GROWTH RATE AND ECOLOGY OF THE GIANT HETEROMORPH AMMONITE DIPLOMOCERAS MAXIMUM USING STABLE ISOTOPES OF ACCRETIONARY SHELL CARBONATE

Diplomoceras maximum is a large, hamitcone heteromorph ammonite with a shell that resembles a giant paperclip. A 1.5 meter long specimen representing more than 3 meters of linear shell growth from the late Cretaceous Lopez de Bertodano Formation of Seymour Island, Antarctica, now resides at the Paleontological Research Institution (PRI) in Ithaca, NY. Its growth rate and ecology have been the subject of much discussion but are completely unknown. Stable carbon and oxygen isotope analysis of serially sampled shell material can provide insight into the growth and habitat of these peculiar cephalopods. A roughly half-meter section of shell containing a hook and both adjoining portions of shafts, with aperture diameter ~15 cm, was sampled at a resolution of 5 samples per sculptural rib on the shell (total of 145 analyses). δ¹⁸O values vary between +1.9 to -1.7‰, roughly the same range expressed by co-occurring benthic mollusks. A 15-point moving average defines a broad sinusoid that likely reflects ~1.5 years of shell growth. If so, and if accretion were constant, the PRI animal would have been about 9 years old at death. Superimposed on this sinusoid is regular variation of up to 2‰ that corresponds to sculptural ribs and covaries with δ¹³C, suggesting the potential for disequilibrium effects during precipitation of ribs or, potentially, repeated vertical migrations through the water column. Carbon isotope values overlap those of typical benthic mollusks and other ammonites, but also include extremely negative values (+1.7 to -30‰). Values are low between ribs and higher on them; regular swings to low values become significantly more extreme in the hook than they are in the shafts. If related to disequilibrium and/or incorporation of metabolic CO₂, this suggests more rapid growth in the hook section. Exceedingly negative δ¹³C values may also reflect precipitation in the presence of methane, as suggested in the literature by unusual carbonate precipitates, chemosymbiotic cold-seep bivalve taxa, and very low δ¹³C values of cemented burrow fills. Taken together, our data suggest a pelagic habit near the bottom, with a vertically oriented shell, closely associated with cold methane seeps. Shell growth is fast, consistent with living coleoids, and large individuals are likely less than 10 years old at death.