Cornulites

The Decorah Shale is the king of Twin Cities rock formations in terms of fossils. You don't have to take my word for it; try these reviews:

• "Nearly optimum conditions for marine life must have prevailed, inasmuch as a large number of marine macro- and microorganisms, as represented by nearly all phyla of marine invertebrates found in the Ordovician of Minnesota, attain maximum abundance and diversity in the Decorah Shale" (Webers 1972);
• "The Decorah Shale is the best formation in southeastern Minnesota in which to collect fossils" (Mossler and Benson 2006);
• "The Decorah Shale contains the most diverse and abundant fauna of the Ordovician System in Minnesota (Webers, 1972). Because of the abundance of fossils and the ease with which they can be extracted from the shale, numerous biostratigraphical and paleontological studies have been conducted on the Decorah Shale" (Mossler 2008).

Fossils are commonly found either loose, having eroded out of the shale, or concentrated in thin limestone beds. One of the fun things about the limestone fossil hashes is how you can keep finding new things in them; there's just so many fossils in some pieces that you can't notice everything at once. The average small sample of Decorah fossil hash, say a rock chunk about as thick as a finger and about palm-size across, will probably have specimens of brachiopods, bryozoans, and crinoids (the typical Decorah "BBC") and often also includes snails and fragments of trilobite exoskeletons. Breaking out a magnifying glass or hand lens sometimes reveals other things. A surprisingly common type of "accessory" fossil are millimeter-scale conical objects, which were once the home of the enigmatic extinct cornulitids.