The function of forks: Isotelus-type hypostomes and trilobite feeding

Despite previous investigations, the function of the forked morphology of asaphid trilobite hypostomes is enigmatic. The focus of this study is the large and robust forked hypostome of the largest known genus of trilobite, Isotelus, and the independently-derived forked hypostome of Hypodicranotus, the longest hypostome relative to body size of any trilobite. Although the trilobite hypostome is analogous to the labrum in other arthropods, forked hypostomes lack an obvious modern functional counterpart. The Isotelus hypostome is distinguished from other trilobite hypostomes by closely-spaced terrace ridges on a greatly thickened inner surface of the forked posterior margin, with the scarp of the terrace facing antero-ventrally. This is compatible with a grinding function, suggesting possible limb differentiation to complement this structure. The inner face of the tine (one of the two, prominent, sub-parallel posterior projections) is also unique in that it has a microstructure which is evident in section, running perpendicular to the surface. Macropredatory and filter-feeder roles are ruled out, and previous characterizations of the hypostome as knife-like or serrated are rejected. Its function is incompatible with that of other non-asaphid trilobites with forked hypostomes, like the remopleuridid Hypodicranotus, which lack similar terrace ridges and thickened inner-edge cuticle.