LATITUDINAL PATTERNS IN PREDATION BY SHELL-DRILLING GASTROPODS: ARE DIFFERENCES IN PRESERVATION POTENTIAL RESPONSIBLE?

Frequency of drilling by predatory gastropods varies with latitude, but some studies report decreased drilling frequency (DF) with latitude and others report an increase in DF. Our previous work surveyed Recent drilling from Maine to Florida and found a peak in DF at mid-latitudes for bivalve assemblages and lower taxa, but the cause of the pattern remains unclear. Could spatial variation in DF be an artifact of latitudinal differences in preservation potential of drilled vs. undrilled shells?

Drilled shells are more prone to breakage in compression experiments, though the importance of such bias in the fossil record is debated. Latitudinal differences in intensity of taphonomic processes could conceivably produce geographic patterns in DF by affecting the degree of bias against drilled shells. At present the degree of latitudinal bias is unclear. Intensity of some taphonomic processes varies with latitude (e.g., rate of microbial ligament decay), but other processes are more closely linked to factors that vary spatially but not necessarily along a latitudinal gradient (e.g., sediment supply, water energy, bioturbation).

Taphonomic bias could also occur if shell thickness varies latitudinally within taxa (e.g., due to ease of calcification or natural selection for shells resistant to physical or biological hazards). Such latitudinal thickness variation could produce taphonomic bias because thin shells with drillholes should break more easily than thicker drilled shells, artificially decreasing DF in areas where shells are thin. A positive correlation between DF and thickness across latitude within a taxon could indicate such a latitudinal bias. To assess this possible bias, thickness was measured relative to length for several wide-ranging taxa along the Atlantic coast of the US and Brazil. Spisula and Crepidula showed geographic variation in DF, with a peak in the Carolinian Province, but in neither case did DF correlate positively with thickness across latitude. For Crepidula and Mercenaria DF and thickness were inversely correlated. In Divalinga from Brazil, DF was significantly greater at 10°S latitude than at 26°S, but thickness did not differ. Though further work is needed, taphonomic bias related to thickness variation does not appear responsible for the latitudinal differences in drilling frequency.