EVOLUTIONARY IMPLICATIONS OF GASTROPOD PREDATION ON IRREGULAR ECHINOIDS REPRESENTING DIFFERENT ECOLOGICAL BURROWING TIERS, LATE EOCENE, FLORIDA

Predation is an important biological interaction that has contributed to ecological community structure and evolutionary innovations. Irregular echinoids (spatangoids, clypeasteroids, and cassiduloids) may have evolved the ability to burrow in the Mesozoic as a result of increasing predation pressure from predatory drilling gastropods. The prima facie expectation would be that species with the ability to burrow deeper in the substrate would be found with fewer gastropod drill holes. To test this hypothesis, museum collections were used to determine if there was a significant difference in gastropod drill-hole frequency among echinoids that represent different burrow depths (burrowing tiers) from the Late Eocene Ocala Limestone.

A predatory drill hole was found on seven of the eleven species, representing 64% of the species. Only 5% of the individuals were drilled (15 out of 321 individuals). To determine if burrow depth affected predation frequency, echinoids were classified into one of three tiers based on test morphology: shallow burrowing (0-5cm), moderate burrowing (0-10cm), and deep burrowing (0-25cm).

For the following, predation frequency was calculated by taking the total number of specimens in each tier and dividing that by the number of specimens in that tier with a predatory trace. The shallow burrowing non-spatangoid irregular echinoids (Rhyncholampas conradi, Rhyncholampas ericsoni¸ Oligopygus wetherbyi, Oligopygus haldemani, Weisbordella cubae and Mortonella quinquefaria) and the shallow burrowing spatangoids (Plagiobrissus dixie, Eupatagus antillarum, and Eupatagus ocalanus) had a predation frequency of 3.2% (n =7). The moderate burrower (Agassizia clevei) had a higher predation frequency of (16.6%, n =2). The deepest burrowing species (Schizaster armiger) had a predation frequency of 6.5% (n = 6).

The results from this study indicate that there is no significant correlation between burial and relative frequency of predation as represented by drill holes (r=-0.368; p=0.27) for tropical echinoids in the Late Eocene of Florida. Burrowing, as an evolutionary innovation in echinoids may not necessarily be tied to escaping predation. We suggest that a reevaluation of the predation hypothesis in light of burial depth is needed to further understand predation and its macroevolutionary effects on echinoids.