Monoplacophorans

There are some things in life that just don't have a handy name. Take for example the subject of this entry, an obscure group of mollusks known from a few modern deep-water species and a more robust fossil record. Various members of this group have labored under the terms "Monoplacophora", "Tergomya", and "Tryblidiida", all of which could be mistaken for a disease, a planet in a science fiction series, or a piece of anatomy you'd rather not know about. (You can combine all three: "I got monoplacophora in my tryblidiida after I visited Tergomya. It's as bad as it sounds.")  At one point someone suggested that their vernacular name should be "gastroverm", which you'll probably agree was no help at all. The reason for this multitude of names has to do with the rules of classification; visit here or here if you want details (and don't say I didn't warn you), but the basic bit is that Tryblidiida includes all the modern forms, and is classified in Tergomya, which is in turn part of Monoplacophora, and "Monoplacophora" is something of a problem because its traditional definition includes some things that weren't all that closely related. However, everyone was kind of used to Monoplacophora, so they kept it around, except now it can have a precise definition, or be a "state of mind" including various odd extinct things from the Cambrian, which I shall omit. If you want to be picky, this is about tergomyans.

So, what is (or was) a monoplacophoran? First of all, both fossil and modern examples tend to get mistaken for snails. Modern and many fossil monoplacophoran shells look a lot like limpet shells, being cap-like circular to ovoid structures with a peak at one end. The peaked end is the front end of the shell. Other fossil forms were more structurally ambitious, with coiled shells (coiling in a flat plane, which is known as planispiral; more free vocabulary words!), and a variety of similarly-coiled shells usually thought to be from snails may actually represent monoplacophorans. As you might imagine, they are not swift creatures. Modern examples are detritivores, although note that they are limited to deep water. Paleozoic forms lived in shallowe water and may have grazed on algae and microbes on the sea floor. Monoplacophorans are also noted for an extravagance of organs, such as three to six pairs of gills, two to three pairs of kidney-like organs, and two pairs of reproductive organs.

Described as snails, but not snails: a plate of (mostly) Minnesotan monoplacophorans and scenellids (31–41) (Ulrich and Scofield 1897, plate LXI).

The Middle–Late Ordovician sea of the Twin Cities metro was populated by a variety of limpet-like and coiled monoplacophorans, as well as "snails" and other things that may be monoplacophorans. Their fossils have been known since the 19th century, but are often described as snails, at least as recently as Stauffer and Thiel (1941), so they're somewhat "hidden" in the literature if you don't know what to look for (if it looks like a little cap with a low point, or the names Archinacella, Cyrtolites, Helcionopsis, Palaeacmaea, Stenotheca, Tryblidium, or [sometimes] Platyceras are used, think monoplacophoran). They aren't particularly common fossils, but they are found in the St. Peter Sandstone, Platteville Formation, and Decorah Shale in the Twin Cities area. Their fossils tend to be small (cm-scale). Identifying them is complicated not only by their resemblance to snails, but to their resemblance to certain brachiopods known broadly as "inarticulate brachiopods". In particular, members of a brachiopod group called Craniata or Craniaforma (see examples here and here) can drive you nuts if you go looking for monoplacophorans on a slab.

Side view of a monoplacophoran (~7 mm long) next to a conical snail; Decorah Shale.

References cited:

Stauffer, C. R., and G. A. Thiel. 1941. The Paleozoic and related rocks of southeastern Minnesota. Minnesota Geological Survey, St. Paul, Minnesota. Bulletin 29.

Ulrich, E. O., and W. H. Scofield. 1897. The Lower Silurian Gastropoda of Minnesota. Pages 813–1081 in Ulrich, E., W. Scofield, J. Clarke, and N. H. Winchell. The geology of Minnesota. Minnesota Geological and Natural History Survey, Final Report 3(2). Johnson, Smith & Harrison, state printers, Minneapolis, Minnesota.