English: Isotelus maximus Locke, 1838 fossil trilobite from the Ordovician of Ohio, USA.

Trilobites are extinct marine arthropods. They first appear in Lower Cambrian rocks and the entire group 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 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.

Classification: Animalia, Arthropoda, Trilobita, Polymerida, Asasphida, Asaphidae

Stratigraphy: Waynesville Formation, Richmondian Stage, upper Cincinnatian Series, upper Upper Ordovician

Locality: type locality of Isotelus maximus - "near Treber's, in Adams County" (see Locke, 1838, p. 247) - apparently referring to the vicinity of Treber at or near Dunkinsville, Oliver Township, central Adams County, southwestern Ohio, USA

Collected & prepared by Tom Johnson.

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Reference cited:

Locke, J. 1838. Isotelus maximus, found near Treber's, in Adams County. Second Annual Report on the Geological Survey of Ohio, pp. 247-249, figures 8-9.