a fleshy organ found in most brachiopods that attaches the brachiopod to a surface. "Pedicle" doesn't quite make sense in strophs, because adult strophs did not have pedicles, but them's the breaks.

Brachiopod valves articulate at a hinge. The hinge end of a brachiopod is frequently either pointed and narrower than the body, as in many of the small Decorah brachiopods, or it is expanded along a straight line, producing a brachiopod that has a D-shape. Strophs belong to this group.

So, a stroph had a brachial valve and a pedicle valve, and each valve had an exterior and an interior surface. That makes four possible views. The brachial valve is convex in exterior view and concave in interior view, and aside from fine ridges radiating from the center of the hinge line, lacks structural features. The pedicle valve is either flat, or concave in exterior view and convex in internal view. A complete stroph therefore would either be convex on one side and flat on the other, or convex on one side and concave on the other, with the pedicle valve nested in the brachial valve like bowls or spoons (with space for the living animal, of course). Even though you might be expecting these brachs to be convex on both sides, like many of their smaller relatives, they were not. Depending on the genus and species, there will be different patterns of ridges, processes, and depressions on the interiors of the brachial and pedicle valves. Strophomena, for example, has a well-developed set of ridges on the interior of the pedicle valve near the hinge that make a diamond shape.

A brachial valve of Strophomena in exterior (A) and interior (B) views. The curvature of the valve affects the depth of field at this range; you can get a sense of the curvature from the shadows in B.

Here are two good pedicle valves, both in interior view and both with well-developed ridges for muscle attachment. The shell on the left (A) is gently curved in profile, but the shell on the right (B) is strongly curved. Because they are in interior view, you are looking at the inside of the shell; don't let the convex surfaces fool you!

Unlike many other brachiopods, stroph valves usually disarticulated upon death, so we typically see single valves. Because they are thin broad objects, stroph valves often broke before fossilization, and frequently break today when not supported by matrix. Good valves are more easily found on slabs or plates than loose, although they are usually partially buried when embedded. Some horizons can be stroph hashes, with complete and partial valves jumbled together.

One of the first things the beginner runs into is learning when you have something really unusual, and when you have partially exposed or fragmentary common fossils. Pieces of broken stroph shells, particularly the area around the interior side of pedicle valves where the muscle attachment ridges are found, are something that can trick you.

Something unusual? No, just a chunk of a pedicle valve in interior view next to a large piece of bryozoan.

Ecologically, strophs must have been doing something different than most brachiopods, because they lacked pedicles and thus were not attached. They are usually thought of as being adapted to resting on soft substrates, convex-side down, which can cause a bit of a problem if a current overturns the brachiopod. Meyer and Davis (2009:112–113) present some discussion and speculation on the problem (note that they reverse "brachial" and "pedicle", taking into account that the "dorsal" valve was down and the "ventral" valve was up), noting the possibility of some mobility by snapping the valves like a scallop. They certainly could have caused a bad day for *other* brachiopods, as documented by Freeman et al. (2013): the authors described a Cincinnatian site where hundreds of concave-down disarticulated stroph shells, redeposited by a storm, trapped lingulid brachiopods that had been trying to burrow up out of their storm burial.

Stratigraphically speaking, strophs can be found through the Platteville and Decorah, and on up (Rice and Hedblom 1987; Rice 1987). The most thorough investigation of brachiopods in the Twin Cities is Rice (1987), in which 69 collections of brachiopods were made at different levels throughout the 27.24 m (89.37 ft) of Decorah Shale at the Brickyard (Carimona not included, because this was pre-2008). This study, which involved a great deal of bulk sampling and screening, resulted in 5,601 brachiopods representing at least 37 species. Bulk sampling is a good way to get small fossils that the average surface collector would overlook or disdain to collect, but large brachiopods were certainly represented as well.

Going by Rice (1987), there are four species of large strophs (> 20 mm [>0.78 in] along the hinge) in the Decorah of the Twin Cities area: Strophomena filitexta, Strophomena septata, Oepikina inquassa, and Rafinesquina trentonensis. This omits several smaller strophs that top out in the 10–20 mm (0.39–0.78 in) range. Strophomena, like all good 19th century fossil taxa, has a dark and troubled past. Traditionally, large Strophomena in the Decorah and other rocks of comparable age were placed in S. incurvata, but Rice (1987) found a number of problems with using that name, and instead placed the largest Decorah stroph into S. filitexta, partially out of convenience. Whatever name chosen, the big Strophomena can exceed 36 mm (1.4 in) along the hinge (the largest cited by Rice had a hinge width of 36.5 mm [1.44 in], but I have some in the 40–45 mm [1.57–1.77 in] range; exact figures can be hard to get if the corners are buried or broken), has well-developed ridges for muscles on the interior of the pedicle valve, and has no median ridge on the interior of the pedicle valve. S. septata is usually smaller and has a well-developed median ridge on the interior of the pedicle valve. Rafinesquina trentonensis can be almost as large as the big Strophomena, but is more delicate, with very thin shells (on the order of 0.25 mm [about 0.01 in] thick at the edges) and delicate structures at the beak of the hinge for muscle attachment, on the brachial valve (known as cardinal processes; the beak makes more intuitive sense with other brachiopods, which come to a point at the hinge instead of having a wide straight hinge line, but there you go). Oepikina inquassa, on the other hand, has robust cardinal processes (Rice 1987).

I don't usually put on my taxonomist's hat, because it doesn't fit very well, but why not give it a shot? A couple of pedicle valves: A is an interior surface, and based on the median ridge I interpret it as a small Strophomena septata. B is an exterior surface, but the profile of the ridges can be seen. The thinness suggests Rafinesquina trentonensis, but the diamond is more of a Strophomena thing. I'd want to see the other side to check how strong the ridges are, but I'd rather have the whole shell in place than inevitably break it up finding out.

A Rafinesquina? A shows the whole valve in a slab, and B shows the extreme thinness of a loose part of the shell (the upper right side in A; the part circumscribed by a crack).

In Rice's sample, S. filitexta was most abundant, with 104 specimens. S. septata and Rafinesquina trentonensis were almost tied, at 56 and 53, respectively. Oepikina inquassa brought up the rear, with only 13. Interestingly, S. filitexta was not found above approximately 17.5 m (57.4 ft) above the shale/Carimona contact, while the other three were all concentrated between approximately 12.3 m (40.4 ft) and 17.5 m (57.4 ft) up (Rice 1987).

References

Freeman, R. L., B. F. Dattilo, A. Morse, M. Blair, S. Felton, and J. Pojeta, Jr. 2013. The "Curse of Rafinesquina": negative taphonomic feedback exerted by strophomenid shells on storm-buried lingulids in the Cincinnatian Series (Katian, Ordovician) of Ohio. Palaios 28(6):359–372.

Meyer, D. L., and R. A. Davis. 2009. A sea without fish: life in the Ordovician sea of the Cincinnati region. Indiana University Press, Bloomington and Indianapolis, Indiana.

Rice, W. F. 1987. The systematics and biostratigraphy of the Brachiopoda of the Decorah Shale at St. Paul, Minnesota. Pages 136–166 in R. E. Sloan, editor. Middle and Late Ordovician lithostratigraphy and biostratigraphy of the Upper Mississippi Valley. Minnesota Geological Survey, St. Paul, Minnesota. Report of Investigations 35.

Rice, W. F., and E. P. Hedblom. 1987. Brachiopods and trilobites of the Sardeson Beds in the Twin Cities. Pages 131–135 in R. E. Sloan, editor. Middle and Late Ordovician lithostratigraphy and biostratigraphy of the Upper Mississippi Valley. Minnesota Geological Survey, St. Paul, Minnesota. Report of Investigations 35.