FEEDING BEHAVIOR OF TWO CINCINNATIAN (ORDOVICIAN) TRILOBITES INFERRED FROM TRACE FOSSILS AND NONMINERALIZED ANATOMY

Historically, most trilobites have been regarded as sediment-deposit feeders. Trace fossils that represent the behavior of two Late Ordovician trilobites from Cincinnatian strata, Isotelus and Flexicalymene, plus appendages and internal soft parts of Isotelus, challenge that historical interpretation. Several specimens of the trace Rusophycus pudicum, which were constructed by Flexicalymene, are preserved in intimate association with Palaeophycus-type traces, which were probably constructed by infaunal worms. Examples typically show truncation of the Palaeophycus by the Rusophycus at the "anterior" end of the Rusophycus trace, suggesting lethal predation by the trilobites on the worms. Similar interactions are shown involving Rusophycus carleyi traces, constructed by Isotelus, and Palaeophycus. These examples provide evidence for a carnivorous feeding habit in these two trilobites, and further provide evidence against the "feeding current burrow" model for Rusophycus traces.

Appendages and mineralized internal guts of Isotelus maximus specimens strengthen the interpretation that this trilobite was a carnivore. Ventral appendages bear strong spines on the endopodites that may have helped restrain prey. A sediment-free mineralized gut in one specimen indicates that Isotelus did not feed by processing sediment. Lack of sclerites in the gut indicate that the trilobite was non-durophagous. This evidence, combined with inferred phylogenetic affinities among the arachnomorph arthropods, suggests that this trilobite fed on live prey in a manner similar to that of other arachnomorphs, including modern chelicerates.