Brachiopods are still around today. They occasionally go by the informal name "lamp shells", because some of them resemble old oil lamps. You may not see it, but there you go. It's a bit like how stegosaurids take their name from Stegosaurus, although most of them are rather different-looking spiky things. Brachs make their living filtering food from the water. Like their relatives the bryozoans, brachiopods have lophophores, tentacle-bearing structures for feeding and respiration. Brachiopod lophophores are set up in two "arms", which is where the name comes from ("brachion" meaning "arm", which you may know better from Brachiosaurus, the "arm lizard"), and the animal has to open its shells for them to work. Interestingly, not all brachs have complete digestive systems (i.e., some of them use the same opening for taking stuff in and sending stuff out).
The art of distinguishing mollusk from brach is often summarized as a matter of symmetry. A single brachiopod half shell usually has bilateral symmetry (which you can see in the following photo). A single bivalve valve is usually skewed in one direction. However, if you take both halves of a brachiopod shell, they will not be symmetric with respect to each other, while a bivalve's valves will be mirror images of each other. In fact, the valves of a brachiopod represent dorsal and ventral (top and bottom) structures, while those of a bivalve represent left and right structures. Another trick is taphonomic (related to the processes of death and decay): bivalve valves are held together by ligaments, which open the shell when they relax upon death, so the valves often separate, while brachiopod valves are held together by muscles that close the shell upon death, so brachiopod valves tend to stay together. Depending on the specific rocks you are looking at, you may have other pieces of circumstantial evidence. For example, in the Platteville and Decorah, brachiopods are generally smaller than a few cm across, are often strongly ribbed, and are usually better preserved than bivalves, which are often just molds or casts (brach shells being made of sterner material when it comes to dissolution and dolomitization), usually have smooth surfaces, and can be much larger. Bivalves are also much rarer in these rocks. The rule of thumb is as follows: unless you are a lucky or saintly person, if you're looking at a shell in the Platteville or Decorah and it's not obviously a snail or nautiloid, the default is brachiopod until proven otherwise.
|We're all brachiopods! Even that guy one down in the second column that you talked yourself into thinking was a monoplacophoran for a while—he's one of those blasted inarticulates!
Even if you're not into the minutiae of brachiopod taxonomy, you can probably pick out several types of clearly different brachiopods after a brief survey of a given collection. The two most obvious are the small, nut-like ribbed shells and the larger, broad, thin, unribbed shells. You may also notice thin smooth roundish shells, often attached to larger shells. Traditionally, brachiopods have been divided into articulate and inarticulate forms. Most of your Platteville and Decorah brachs would be articulates, but those thin roundish attached shells and a few others would be inarticulates. The terms refer to how the valves go together: the articulates use a tooth and socket hinge, while the inarticulates use a purely muscular system. Further research has shown that brachiopod classification is more complex, but the inarticulate/articulate division is a handy shorthand, especially given that the two types of hinge systems have led to differing lifestyles. The majority of brachiopods use a stalk (called a pedicle) to anchor themselves to their substrate, but some don't, including the broad smooth forms noted above (the extinct strophomenids) and certain inarticulates, which cement themselves directly to their substrate (often another brachiopod for our Ordovician friends). It may take a bit of practice to recognize inarticulates attached to other brachiopods; one thing to look for are little "pepperoni"-like forms on larger brachs. Other inarticulates, not attached to larger brachs, look something like circular caps, and can be mistaken for monoplacophorans (particularly if you are wanting to see monoplacophorans).
Something like 80 to 90 percent of the fossils you casually run across in the Platteville are brachiopod molds and casts. (I may be exaggerating.) It's not uncommon to see thin shell beds of brachs in this formation, particularly in the upper Platteville (the Magnolia Member). For the Decorah, it is common for small brachiopods to erode out of the less-resistant shale, producing vaguely nut-like loose fossils (see the first photo). Larger shells can erode out as well, but they don't tend to survive as well, being thin and broad. Alternately, the thin limestone beds of the Decorah often weather out in chunks which frequently host both large and small brachiopods.
|Platteville shell pavement
|Decorah Shale limestone block, featuring a large brachiopod, bryozoan chunks, snails, trilobite exoskeleton pieces, crinoid columnals, your usual Decorah et cetera.
Brachiopod nomenclature can make you want to tear your hair out. People have loved naming new brachiopods, shuffling old brachiopods between genera, and sinking old species. Most of the local forms were named by about 1900, leaving plenty of time for taxonomic second thoughts. In light of the fact that the brachiopod lists in Stauffer and Thiel (1941) are looking a bit creaky, my suggestion for the discerning fan of Twin Cities brachiopods is to look into Rice (1987), an illustrated taxonomic revision for the Decorah based primarily on collections from the Brickyard. You don't get the Platteville brachs, but on the other hand they're just the next formation down so there's a lot of overlap, and the Decorah has more diverse brachs anyway. The other document to look into is Winchell and Schuchert (1895), the grand monographic treatment in "The Geology of Minnesota" with all of its plates (if you can handle some archaic stratigraphy).
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.
Stauffer, C. R., and G. A. Thiel. 1941. The Paleozoic and related rocks of southeastern Minnesota. Minnesota Geological Survey, St. Paul, Minnesota. Bulletin 29.
Winchell, N. H., and C. Schuchert. 1895. The Lower Silurian Brachiopoda of Minnesota. Pages 333–474 in Lesquereux, L., C. Schuchert, A. Woodward, E. Ulrich, B. Thomas, and N. H. Winchell. The geology of Minnesota. Minnesota Geological and Natural History Survey, Final Report 3(1). Johnson, Smith & Harrison, state printers, Minneapolis, Minnesota.