ESCALATORY INCREASE IN DUROPHAGOUS PREDATION DURING AN EXTINCTION EVENT

A Late Pliocene extinction event in tropical American marine ecosystems selected against seagrass-associated gastropods with narrow or thick-lipped apertures (Vermeij and Petuch 1986). Whether selection by enemies in this habitat was actually disrupted, however, has not been investigated. Our work examined a seagrass-associated predatory snail Phyllonotus (Muricidae) from the middle Pliocene Tamiami Fm., the late Pliocene Caloosahatchee Fm., the early Pleistocene Bermont Fm., and the late Pleistocene Fort Thompson Fm. Fossilized Phyllonotus show trace fossil evidence of predation attempts by crabs, octopi, and other gastropods that change frequency though the extinction period. We measured body size and analyzed predation trace fossil data for 1591 Phyllonotus shells. Data from each interval were organized into two shell length classes (class 1: 20-40 mm, class 2: 40-60 mm), so that trends within larger or smaller classes could be examined independently.

Octopus drill holes decrease through the extinction from 17% in the Pinecrest to 11% in the Caloosahatchee, 6% in the Bermont, and 3% in the Fort Thompson. This decrease in octopus drilling attacks on Phyllonotus is consistent with a decrease in selection by enemies, but patterns of shell repair are not. Shell lip repair increases from a frequency of 9% in the Pinecrest to 32% in the Fort Thompson. Siphonal canal repair scars show a similar increase, with 10% of shells scarred in the Pinecrest increasing to 16% in the Fort Thompson.

Increased repair from durophagous predators could mean that snails became more proficient at surviving attacks (or predators became less adept at prey handling) after the extinction. However, Phyllonotus snails also decreased in size and thickness (i.e., became more poorly defended) after the Pliocene. A more likely explanation is that attacks by durophagous predators actually intensified during and immediately after the extinction, in contrast to our initial prediction. Similar trends in repair frequencies in other gastropod lineages suggest to us that this pattern of escalation during an extinction is real and a pervasive feature of this event.