LIFESPAN, GROWTH RATE, AND ECOLOGY OF A GIANT HETEROMORPH AMMONITE FROM ANTARCTICA (Invited Presentation)

Shells of heteromorph ammonites (Suborder Ancyloceratina) encompass a surprising array of bizarre forms that defy even the simplest of ecological inference. The large (~1.5 m), paper-clip shape of the hammitcone heteromorph Diplomoceras maximum from the late Cretaceous Lopez de Bertodano Fm of Antarctica is no exception. Benthic/nektonic/planktonic? Predator/herbivore/planktivore/scavenger? Orientation of the shell in life? Long-lived and slow growing or short-lived and fast? All are fraught with significant uncertainty. Here, we use stable oxygen and carbon isotope analysis of serial samples collected along a .5m section of shell later in ontogeny in an attempt to shed light on how this enigmatic animal might have lived. The data reveal that 1) δ¹⁸O and δ¹³C covary; 2) both are consistently depleted in shell material between the pronounced sculptural ribs on the shell surface; 3) δ¹⁸O values are unremarkable and consistent with other well-preserved skeletal benthos from the formation; and 4) δ¹³C values are exceptionally negative, as low as -34‰, between adjacent ribs, with values on ribs near 0‰. Data suggest precipitation in close association with the methane seeps documented from the unit and/or significant disequilibrium precipitation. Interpretation of a broad sinusoid in δ¹⁸O superimposed on intra-rib variation is key in evaluating the two possibilities. If reflecting the annual temperature cycle, growth was very fast, episodic, and consistent with disequilibrium between the ribs. If simply coincidental and the intra-rib variation itself reflects the annual cycle, then methane venting happened preferentially during the austral summer and methane-derived dissolved inorganic carbon (DIC) was regularly incorporated into shell material. The former implies a lifespan around 9 years, while the latter demands a lifespan in the neighborhood of 200 years. Both are remarkable, either in requisite precipitation rate or longevity, and both could be considered consistent with living cephalopod analogs. Recently published data from co-occurring bivalves exhibit very low δ¹³C values in association with clear annual growth bands, thereby requiring that intra-rib depletions are annual. Ammonite paleobiologists must therefore find ways to accept the reality of extreme longevity in this enigmatic animal.