DRILLING PREDATION ATTRIBUTED TO OCTOPODS IN CENOZOIC MOLLUSKS FROM JAPAN

Studies on drilling predation have focused primarily on drill holes attributed to gastropods. Although this type of drilling predation is and probably was most common in the Cenozoic, other animals are able to produce predatory drill holes as well. One such an organism is octopods. Drill holes produced by modern octopod holes tend to be either oval or circular, but their outer outline is characteristically irregular and may include a gutter. Such holes attributed to octopods have only been found occasionally in the fossil record from the Eocene – Pleistocene. The limited number of previous studies has focused on a restricted stratigraphic interval. Consequently, trends through geologic time of drill holes attributed to octopods have not been reconstructed thus far. Based on Cenozoic mollusks from Japan, we report on such drill holes in Miocene – Holocene gastropods and bivalves, especially in pectinids. At the subepoch level, drill hole percentages in mollusks increase toward the modern, from < 0.5% in the early Miocene to ~5.0% today. More variability is observed when individual localities are studied for all mollusks combined, but also when gastropods and bivalves are analyzed separately. As has been reported for drill holes attributed to gastropods, the drill hole diameter increases in larger mollusks. More drill holes are found in epifaunal mollusks than in infaunal mollusks. This study suggests that much more research can be done on drilling by octopods.