SHELL FRAGMENTS IN FOSSIL SHELL BEDS AS A RESULT OF DUROPHAGOUS PREDATION: RESULTS FROM FIELD OBSERVATION AND TUMBLING EXPERIMENTS

Mesozoic and Cenozoic shallow-marine shell beds were examined in various localities of Japan, in order to identify if these contain fragmented shells of bivalves and gastropods. The number of formations surveyed is nine for the Mesozoic and twelve for the Cenozoic. Many of these shell beds are associated with storm-dominated deposits and they are reasonably interpreted to be deposits above the wave base. As a result, there is a clear increase in the degree of shell fragmentation in the Late Cretaceous and the Tertiary. This is approximately the same period with the increased durophagy, known as the Mesozoic marine revolution. Dominant occurrence of such shell fragments in the Cenozoic is in favor of its biological origin. In order to confirm such an interpretation, we conducted tumbling experiments with shells with sand and artificial seawater in a container. After tumbling in twelve days, shells of a trochid gastropod (Umborium (Suchium) giganteum) and a venerid bivalve (Ruditapes philippinarum) showed considerable abrasion, but no fragmentation was observed in the shells of Umborium (Suchium) giganteum and only a small number of Ruditapes philippinarum showed breakage of shells after their severe abrasion. These results also suggest that shell fragmentation in fossil shell beds is regarded as the result of durophagous predation, rather than that of physical agents such as wave agitation and water currents that primarily induce shell abrasion. The presence or absence, or the degree of shell fragmentation can potentially be a good indicator of durophagous predation.