DescriptionIsotelus maximus fossil trilobite (Upper Ordovician; southwestern Ohio, USA) 1 (15076169569).jpg
Isotelus maximus Locke, 1838 trilobite (~40 cm long) - 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.
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.
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