DEATH BY COMMON HOUSEHOLD TOOLS: MECHANICAL ANALOGY AND THE FUNCTIONAL MORPHOLOGY OF THE HYPOSTOME IN GENUS ISOTELUS (DEKAY) EVIDENCE FROM ISOTELUS IOWENSIS (OWEN)

The forked morphology of the hypostome in the Genus Isotelus (DeKay) is analogous to the common straight claw hammer. The claw portion of the hammer is strikingly similar in form and function to that of the isotelid hypostome. Cross-sections of both isotelid hypostome and claw hammer reveal a flat to slightly curved ventral surface and a beveled, dorsal surface on the inner side of each tine. The notch of the fork in the hypostome narrows toward the anterior end of the structure, identical to a claw hammer. This morphology implies a prying function for the isotelid hypostome. The forked morphology together with the partially arched, enrollment habit observed in Isotelus iowensis(Owen) fossils, suggests a dual prying/digging habit for feeding purposes. All isotelid cephalons are convex, spade-like and were well adapted for digging in soft, lime-mud environments where they are typically preserved. This idea supports the predatory habits for asaphid trilobites proposed by Forety and Owens.

Isotelid species including I. maximus, I. gigas, I. rex and I. iowensis all attain a relatively large size compared to other trilobites. This relates to food intake that had high nutritional value. A number of specimens of I. iowensis have been found in association with Chondrites isp. burrows. In the Maquoketa Shale, a distal tempestite bed containing anoxic mud (now pyritic shale) yields trilobites “frozen” in time. They appear to have been feeding at the level of an abundance of Chondrites isp.burrows.

The forked shape of the isotelid hypostome was an adaptation for infaunal polychete worm extraction. The flattened shovel-like cephalon was well adapted for digging into soft sediment. The morphological fits between common household tools i.e. claw hammer and spade and the hypostome and cephalon in the Genus Isotelus is remarkable. This strategy made the isotelids highly successful as predators on and in muddy infaunal environments.