USING SYSTEMATIC δ13C DIFFERENCES BETWEEN AMMONITES AND BENTHIC MOLLUSKS FROM SEYMOUR ISLAND, ANTARCTICA TO BETTER UNDERSTAND AMMONITE LIFESTYLES

Reconstruction of ammonite lifestyle has been attempted through investigations of their morphology, preservation, and geochemistry. One of the primary motivations behind understanding the mode of life of ammonites is that, unlike other mollusks, including the morphologically similar Nautiloids, ammonites went completely extinct at the Cretaceous – Paleogene (K-Pg boundary). Geochemical studies using δ¹⁸O of well-preserved shell material have compared their habitat water temperature (and presumably depth) relative to other molluscan groups with mixed results, but δ¹³C values from ammonite shells have been analyzed less frequently due to the complexity of inputs to the carbon isotopic system. Here we report a ~4‰ δ¹³C difference between aragonite from ammonites and benthic mollusks from the very latest Cretaceous of Seymour Island, Antarctica. The magnitude and direction of this offset is present in data sets from other studies at locations around the world, though the causes are rarely explored. We explore some explanations of this offset given current understanding of living molluscan shell secretion, and compare our observations with carbon isotopic studies of extant shelled cephalopods. We propose that ammonites have more depleted carbon isotope values due to an increased incorporation of metabolic carbon into their shell material, potentially due to a greater metabolic rate than benthic mollusks. We also explore other plausible explanations we consider unlikely, but welcome input from other researchers into the cause of this phenomenon.