Midwest Ordovician craters: Decorah and Rock Elm

Life in the Ordovician of the Upper Midwest wasn't all warm tropical seas and all the organic particles you could filter. We've already run into giant volcanic eruptions spreading ash far and wide. There was also a significant glaciation at the end of the Ordovician; the ice itself didn't get to tropical North America, but it did lead into major extinctions. We don't have to go to the poles or volcanoes off the coast of North America for dramatic geologic events, though: we only have to go as far as west-central Wisconsin and northeastern Iowa.

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.