Isotelus rex is a moderately effaced asaphid trilobite lacking terminal spines or prolongations, and the holotype specimen was found in a carbonate unit showing little evidence of distortion or compaction. All dorsal sclerites of the holotype are closely articulated, suggesting this is not an exuvium. At about 720 mm long, 400 mm in maximum width (across the cephalon), and 70 mm in height (at the posterior midpoint of the cephalon), it is the largest complete trilobite specimen ever found. Large representatives of Isotelus occur elsewhere in the Late Ordovician succession of North America and some of the earliest described species were considered to be among the biggest trilobites then known. Indeed, several specific epithets, including those of I. gigas Dekay, 1824 and I. maximus Locke, 1838, were coined in reference to their comparatively large size. Hansen (1989) reported complete specimens of I. brachycephalus Foerste, 1919 from Ohio with lengths up to 410 mm. Prior to the discovery of the 720 mm Isotelus rex, the largest confirmed complete trilobite specimen was a fully articulated isoteline measuring 430 mm in length (Whittington in Kaesler, 1997). It is from the same locality as Isotelus rex and is considered a smaller individual of that species.