ICHNOLOGIC EVIDENCE FOR PREDATORY TRILOBITES: HOW LITERALLY CAN WE READ THE RECORD?

Superimposition of trilobite trace fossils (Rusophycus) with “worm” burrows (e.g, Paleophycus) is used to support interpretations of trilobite predatory behavior. However, the stratigraphic range of these “trilobite hunting burrows” is not entirely coincident with the stratigraphic range of trilobite taxa presumed to have been predatory, based on the morphological criterion of having had spinose gnathobases. Trilobites with spinose gnathobases reportedly range from the Cambrian through the Permian; Rusophycus is also known over the same range. Trilobite/worm burrow associations are relatively rare. Specimens interpreted as showing this predator/prey interaction have been described from four different stratigraphic localities ranging from the Cambrian to the Silurian. If trilobites with spinose gnathobases were predatory, why are there no trilobite hunting burrows from post-Silurian strata? If the correlation of possession of spinose gnathobases with a predatory habit is causal, then a literal reading of the fossil record of trilobite hunting burrows suggests that our characterization of post-Silurian trilobite limbs is incomplete, and that, after the Silurian, trilobites did not occupy the predator niche. The elimination of this trophic option would have reflected the reduction of trilobite diversity through the Mid-to-Late Paleozoic.