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