Officially the state fossil of Wisconsin, as of 1985, and decades ago was a common find for collectors - as It is found in Illinois, Indiana, and Wisconsin. In recent years, the quarries where these trilobites were found slowly closed down and access to these fossils has diminished. Found in Dolomite quarries (or Dolostone), these trilobites are an internal cast of the original shell, a slow process thought to have been caused by a transition from a rich lime mud and limestone by a magnesium rich water supply.
Living during the Silurian Period, roughly 425 million years ago in an ancient sea, at that time Illinois would have been about 30º south of the equator. These trilobites lived in a shallow, warm water coral reef environment alongside brachiopods, mollusks, crinoids, and other trilobites.