PLATYCERATID GASTROPODS AS PARASITES, PREDATORS, AND PREY AND THEIR POSSIBLE EFFECTS ON ECHINODERM HOSTS: COLLATERAL DAMAGE AND TARGETING

Platyceratid gastropods present excellent opportunities for the study of Paleozoic organism interactions. Almost uniquely among gastropods, the Platyceratidae possessed readily preserved, thick, calcitic shells. Members of this family show direct attachment over the anal vents of echinoderm hosts from Middle Ordovician to Permian time. Although they have long been considered merely commensal (coprophagous), there is increasing evidence that platyceratids may have had negative effects upon their echinoderm hosts and indeed drilled them. The presence of single complete boreholes, stereotypical positioning, and species preferences suggest predation and/or parasitism. The size and morphology of boreholes on crinoids directly associated with attached gastropods matches contemporaneous drill holes in Paleozoic echinoderms, brachiopods, and mollusks.

In turn, platyceratids may have presented a delectable “escargot” for varied predators. Platyceratids themselves may possess drill holes, suggesting cannibalistic behavior, as in later naticids. Cases of shell repair may also indicate durophagous predation. It is particularly notable that platyceratids were among the only Paleozoic gastropods to evolve spines. In the Middle Devonian some individuals feature spines up to 5 cm long. This attribute is likely related to the rise of durophagous predators (Middle Paleozoic Marine Revolution) and to the sessile, elevated mode of life of the gastropods, which made them easy targets for nektonic predators. Spiny platyceratids are commonly found attached directly to crinoid hosts, some of which are also spiny. Attacks on symbiotic platyceratids likely would have produced collateral damage on host crinoids, possibly reflected by a high proportion of regenerated arms. Abrupt co-evolution of spinosity in these gastropods and their hosts during the Middle Devonian may reflect targeting by newly evolved durophagous fishes at this time.