ENVIRONMENTALLY AND ECOLOGICALLY CONTROLLED VARIATION IN PREDATORY AND EPIBIONT PATTERNS IN CHIONE CANCELLATA FROM THE FORT THOMPSON FORMATION (PLEISTOCENE) OF FLORIDA

The biotic interactions of an ancient organism with other organisms can sometimes be inferred from evidence left on its fossil remains. We examined valves of Chione cancellata (Linné 1767) from the Fort Thompson Formation (Pleistocene) of Florida for these traces.

Our samples came from two different Fort Thompson facies. The lower facies contained a molluscan fauna dominated by epifaunal species which most likely lived in a frequently disturbed sea grass meadow. The molluscan fauna of the upper facies was dominated by infaunal burrowers such as C. cancellata, and was more ecologically complex. It was probably deposited under more stable paleoenvironmental conditions, and contained both abundant sea grass meadow and open sand species. C. cancellata were ten times more abundant in this facies; a single 2 kg sediment sample routinely yielded over 600 specimens. While C. cancellata is present in both sampled facies, there are differences between samples from the two facies.

Predatory boreholes, which record the activity of naticid gastropods, were stereotypically located in the center of both left and right valves of C. cancellata. In the upper facies, which had more abundant C. cancellata, the larger valves were more likely to be bored than in the lower facies, where all size classes were bored equally. Various hypotheses for explaining this pattern will be examined further by studying the sizes of the boreholes from the two facies.

Epibionts on C. cancellata such as corals, barnacles, and kitten paws were more common in the upper facies. While many of the epibionts were clearly post-mortem infestations, the positions of other epibionts indicate that both clam and epibionts lived at the same time. These life associations were only recognized from the facies with abundant C. cancellata, and were absent from the other facies. Thus, C. cancellata interacted with these epibionts differently under different paleoenvironmental conditions.