Ceraurinus icarus (Billings, 1860) - fossil trilobite from the Ordovician of Ohio, USA. (scan from a slide; Dan Cooper collection) (~3.0 centimeters across at its widest)

The famous Cincinnatian Series of the tristate area of Ohio-Kentucky-Indiana has some of the richest fossiliferous rocks on Earth. Cincinnatian rocks were deposited in relatively shallow marine facies during the Late Ordovician. The Cincinnatian succession is mostly interbedded limestones and shales. Most of the limestones are event beds (= tempestites), deposited during ancient storms.

Seen here is a trilobite from the Cincinnatian of Ohio. Trilobites are extinct marine arthropods. They first appear in Lower Cambrian rocks and went extinct at the end of the Permian. Trilobites had a calcitic exoskeleton and nonmineralizing parts underneath (legs, gills, gut, etc.). The calcite skeleton is most commonly preserved in the fossil record, although soft-part preservation is known in some trilobites (Ex: Burgess Shale and Hunsruck Slate). Trilobites had a head (cephalon), a body of many segments (thorax), and a tail (pygidium). Molts and carcasses usually fell apart quickly - most trilobite fossils are isolated parts of the head (cranidium and free cheeks), individual thoracic segments, or separated pygidia. The name "trilobite" was introduced in 1771 by Johann Ernst Immanuel Walch and refers to the tripartite division of the trilobite body - it has a central axial lobe that runs longitudinally from the head to the tail, plus two side lobes (pleural lobes).

This is a complete exoskeleton of Ceraurinus icarus, a rare Cincinnatian trilobite species. It has been reported from the upper Cincinnatian, in the Arnheim, Liberty, and Whitewater Formations.

Classification: Animalia, Arthropoda, Trilobita, Polymerida, Cheiruridae

Stratigraphy: Richmondian Stage (maybe the Liberty Formation), upper Cincinnatian Series, upper Upper Ordovician