SIZE- VS. AGE-STANDARDIZATION OF PREDATION FREQUENCY IN THE FOSSIL RECORD: A FIRST LOOK

Indirect proxies for predation frequency, such as drillholes and shell repairs, are often standardized by size of the prey in tests of the escalation hypothesis. The working assumption in such standardizations is that prey individuals of similar size are equivalent in ontogenetic age, and thus exposure time to predators. Here we evaluate the potential bias associated with this assumption for a predator-prey system from the Plio-Pleistocene of Florida. We focused on the muricid gastropod Phyllonotus, because species of this genus display a significant trend of decreasing maximum body size between the Pliocene and Pleistocene. Previous isotopic studies (Ford et al. 2005) also showed that this change in body size was due largely to differences in shell growth rate instead of lifespan. In other words, Pliocene Phyllonotus of a given size are younger than individuals of the same length in the Pleistocene. This result implies that snails of different ontogenetic age, that is, different exposure time to predators, are compared in a size-standardized analysis in this system.

Predation traces from a variety of predators, such as drilling predators (e.g., muricids and octopods) and shell-crushing predators (e.g. decapods), are also common on Phyllonotus shells. We compared the frequency of these predation traces on a sample of nearly 1600 Phyllonotus prey from 102 localities and four stratigraphic horizons in three ways: 1) pooled (no standardization), 2) two standardized size classes (20-40 mm, 40-60mm), and 3) two age-standardized classes (1yr. and 2yrs.). Preliminary results indicate that all three analyses capture the same directional trends in predation frequency between the Pliocene and Pleistocene for both shell-drilling and shell-crushing predators. Despite this encouraging result, we caution against simple extrapolation to other systems.