Temporal change of life-history traits in fossil bivalves: an example of Phacosoma japonicum from the Pleistocene of Japan

Abstract

Shell microgrowth and oxygen isotope patterns were examined in fossil specimens of a venerid bivalve Phacosoma japonicum from the Kami-iwahashi shell bed (c. 2.5 m thickness) in the middle Pleistocene Shimosa Group (c. 0.2 Ma) in central Japan. Winter and spawning breaks in this species can be distinguished by shell microgrowth analysis, and oxygen isotope values are high near winter breaks and low around spawning breaks. These breaks, therefore, can be used to estimate age of sexual maturity and shell growth rate in fossil specimens. Shell microgrowth analysis in the three fossil samples from the lower, middle and upper horizons in the Kami-iwahashi shell bed reveals that the maximum shell height and age of sexual maturity decrease gradually toward the top of this shell bed. Extant populations around the Japanese coast show a progressive change of life-history traits along a north–south gradient. Northern populations generally exhibit more delayed sexual maturity and larger shell size at a given age than southern ones. Life-history traits of the fossil sample from the lower horizon of this shell bed resemble those of the extant population from Ishikari Bay, Hokkaido, northern Japan, and those from the upper horizon of this shell bed are similar to those of the extant population in Tokyo Bay, central Japan. Sequence stratigraphic analysis has regarded this shell bed as a part of the transgressive systems tract, and the observations of the warm- and cold-water bivalve species suggest that mean annual water temperature in Paleo-Tokyo Bay increased during deposition of this shell bed. The geographic variation of life-history patterns among the living populations is related to the difference of mean water temperature during the growing season for each population. Judging from these facts, the temporal change of life-history traits observed in the fossil samples reflects the increase of mean annual water temperature during the deposition of this shell bed.