GEOCHEMICAL (C, O, AND N) AND BIOGEOGRAPHIC EVIDENCE FOR LONG TERM HABITAT CHANGE OF THE POST-JURASSIC NAUTILIDAE: FROM JURO-CRETACEOUS GLOBAL, SHALLOW SHELF PREDATORS TO NEOGENE, DEEP WATER, INDOPACIFIC SCAVENGERS

We present new fossil discovery and geochemical data concerning the comparative habits and habitats of post- Jurassic nautilid cephalopods. Using oxygen, carbon, and nitrogen isotopes as well as extensive new nautilid discoveries preserved as aragonite from the Cretaceous and Cenozoic of North America, Antarctica, and Australia, we document a progressive decrease in temperature of shell formation in post-Cretaceous nautilid species. We also note changes in both carbon and nitrogen isotopes over time. While most extant nautilids show carbon isotope values that are negative (averaging -1), we note that virtually all specimens tested from Aturia species are positive, while other nautilids range from 1 to -1. Nitrogen isotopes recovered from pristine aragonite of Cretaceous nautilids and heteromorphic ammonites from the Turonian of Madagascar, as well as from fossil Nautilus praepompilius from Upper Cretaceous of California yield nitrogen values of about 6, while extant Nautilus (from 6 species) and Allonautilus (from 2 species) range from 10 to 13, although when raised in aquaria on one species of shrimp, the same specimens change from 12 to 7. The ratios of carbon to nitrogen isotopes in this study are compared to the same and newly collected values from living Sepia and other dibranchiates from South Australia; one possibility is that the extant Nautilus and Allonautilus are on the edge of starvation in the wild at all times, which would produce higher levels of nitrogen values. We conclude that these data are consistent of not only a biogeographic contraction of world wide Nautilus in the Late Cretaceous, all inhabiting shallow water as active carnivores, to todays ranges only in the fore reef slopes of the Indopacific at depths averaging 250m.