COMPARISON OF SR/CA RATIO OF FOSSIL AND RECENT MOLLUSC SHELLS

BIOCHEMICAL evolution has occurred in mollusc shells with respect to their Sr contents, according to Lowenstam1, and this thesis has subsequently been elaborated2,3. Contemporary forms are thought to be able to discriminate more efficiently than ancient molluscs against Sr relative to Ca in seawater, so that modern mollusc shells generally contain lower Sr/Ca ratios than ancient shells. Evidence for this is a systematic decrease in the Sr/Ca ratio in unrecrystallized aragonitic gastropod shells from the Pennsylvanian to present1 and in unrecrystallized aragonitic cephalopods from Carboniferous to present3. Hallam and Price3 do not seriously consider the possibility of post-depositional alteration of shells without recrystallization as an explanation for the observed relationship between the Sr/Ca ratio and age, and it may be that the Sr content of calcareous shell material is altered before recrystallization4-6. If so, it may be difficult to differentiate the role of biochemical evolution from post-depositional alterations caused by diagenesis or weathering. © 1969 Nature Publishing Group.