An asaphid trilobite from the middle and upper Ordovician Period fairly common in the Northeast U.S., northwest Manitoba, southwestern Quebec and southeastern Ontario. A specimen of Isotelus rex, from Churchill, Manitoba, is the largest complete trilobite ever found. Large specimens have also been found in Ohio, where Isotelus maximus is the state fossil.
Articulated specimens are uncommon in most exposures, as it was not as robust as Isotelus gigas or Flexicalymene senaria found in the same rocks.
Small Isotelus gigas, particularly those under 50mm long, often have genal spines but these are lost as the trilobite became larger.
Thus, the adult Isotelus iowensis has long genal spines, extending to the fifth, sixth, or seventh segment, an axial lobe a little less than half the width of the thorax, and pygidia show a fairly convex axial lobe, and traces of ribs on the pleural lobes.
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