Isotelus  maximus

 

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

 


 

Isotelus maximus Locke, 1838 trilobite (~1.8 cm long) - small holaspid from the Upper Ordovician of southwestern Ohio, USA.  Notice that the genal spines are longer than the entire thorax.

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

Locality: Flat Run Trilobite Quarry, far-southwestern Highland County, southwestern Ohio, USA.

Collected & prepared by Dan Cooper.

 


 

Isotelus maximus Locke, 1838 trilobite (~5.2 cm long) - holaspid from the Upper Ordovician of southwestern Ohio, USA.  Notice that the genal spines are still longer than the entire thorax.

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

Locality: Flat Run Trilobite Quarry, far-southwestern Highland County, southwestern Ohio, USA.

Collected & prepared by Dan Cooper.

 


 

Isotelus maximus Locke, 1838 trilobite (~11.2 cm long) - holaspid with pygidial appendage attachment scars visible through the exoskeleton from the Upper Ordovician of southwestern Ohio, USA.  Notice that the genal spines are about 7/8th of the length of the thorax.  The relative genal spine length decreases with ontogeny (growth) - see also the larger holaspid specimens below.

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

Locality: Flat Run Trilobite Quarry, far-southwestern Highland County, southwestern Ohio, USA.

Collected & prepared by Dan Cooper.

 


 

Isotelus maximus Locke, 1838 trilobite - large holaspid with missing right eye (= healed injury) from the Upper Ordovician of southwestern Ohio, USA.  The genal spines are less than half the thoracic length.

Collected & prepared by Tom Johnson.

 


 

Isotelus maximus Locke, 1838 trilobite (cast, ~39 cm long) - very large holaspid from the Richmondian Stage (Upper Ordovician) of southwestern Ohio, USA.  The genal spines are relatively short.

Original collected, prepared & owned by Tom Johnson.

 


 

Home page