See text Isotelus is a genus of asaphid trilobites from the Middle and Late Ordovician Period, fairly common in the northeastern United States, northwest Manitoba, southwestern Quebec and southeastern Ontario.

Specimens of the species I. rex were found in the Churchill River Group, in sediments that were once a warm, rocky, shallow subtidal zone along an Ordovician shoreline.

Asaphidae, like all other derived asaphide trilobite families, are inferred to have a unique and lengthy planktonic larval phase, only becoming benthic like adults after metamorphosis.

As there is a wide diversity of adult morphology in the aforementioned trilobite families, this tiny planktonic larval phase that they share may be the link to their extinction.

During the Ordovician-Silurian extinction event, the widespread onset of cold water conditions and anoxia may have instead favoured species that produced small numbers of large eggs, from which hatched larvae that were already benthic and adult-like in morphology.