An internal fossil cast of the trilobite, Anebolithus simplicior (Whittard), from the Ordovician period (500 to 435 million years ago), found in the Lower Llanvirn sediments, Builth Wells inlier, Powys, Wales. The trilobites are an extinct group of marine arthropods, which were distributed throughout the prehistoric oceans.
Fossils aren't just the remains of plants or animals. This is a Paleocastor burrow fossil, the preserved remains of a corkscrew den up to 9 feet in length, occupied by an early type of beaver. From Sioux County, Nebraska.