Predator-prey interactions based on drillholes: A case study of turritelline gastropods from the Pleistocene Szekou Formation of Taiwan

Drillholes on shells provide a useful way to investigate prey and predator relationships. The current study documents predator-prey interactions exemplified by a faunal assemblage of the fossil gastropod Turritella cingulifera from the Pleistocene Szekou Formation in Hengchun Peninsula, Taiwan. All recognisable skeletal and shell fragments that are larger than 3 mm in size were collected and recorded. Processed bulk sediments (5.24 kg) contained 1462 molluscan shells, including 824 specimens of T. cingulifera, and 27 non-molluscan invertebrates. In the current study, approximately 41.6% (609/1462) of molluscs are drilled with at least one hole. Drilling intensities (DIs) regardless of shell completeness in all gastropods, bivalves and the turritelline gastropod T. cingulifera are 0.546, 0.060 and 0.413, respectively. DI on turritellids is significantly lower than that on other gastropods (χ2= 21.039, P < 0.001). Furthermore, the percentage of drillholes that occur in multiply drilled specimens is 34.7% (95/275) for turritelline gastropods based on complete to nearly complete specimens (n = 588). Our study shows no significant preference of drillhole position either on the suture or on the whorl (χ2= 0.055, P = 0.814). Most drillholes are located in whorls two to four proximal to the aperture. Drillhole diameters of the shells with one drillhole and ones with multiple drillholes are 1.0 and 0.5 mm on average, and the results of Mann-Whitney tests indicate that they are significantly different (P < 0.001). The first turritelline gastropod shell with an incomplete drillhole from Taiwan is documented here. The dominant drilling predators were naticids based on the drillhole morphology and the presence of naticids in the same assemblage. No apparent prey size selectivity is observed, so a 'size refugium' does not exist for the turritellids in the current study.