Abstract

No temporal trend in the intensity of drilling by naticids on Glycymeris yessoensis can be recognized during the late Cenozoic. Drilling sites shifted from the umbo to the center of the valve during the late Cenozoic. This shift might reflect the change of predators from Glossaulax in the Miocene to Cryptonatica or Euspira in the Pleistocene. Borehole sites in the middle Pleistocene were more stereotyped than in the early Pleistocene population despite the same predators. Edge drilling, which is a faster drilling method, first appeared in the population showing high drilling intensities in the early Pleistocene. Because a prediction of the hypothesis of escalation is that changes in predators' behavior developed through time, these changes in drilling location may be the results of escalation. In contrast with the stereotypic trend of borehole sites, correlation coefficients of predator-prey size decreased from the early to the middle Pleistocene.

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