Isotelus maximus (fossil trilobite) (Flat Run Trilobite Lagerstätte, Arnheim Formation, Upper Ordovician; Flat Run Quarry, Highland County, Ohio, USA) 6

            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: Arthropoda, Trilobita, Polymerida, Asaphidae

Stratigraphy: Flat Run Trilobite Lagerstätte, mid-Arnheim Formation, lower Richmondian Stage, upper Upper Ordovician

Locality: attributed to "Mt. Orab", which means the Flat Run Trilobite Quarry, far-southwestern Highland County, southwestern Ohio, USA

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Taken on: May 4, 2024

Tags:

Isotelus

maximus

Ordovician

Ohio

Cincinnatian

Series

Flat

Run

Quarry

Highland

County

Mt.

Orab

Lagerstätte

Arnheim

Formation

Richmond

Richmondian

Stage

Group

Asaphidae

Sunset

Member

trilobites

fossil

fossils « less

Sets appears in: • Ohio Rocks, Minerals, Fossils • Arnheim Formation (Upper Ordovician; Ohio-Indiana-Kentucky, USA) more » • Ordovician Rocks & Fossils • Cincinnatian Fossils and Rocks (Upper Ordovician of the Cincinnati, Ohio region, USA) • Trilobite Fossils « less

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