I am going to make the assumption that we are all familiar with snails. In technical terms, snails are members of the class Gastropoda, or Gasteropoda in older references or for when you want to make them sound particularly disgusting. Some older references also use "univalves", because there is only one shell. "Univalve" also makes them sound a bit zippier, so maybe you'd prefer that. "Snail" in general usage denotes gastropods with a prominent shell, which leaves out their nudist cousins the slugs. As you might guess, slugs are not noted for their fossil record. Slugs also do not form one group; instead, shell-less gastropods have evolved many times. Describing modern snail biology in a couple of sentences is not easy, considering they include forms with lungs and forms with gills, and include dietary specializations from grazing on land plant to drilling the shells of marine invertebrates. Suffice it to say that your basic snail comes equipped with the ability to move, its own protective bunker, and an adaptable feeding organ (the radula), which is a pretty good set of features. Snails have gone far with them, albeit slowly.
Unsurprisingly, snails have an excellent fossil record, although as discussed with bivalves their shells are susceptible to chemical replacement, often leaving us with molds and casts. Snails go back into the Cambrian, where (when?) things get tricky: the basic snail form is not exclusive. We've already seen the limpet-like monoplacophorans and the cap-like scenellids, and the hyoliths and conulatans have also sometimes been listed as gastropods (Stauffer and Thiel 1941, for example). Helcionelloids, of the Cambrian and Early Ordovician, can also look like snails. There are also other forms that are usually tucked in with the snails, but which may not be. For our purposes, the most important are the bellerophonts, a group of mollusks with planispiral shells (coiled in a flat plane, like a garden hose) that are bulbous and generally feature broadly flared apertures (the opening for the critter), sometimes like a horn (the musical kind). They can sometimes look like nautiloids or ammonites. You might also say that they look like they are trying to swallow themselves. I don't wish to burden you with terminology, but the bellerophonts are quite common in the Decorah Shale, so it's nice to have a name to go with the face.
|Loose snails of the Decorah. The four on the right are bellerophonts (there's a trilobite pygidium or tail segment stuck to the bellerophont near the center of the photo).|
Snails are found in all of the rock units of the Twin Cities, even those with few fossils like the St. Peter Sandstone and the Prairie du Chien Group.
|Snail molds in the Shakopee Formation, including one distinct example left of center.|
|A snail mold in the Platteville Formation at Minnehaha Park.|
The canonical work on Minnesota's Ordovician snails is Ulrich and Scofield (1897), which is also the canonical work on Minnesota's Ordovician monoplacophorans, scenellids, and other such snail imitators because they weren't distinguished at the time. This is still an important work today. Stauffer and Thiel (1941), our go-to for formation lists, also did not separate monoplacophorans and friends from snails. After separating snail from definite non-snails, I came up with 156 species in 37 genera in their St. Peter (Sp), Glenwood (Gl), Platteville (Pl), Carimona (Ca), Decorah (De), and "Prosser" (Gp), which could further be divided among bellerophonts (34 species in 10 genera), other questionable snails (14 species in 5 genera), and solid snails (108 species in 22 genera). The most species-rich were Fusispira (smooth and steeply coiled), Holopea (low coiled, mostly in the "Prosser"), Hormotoma (steeply coiled), Liospira (very low coiled), Lophospira (screw-like), Subulites (smooth and steeply coiled), and Trochonema (low coiled), and the bellerophonts Bucania, Phragmolites, and Tetranota. The genera lists are as follows:
Bucania (Pl, De, Gp)
Carinaropsis (Ca, De)
Oxydiscus (Pl, Gp)
Phragmolites (Pl, Ca, De, Gp)
Pterotheca (Pl, Ca)
Sinuites (Pl, Ca, De, Gp)
Salpingostoma (Pl, De, Gp)
Tetranota (Pl, Ca, De, Gp)
Other possible monoplacophorans
Eccyliopterus (Pl, Gp)
Maclurites (Pl, Gp)
Ophiletina (Pl, De, Gp)
Clathrospira (Pl, Ca, De, Gp)
Ecculiomphalus (Pl, Gp) (their Eccyliomphalus, which looks better but is incorrect)
Eotomaria (Pl, De, Gp)
Eunema (De, Gp) (their Trochonema (Eunema))
Fusispira (Pl, Ca, De, Gp)
Gyronema (Pl, De, Gp)
Holopea (Sp, Pl, Gp)
Hormotoma (Sp, Pl, Ca De, Gp)
Hormotoma? major (Gp)
Liospira (Pl, De, Gp)
Lophospira (Sp, Gl, Pl, Ca, De, Gp)
Raphistoma (Ca, De, Gp)
Strophostylus (De, Gp)
Subulites (Pl, De, Gp)
Trochonema (Pl, De, Gp)
|One of the many plates of snails from Ulrich and Scofield (1897), featuring examples from Minnesota, Wisconsin, Tennessee, and Kentucky.|
As a bonus, here are Stauffer and Thiel's genera for the Prairie du Chien and Jordan. For the Prairie du Chien, the Oneota Dolomite and Shakopee Formation are denoted, logically enough, as On and Sh. The Jordan is Jo.
Euomphalopsis (On, Sh)
Dirhachopea (Jo, On)
Hormotoma (On, Sh)
Ophileta (On, Sh)
Platyceras sp. (may include monoplacophorans that resemble Platyceras) (On)
Raphistoma (On, Sh)
Rhachopea (Jo, On)
Sinuopea (Jo, On)
|A Decorah hash slab featuring numerous snails, including a conical snail on the left, a more bulbous "snowman"-like snail on the right, and several bellerophonts.|
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.