File:Isotelus maximus (fossil trilobite genal spine) (Liberty Formation, Upper Ordovician; Caesar Creek Lake's emergency spillway, Warren County, Ohio, USA).jpg

English: Isotelus maximus Locke, 1838 - trilobite genal spine from the Ordovician of Ohio, USA.

This fossil is from the famous Cincinnatian Series of the tristate area of Ohio-Kentucky-Indiana. Rocks in the Cincinnatian 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.

The specimen is part of the skeleton of a trilobite, an extinct marine arthropod. Trilobites first appear in Lower Cambrian rocks and the entire group went extinct at the end of the Permian. They 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 (Examples: 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 isolated 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).

The “official” state fossil of Ohio is the trilobite Isotelus, which attained impressively large sizes (up to 15 inches for complete Ohio specimens). Two species of Isotelus are known in the Ohio-Indiana-Kentucky Upper Ordovician outcrop belt: Isotelus maximus Locke, 1838 and Isotelus gigas Dekay, 1824.

The key differences between these two species are:

Isotelus maximus - genal spines in all holaspid stages, a larger maximum holaspid size, and a semicircular cephalic & pygidial outline.

Isotelus gigas - lack of genal spines in large holaspids, a smaller maximum holaspid size, and a subtriangular cephalic & pygidial outline.

The Isotelus maximus specimen seen here is a genal spine - a "cheek spine" from one of the posterolateral corners of the head.

Classification: Animalia, Arthropoda, Trilobita, Polymerida, Asaphidae

Stratigraphy: float from the Liberty Formation, Richmondian Stage, upper Cincinnatian Series, upper Upper Ordovician

Locality: Caesar Creek Lake's emergency spillway, northeastern Warren County, southwestern Ohio, USA