THERMAL POTENTIATION AND MINERALOGICAL EVOLUTION IN THE BIVALVIA (MOLLUSCA)

The most important factor controlling the timing of Phanerozoic minera logical evolution in the Bivalvia appears to be thermal potentiation o f calcite deposition in colder marine and estuarine environments. Cold temperature has promoted mineralogical evolution in the Bivalvia by k inetically facilitating (potentiating) initially weak biological contr ols for calcite, thereby exposing their genetic basis to natural selec tion. Calcite has evolved in bivalve shells for a variety of selective advantages, including resistance to dissolution; resistance to chemic al boring by algae and gastropods; reduced shell density in swimming a nd soft-bottom reclining species; enhanced flexibility in simple prism atic shell layers; and fracture localization and economy of secretion in association with certain foliated structures. Endogenous calcite in bivalve shells varies from biologically induced to weakly and strongl y biologically controlled. Biologically controlled calcite generally f irst appears in bivalve shells as an impersistent component of the out er shell layer, only later, in some groups, expanding to include the e ntire outer and then part or all of the middle and inner shell layers. The initial stages of mineralogical evolution are shown by certain mo dern Mytilidae, Veneridae and Petricolidae. In the latter two families , the calcite occurs as conellae in the outer part of the outer shell layer. Calcitic conellae in the inner shell layer of Pliocene Mercenar ia are not barnacle plates, as previously indicated, but endogenous ca lcite comparable in origin to other venerid conellae. Their occurrence in Mercenaria may reflect thermal potentiation of weak controls for c alcite, as well as local detachment of the secretory mantle epithelium near the pallial and adductor musculature.