A CALL TO ARMS: TESTING ECOLOGICAL AND EVOLUTIONARY HYPOTHESES OF PREDATION USING CRINOID ARM REGENERATION FREQUENCY

Arguably, predation has shaped marine communities since the Precambrian. Many ecological and evolutionary trends in Phanerozoic marine invertebrates have been explained by punctuated increases in predation intensity (e.g. Paleozoic and Mesozoic Marine Revolutions). However, many such hypotheses are grounded in anecdotal evidence or based on correlations that need not be causal. For instance, the Middle Paleozoic Marine Revolution (MPMR), which predicts an increase in predation during the Devonian, is supported, in part, by coincident increases in durophagous predator diversity and antipredatory features in crinoid prey. Although such correlations are compelling, is there more direct evidence for predation in fossil crinoids?

In extant crinoids, arm loss and regeneration is attributed directly to predation, mostly by fishes. In addition, regeneration frequency in living crinoids is considered to be a reliable proxy for predation intensity. Regenerating arms in fossil crinoids, though rarely reported, are present in fossil assemblages and suggest that crinoids have been subject to predation since the Ordovician. Thus, it seems reasonable to compile data on fossil crinoid arm regeneration frequency, and to use such evidence for testing predatory hypotheses, a task that we have begun.

Data on arm regeneration frequency can be used for hypothesis testing at scales ranging from the entire Phanerozoic to single Lagerstätten. For example, analysis of arm regeneration in over 2,500 Ordovician-Pennsylvanian crinoids reveals a sharp increase in arm regeneration frequency during the Siluro-Devonian, the only significant change over the study interval. This result is consistent with an increase in predation intensity as predicted by the MPMR. Furthermore, regeneration at the scale of a single assemblage, the Early Mississippian Lagerstätten from Le Grand, Iowa, indicates that the most abundant and tallest species, Rhodocrinites kirbyi, also had the highest regeneration frequency. This result is consistent with attacks by a predator that targets the most apparent prey. Together, these studies demonstrate that arm regeneration in the fossil record is more common than generally believed and that such evidence provides a rich source of data for testing ecological and evolutionary hypotheses of predation.