Article: Palaeotidal characteristics determined by micro-growth patterns in bivalves
SEM studies of growth patterns in the ligament groove of late Pleistocene oysters and in unidentified Miocene shell fragments have enabled the fossils' position within the intertidal zone and the tidal regime to be reconstructed. Whereas the present-day tidal regime in Osaka Bay has a strong diurnal inequality seen especially in the heights of low water, 70 000 years ago it possessed only weak diurnal inequality, as in the Miocene at another locality, where it was weakly discernible in the heights of high water. The late Pleistocene borehole material represents a mixed assemblage for it includes individuals that lived at various intertidal levels. The majority of the Miocene individuals lived at mean tide level. The arrangement of growth lines of alternating thickness inverts every 2 weeks. Sequences are plotted with successive days on the ordinate and the time of day on the abscissa. For the Recent this results in a vertical partition of thicker and thinner growth lines. If the partition is not vertical when fossil growth lines are plotted, a change in the number of days per month is indicated. The plots for the Miocene material indicate that the length of a synodic month in terms of the number of synodic days of that time was essentially the same as the present.