A few basic pieces of bivalve mollusk anatomy were already covered under the general rules of telling brach from mollusk back in the brachiopod post, and the UCMP and Wikipedia do a good job of covering the squishy edible bits. Your basic bivalve has, of course, the two half-shells, which represent left and right sides. The shells are composed of calcium carbonate minerals, in some groups just calcite, and in other groups calcite and aragonite. "Big deal," you say. Well, this is important because aragonite is not as stable over geologic time scales and thus readily alters to calcite. This is a problem because you lose the original shell structure, and also means that bivalves (and other mollusks, which also use aragonite) tend to be particularly susceptible to replacement, leaving behind natural molds and casts. Mollusks are prone to creating steinkerns, natural internal casts that result when a shell is filled with sediment, the sediment lithifies, and the shell disappears. The shell structure of mollusks also makes them susceptible to dolomitization, where calcite (calcium carbonate) is replaced by dolomite (calcium magnesium carbonate). Under dolomitization, fossils can be lost entirely, or replaced by poor casts and molds. This is a problem in the Ordovician of Minnesota, where limestones are generally altered to dolomite. Fossils of things primarily composed of calcium carbonate are often represented by natural molds and casts.
|A natural cast of the bivalve Vanuxemia from the Platteville Formation, University of Minnesota collections.|
Anyway, I digress, as so frequently happens. Aside from the shell, your standard-issue bivalve comes equipped with the molluscan mantle and foot, a set of muscles and ligaments to operate the two halves of the shell, a complete one-way digestive system, structures called ctenidia which are used both as gills and in filter-feeding, and siphons to draw a water current across the ctenidia. The habits of the Ordovician and the modern bivalve are probably quite similar in general. Bivalves typically bury themselves into substrate, or bore into or attach onto hard objects. From there, they filter-feed, respire, excrete, reproduce, and generally are happy as clams. In truth, they probably feel little happiness, getting by as they do without much of a nervous system, but on the other hand, their capacity for negative emotions is similarly minimal. As you might have suspected, bivalves and brachiopods do a lot of the same general things, and it's tempting to infer that bivalves are somehow better at it than brachiopods given their current dominance, but it instead appears that bivalves simply came out better-off from extinction events instead of out-competing brachiopods day-to-day through superior pluck, ambition, and moral fibre, or whatever.
Bivalves in the Ordovician rocks of Minnesota are a bit disappointing, primarily because of those preservational issues mentioned above. They don't look so hot and are not especially common to boot. If you've got something that is bivalved, it's almost certainly a brachiopod, or perhaps an ostracode if you are nearsighted. Stauffer and Thiel (1941) listed 124 bivalve species for the St. Peter (Sp), Glenwood (Gl), Platteville (Pl), Carimona (Ca), Decorah (De), and "Prosser" (GP), but I have chosen to omit the species, which brings us down to a more manageable list of 26 genera. (Also, I'm not entirely confident about the species, given the whole "natural molds and casts" thing, and the elapse of 70+ years.) I'd pull their "Prosser" on the grounds that the Cummingsville is only present in the metro in a negligible amount, but their definition probably includes some upper Decorah, so it'll stay for now. The genera are as follows:
Ambonychia (Pl, Ca, Gp)
Clionychia (Pl, Gp)
Ctenodonta (Sp, Pl, Ca, De, Gp)
Cyrtodonta (Sp, Pl, Ca, De, Gp)
Endodesma (Pl, Gp)
Lyrodesma (De, Gp)
Matheria (De, Gp)
Modiolodon (De, Gp)
Modiolopsis (Sp, Pl, Gl, Ca, De, Gp)
Orthodesma (Ca, De, Gp)
Plethocardia (De, Gp)
Psiloconcha (Sp, Gp)
Vanuxemia (Sp, Pl, Ca, De, Gp)
Whiteavesia (Pl, Gp)
Whitella (preoccupied by a previous usage and replaced with Cyrtodontula) (Pl, Ca, De, Gp)
Most of these genera are also represented in the Cincinnatian, and you can find photos of many here.
A couple of interesting points:
Showing only genera hides it somewhat, but this is definitely a Platteville and "Prosser" show, making bivalves one of the few Ordovician groups where the Decorah is not the diversity champ. The Decorah is only represented by 44 of the species. Of course, if you were going by what I've seen in the field, it might have one species; I have one lonely spot that looks vaguely like a shell impression. Presumably bivalves were more amenable to the carbonate seafloors of the Platteville, Carimona Limestone, and upper Galena Group, than they were to the muddier Decorah seafloor.
The diversity is dominated by species of five genera: Ctenodonta (a burrower), Cyrtodonta (at the surface or shallow burrower), Cyrtodontula, Modiolopsis (attached), and Vanuxemia (at the surface or shallow burrower) (habits after Carriker and Yochelson 1968). Furthermore, these genera show up in just about all of the stratigraphic groupings. Most of the rest of the species are taken up by Ambonychia, Clionychia, Endodesma, Lyrodesma, Orthodesma, and Whiteavesia, leaving behind a bunch of one- or two-hit wonders.
|A plate's worth of Minnesota Ordovician bivalves, from Ulrich (1897). Avoid in months that do not include the letter "R."|
Carriker, M. R., and E. L. Yochelson. 1968. Recent gastropod boreholes and Ordovician cylindrical borings. U.S. Geological Survey, Washington, D.C. Professional Paper 593-B.
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. 1897. The Lower Silurian Lamellibranchiata of Minnesota. Pages 475–628 in Ulrich, E., W. Scofield, J. Clarke, and N. H. Winchell. The Minnesota Geological and Natural History Survey, Final Report 3(2). Johnson, Smith & Harrison, state printers, Minneapolis, Minnesota.