Unlike the tropical to subtropical areas where reef-building corals are distributed, high latitude paleoenvironmental records with high temporal resolution over the past centuries are scarce. Long-living bivalve shells can be a good candidate for paleoenvironmental recorders. In such a situation, it has recently been revealed that modern cold-water bivalve Mercenaria stimpsoni (Stimpson's hard clam) living in shallow seafloors of North Japan have a lifespan of over 100 years. In this review, details of sclerochronological analysis and analytical procedures oxygen stable isotope and radiocarbon measurements of M. stimpsoni shells collected from shallow seafloors of Funakoshi Bay and Otsuchi Bay are described. Then, ecological (e.g., shell growth patterns) and paleoenvironmental (e.g., oceanic currents, tsunamis) findings that were obtained through these analyses are described.
In order to clarify the adaptive strategies of Japanese scaphitids, Cretaceous heteromorph ammonites, we restored the population dynamics of the hypothetical scaphitid species by computer simulations, and concluded as follows: (1) Various patterns of size distributions observed in the immature scaphitid shells are interpreted unified as the mixture of the random-death daily accumulated and death from mass starvation eventually occurred. (2) The biased sex ratio actually observed in the mature scaphitids is caused probably by the difference of duration in which the dimorphs are exposed under the high risk of starving. (3) These ammonites may have been opportunistic species that migrated to fertile sea grounds and rapidly increased their populations there. The thanatocoenosis actually observed is well restored by hypothetical scaphitids breeding under the conditions of more or less limited food supply and poor random-death factor.