EFFECT OF SEAWATER CHEMISTRY ON NAUTILOID SHELL MINERALOGY DURING CALCITE SEAS

Governed by rates of seafloor spreading along midocean ridges, secular variation in the Mg/Ca ratio of seawater has led to large-scale oscillations between aragonite and calcite seas throughout geologic history. The seawater chemistry characteristic of these intervals has exerted a profound influence on the evolution of calcareous organisms, controlling, for example, the carbonate mineralogies of major reef builders and sediment producers. Most nautiloid mollusks are believed to have possessed originally aragonitic shells. However, as in some bivalve and gastropod taxa, a few species had shells composed of both aragonite and calcite. This bimineralic construction arose independently across several nautiloid stocks during times of early and middle Paleozoic calcite seas (Calcite I). Traditional views on the functional significance of calcitic shell layers generally involve temperature, with calcite secretion considered either as a physiological requirement at low temperatures or as an adaptation against shell dissolution in cold waters. These arguments are untenable because all known bimineralic nautiloids originated in warm Paleozoic greenhouse climates with little or no polar ice. Alternatively, the shell’s calcitic outer layers likely confer significant protection from the highly corrosive effects of calcite seas, in which biogenic aragonite is more vulnerable to pre-diagenetic chemical dissolution. In contrast, nautiloids of post-Paleozoic calcite seas (Calcite II) were not similarly reinforced. I consider an ecological explanation: aragonite shells offer greater mechanical strength than their calcite counterparts, potentially enhancing resistance to the marine durophagous predation that intensified in the Mesozoic Era.