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.