CALCITIC JAWS OF AMMONOIDS AND NAUTILIDS: A NEW PROXY FOR RECONSTRUCTING HABITATS OF FOSSIL CEPHALOPODS

Paleotemperatures calculated on the basis of the oxygen isotopic composition of the shell of ammonoids and nautilids can provide insights into ancient marine systems as well as into the ecology (depth distribution and habitat) of these important groups of organisms. In some sedimentary deposits, however, the aragonitic shell of the ammonoid or nautililid is poorly or not preserved at all, while the calcitic structures belonging to the jaws are present (e.g., Jurassic Solnhofen Plattenkalk). The aim of our study is to test for the first time if the calcitic jaw structures in fossil cephalopods can be used as a proxy for paleotemperature. In order to test this hypothesis, we first analyzed the calcitic structures on the upper and lower jaws of Recent Nautilus and compared their isotopic values with those of the aragonitic shell in the same individuals. Our results indicate that the jaws of Recent Nautilus are secreted in isotopic equilibrium, and the calculated temperatures of secretion for the jaws approximately match those of the shell. We then extended our study to ammonites from the Upper Cretaceous (Campanian) Pierre Shale of the U.S. Western Interior and the age-equivalent Mooreville Chalk of the Gulf Coastal Plain. In the Pierre Shale, jaws occur in situ inside the body chambers of the ammonites while in the Mooreville Chalk, the jaw elements appear as isolated occurrences in the sediment. The isotopic composition of well-preserved jaw material (displaying the original lamellar mircrostructure) matches that of well-preserved outer shell material (displaying the original nacreous microstructure) in the same individual, suggesting that calcitic jaw elements can, indeed, provide a reliable geochemical archive of the habitat of fossil forms.