SPATIAL AND TEMPORAL VARIATION IN DRILLING PREDATION ON THE GASTROPOD CREPIDULA FROM U.S. ATLANTIC COAST BEACH ASSEMBLAGES, WITH IMPLICATIONS FOR THE FOSSIL RECORD

Paleontologists often use modern analogs to interpret the fossil record of predation. Spatial and temporal variation in predation on modern beach assemblages can potentially inform fossil studies. To test the utility of such assemblages, we compared drilling predation metrics among years in two local areas and tested whether local temporal variation is substantial enough to confound latitudinal drilling patterns.

We collected beach samples from Oak Island, NC, in 2003, 2005, 2010, 2014, and 2015, and from Long Island, NY, in 2003, 2015, and 2018. The NC assemblages were collected at one beach; NY materials derive from three beaches within 12 miles of each other on Peconic Bay, north fork of Long Island. To control for variation in drilling among taxa, study focused on the gastropod Crepidula, primarily represented by C. fornicata (60% of NC and >90% of NY Crepidula). Bulk samples were collected in all years but 2005; target samples were collected in 2005, 2015, and 2018. For all Crepidula (N = 2449), we tallied complete (fully penetrative), incomplete, and multiple drillholes. We calculated drilling frequency (DF = proportion of shells with ≥1 complete drillhole), prey effectiveness (PE = proportion of attempted holes that were incomplete), and the proportion of holes in multiply bored specimens (MULT). Fragmentation and specimen size were also compared between NC and NY bulk samples.

Fisher exact and χ² tests revealed no differences in DF among years for NC Crepidula samples (bulk, target, and combined). NY samples also showed no significant differences in DF among years, except for the 2015 bulk sample. In contrast, DF was significantly greater for NC than NY samples (0.12 vs 0.03). The difference is not an artifact of bias due to size differences (nonsignificant t-test) or shell breakage by durophages or surf; 8.0% of NC and 9.0% of NY specimens were fragments. Although water energy was greater for NC than NY, the difference in DF was in the opposite direction from that expected if surf preferentially destroyed drilled shells. Incomplete and multiple drilling were too rare for temporal comparisons at a site; PE did not differ, but MULT decreased, with latitude. Latitudinal variation in drilling predation exceeded temporal variation at a locality, supporting the utility of beach assemblages in understanding fossil patterns.