Conodonts

A common yet easily overlooked type of fossil is the conodont (or "conodont element", should we choose to be picky and/or technical). They are easy to overlook because they are sub-millimeter-scale fossils, which makes them just barely visible to the naked eye if you look really hard. Most of the time, though, all you can do is to take bulk samples of rock and run them acids to liberate these tiny fossils; conodont fossils are made of calcium phosphate, so they will not be dissolved by some acids that do dissolve calcium carbonate, i.e. limestone. Having dissolved your limestone, you can then inspect the remainder under a microscope. Ideally, you will have some little pointy toothy bits, which are the conodont elements.

In life, conodonts were small eel-like animals that had large laterally-facing eyes, long low dorsal and ventral fins near the tail, and a mouth full of those pointy toothy bits. These did not form a simple pair of jaws, but a multi-element array of different pieces. At the same time, the rest of the body had no bones or armor or anything else favorable to fossilizing, so until a few decades ago all we had were these jaw elements. As you might suspect, the combination of differently structured jaw pieces and nothing to keep them associated after death encourages many names for things that were in the body of one animal. Imagine if we didn't have bony skulls but still had several different types of teeth; how easy would it be for paleontologists of the future to describe different genera and species for incisors, canines, and molars? The exact function of the business end of a conodont is not certain. Straining and seizing functions have been suggested, and it is certainly possible that over the hundreds of millions of years conodonts existed both functions and more were evolved. They were certainly free-swimming animals, at any rate.

From Stauffer (1935a): Glenwood conodont elements.

After having made a long run, beginning at least in the Cambrian, and weathering the great Permian–Triassic extinction, conodonts rather inconveniently passed out of existence at the end of the Triassic. The absence of a body or living relatives left conodonts as an enigma for decades. The lack of detailed anatomical knowledge did not hinder scientific appreciation for conodonts, though: like graptolites, they make useful index fossils for relative dating. At last, though, in the 1980s we got our first glimpse at the rest of a conodont, and it turned out to be the eel-like critter described above. They are now thought to be chordates, and perhaps even vertebrates.

Conodont elements are present in all of the local formations, from the Jordan Sandstone (Mossler 2008), to the Prairie du Chien Group (Smith and Clark 1996), to the St. Peter Sandstone (Witzke 1980), to the Glenwood Formation (Stauffer 1935a, Webers 1966), to the Platteville Formation (Stauffer and Thiel 1941; Webers 1966), to the Decorah Shale (Stauffer 1930, 1935b; Webers 1966). The Stauffer and Thiel (1941) lists include well over a hundred species for the Upper Ordovician rocks. Although they noted the presence of conodonts in the Platteville Formation at several localities, they did not include any conodonts in their Platteville taxonomic lists (and it's not simply a shuffling-the-Carimona-around thing); probably Stauffer hadn't been able to study the Platteville forms taxonomically. This leaves us with only the Glenwood, Carimona, and main Decorah. More recent considerations are in Webers (1966) and Sweet (1987).

Acontiodus (Ca)
Anchyrognathus? (Ca)
Barbarodina (Ca, De)
Belodus (Gl, Ca, De)
Bryantodina (Gl)
Chirognathus (Gl)
Cordylodus (Ca)
Cyrtoniodus (Gl, Ca, De)
Dichognathus (Gl, Ca, De)
Distacodus (Gl, Ca, De)
Erismodus (Gl)
Euprioniodina (Gl, Ca)
Gyrognathus (Gl, Ca, De)
Heterognathus (Ca, De)
Hibbardella? (Ca)
Lonchodus (Gl, Ca, De)
Microcoelodus (Gl, Ca, De)
Neocoleodus (Ca, De)
Oistodus (Gl, Ca, De)
Ozarkodina (Gl, Ca, De)
Paltodus (Gl, Ca, De)
Phragmodus (Gl, Ca, De)
Plectodina (Gl, Ca, De)
Polycaulodus (Ca, De)
Polygnathus? (Ca)
Polyplacognathus (Ca, De)
Prioniodus (Gl, Ca, De)
Prioniodus? (Ca)
Pteroconus (Ca, De)
Scyphiodus (Ca)
Stereocornus (Gl)
Subcordylodus (Gl, Ca, De)
Subcordylodus? (Ca, De)
Subprioniodus (Ca, De)
Tortoniodus (Gl)
Trichognathus (Gl, Ca, De)
Egg cases? (Ca) [I think this is a reference to Stauffer's chitinozoans, which have been mooted as eggs at various times]

As a bonus, the Prairie du Chien conodonts from Stauffer and Thiel (1941), Oneota Dolomite and Shakopee Formation:
Acanthodus (On)
Acodus (On)
Acontiodus (On)
Clavohamulus (On)
Cordylodus (On)
Distacodus? (On, Sh)
Drepanodus (On, Sh)
Loxodus (On)
Oistodus (On)
Oistodus? (On)
Paltodus (On)
Scolopodus (On, Sh)
Ulrichodina? (Sh)

References:

Mossler, J. H. 2008. Paleozoic stratigraphic nomenclature for Minnesota. Minnesota Geological Survey, St. Paul, Minnesota. Report of Investigations 65.

Smith, G. L., and D. L. Clark. 1996. Conodonts of the Lower Ordovician Prairie du Chien Group of Wisconsin and Minnesota. Micropaleontology 42(4):363–373.

Stauffer, C. R. 1930. Conodonts from the Decorah Shale. Journal of Paleontology 4(2):121–128.

Stauffer, C. R. 1935a. Conodonts of the Glenwood beds. Geological Society of America Bulletin 46(1):125–168.

Stauffer, C. R. 1935b. The conodont fauna of the Decorah Shale (Ordovician). Journal of Paleontology 9(7):596–620.

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

Sweet, W. C. 1987. Distribution and significance of conodonts in Middle and Upper Ordovician strata of the Upper Mississippi Valley region. Pages 167–172 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.

Webers, G. F. 1966. The Middle and Upper Ordovician conodont faunas of Minnesota. Minnesota Geological Survey, St. Paul, Minnesota. Special Publication 4.

Witzke, B. J. 1980. Middle and Upper Ordovician paleogeography of the region bordering the Transcontinental Arch. Pages 1–18 in T. D. Fouch and E. R. Magathan, editors. Paleozoic paleogeography of the west-central United States. West-central United States paleogeography symposium. Society of Economic Paleontologists and Mineralogists, Rocky Mountain Section, Denver, Colorado.