Elvinia
Our modern concept of geologic time is inextricably linked to paleontology. Fossils are commonly used to correlate layers of rock from one region to the next, a practice known as biostratigraphy. If the same species is found in two separate rock formations, and that species only existed for a limited period of time, one can argue that the age of the rocks is similar within some margin of error. So-called index fossils can be used for relative dating (clarifying whether one layer is older or younger than another) as well as absolute dating, where numeric dates can be assigned based on co-occurring data. For example, radiometric dating of volcanic ash beds that appear alongside index fossils can be used to pin down their age to within a few million years or perhaps even a few hundred thousand years.
In Ordovician rocks, such as those around Cincinnati, small planktonic organisms such as graptolites, conodonts, and chitinozoans are the index fossils preferred by stratigraphers. However, these fossils are absent or less abundant in the older strata of the Cambrian. Fortunately, Cambrian trilobites were abundant, widespread, diverse, and rapidly evolving, and thus useful index fossils for the period. As a bonus, trilobite fragments can often be identified in hand samples without the need for onerous preparation techniques such as acid digestion. Stratigraphers have identified a series of assemblages of co-occurring species to delimit specific biostratigraphic zones, or intervals of time, within the Cambrian.
One good example is the Elvinia Zone of the Upper Cambrian, part of the Franconian Stage of North America (Paibian Stage of international usage) that was deposited roughly 495 million years ago. Its nominal species is Elvinia, a small, average looking trilobite described by Charles Doolittle Walcott (of Burgess Shale fame) a little over a hundred years ago. Elvinia is a ptychopariid, a distant relative of the Elrathia kingii that are found in abundance in Utah and various museum gift shops. As it turns out, Walcott first described both Elrathia and Elvinia on the very same page (Walcott, 1924, p. 56).
The genus Elvinia was likely named after the Elvins Shale, itself named for the town of Elvins, Missouri. Ironically, Elvins no longer exists, having been since subsumed into a neighboring municipality. However, its legacy lives on in its namesake formation, trilobite, and biozone, which is still used to correlate a slice of Cambrian time across North America. This includes our region, where Elvinia has been found in a drill core cut through the Upper Cambrian Davis Formation, 4,500 feet beneath the surface of western Indiana.