CINCINNATIAN ESCARGO: GASTROPOD-BRYOZOAN TAPHONOMY DEMONSTRATES PRESENCE OF AN UNDEFINED, LATE ORDOVICIAN, SHELL-PEELING PREDATOR

ESCALATION Theory is still under scrutiny and debate (Madin,et al, 2006). Predation on gastropods has provided a primary focus for studies of co-evolution driven by prey choices and killing techniques employed by Phanerozoic marine predators. Drilling, crushing and peeling are the methods of opening shells and dispatching snails that leave taphonomic imprints. Each of these shell-damaging conditions may also be imposed on gastropod shells post mortem by non-predatory behaviors of organisms or by violent physical processes of local environment. Repaired breaks are recognized in Middle and Late Ordovician snails, but those demonstrate survival rather than mortality, and therefore they don't influence Escalation Theory.

In the type Cincinnatian Fairview and Waynesville Formations of Ohio and Kentucky, fusiform and trochiform snails were peeled by one or more undefined predators in the stereotypical spiral pattern posteriorad from the aperture as used by some modern decapod crustaceans. Predation was successful and fatal. Spent shells were colonized by encrusting Bryozoa which overgrew each shell completely, including the apertural region and the peeled body whorl, thus preserving the morphology of the gastropod shell at its time of death. The bryozoan encrusted the exposed parietal wall (interior) of the body whorl closing further access to the interior in all but two of the cases.

During the Cincinnatian in the Ohio region, aragonitic molluscan shell material underwent hyperdissolution, often on the pre-burial sea floor, so that a majority of gastropods occur as molds of the interior. Peeled portions of shell did not hold sediment and are not recoverable as molds. Encasing by the bryozoan encruster was essential for preservation of these predated snails which demonstrate successful predation by peeling. But by which organism(s) was predation done? A chelate or chelicerate arthropod is strongly suggested by the stereotypical pattern of breakage, but such arthropods, xiphosurans being the most obvious candidates, are barely represented in the fossil record of the region and are poorly known from the Late Ordovician in general, usually in a brackish water setting not ascribed to these rocks. Decapods first appeared in the Devonian. The peeling perpetrator remains unknown.