Sr, Mg, Ca and Mn chemistry of skeletal components of a Pennsylvanian and recent nautiloid

Abstract

The atomic ratios of Sr/Ca, Mg/Ca, and Mn/Ca were determined for mineralogically and chemically unaltered shells and cameral deposits of three species of the orthoconic nautiloid Mitorthoceras Gordon (Pennsylvanian, Desmoinesian—Westphalian C) and for shells of four species of Nautilus Linnaeus. These data are used to investigate and document three aspects of nautiloid physiochemistry: (1) differences in the Sr/Ca ratios of shell and cameral deposit aragonite of Mitorthoceras species indicate that the fluids from which shells and cameral deposits were precipitated were of different compositions; and (2) although Sr/Ca ratios of shells are the same for species of Mitorthoceras, significant differences in the Mg/Ca ratios, and to some extent the Mn/Ca ratios, of shells of these species conform to the specific taxonomy of the genus and reflect physiochemical differences between these species. Among species of Nautilus it is the Mg/Ca and Sr/Ca ratios of shells that conform to the accepted specific taxonomy of the genus while Mn/Ca ratios are invariant among species. Third, differences in the Sr, Mg, and Mn content of shells of Mitorthoceras and Nautilus may reflect evolution of the physiochemical system of nautiloid cephalopods or differences in seawater chemistry of their respective premortem habitats.