Abstract
Externally shelled cephalopods were important elements in open marine habitats throughout Earth history. Paleotemperatures calculated on the basis of the oxygen isotope composition of their shells can provide insights into ancient marine systems as well as the ecology of this important group of organisms. In some sedimentary deposits, however, the aragonitic shell of the ammonite or nautilid is poorly or not preserved at all, while the calcitic structures belonging to the jaws are present. This study tests for the first time if the calcitic jaw structures in fossil cephalopods can be used as a proxy for paleotemperature. We first analyzed the calcitic structures on the jaws of Recent Nautilus and compared the calculated temperatures of precipitation with those from 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 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 well-preserved Baculites while in the Mooreville Chalk, the jaw elements appear as isolated occurrences in the sediment and the aragonitic shell material is not preserved. For the Pierre Shale specimens, the calculated temperatures of well-preserved jaw material match those of well-preserved shell material in the same individual. Analyses of the jaw elements in the Mooreville Chalk permit a comparison of the paleotemperatures between the two sites, and show that the Western Interior is warmer than the Gulf Coast at that time. In summary, our data indicate that the calcitic jaw elements of cephalopods can provide a reliable geochemical archive of the habitat of fossil forms.
Highlights
Ammonoids, an extinct class of cephalopods, constitute one of the best documented fossil groups [1]
Studies of the stable isotope composition of the Recent ectocochleate cephalopod Nautilus have demonstrated that the d18O of the aragonitic shell accurately reflects the temperature of the sea water during the secretion of the shell [12], [13]. [14], [15], [16], [17]
Despite the difference in mineralogy, our results suggest that the jaw is secreted in isotopic equilibrium, and that the temperature calculated for the upper and the lower jaw is the same
Summary
An extinct class of cephalopods, constitute one of the best documented fossil groups [1]. They are restricted to a marine habitat and exhibit a broad geographic and stratigraphic range. Because the shell is composed of calcium carbonate, ammonites can be used to provide insights into ancient seawater temperatures [2], [3], [4], [5], [6], [7], [8], [9]. The aragonitic shell of the ammonoid or nautilid is poorly or not preserved at all, while the calcitic structures belonging to the jaws are present
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
