STABLE ISOTOPE SCLEROCHRONOLOGY OF BACULITES REVEALS POTENTIAL GROWTH RATE ESTIMATES

Many aspects of ammonite life histories are either unknown or poorly constrained, despite their frequency in the fossil record. It is currently unknown whether ammonites lifespans are more similar to most squid and octopus, which have short (1-2 year) lifespans, or more similar to Nautilus which live closer to 20 years. These groups, which are distinct in many other important ways, often serve as an extant phylogenetic bracket for ammonites and have both been used to argue for ammonite characteristics that are otherwise generally unknown. Here we present preliminary data suggesting that Baculites may have had shorter lifespans – less than 5 years. This short lifespan implies that ammonites utilized an r-selection life strategy, in which they produce large numbers of offspring. Massive spawning events could explain the dense concentrations of Baculites shells found throughout Late Cretaceous marine deposits.

In some ways, Baculites are ideal targets for sclerochronological approaches because of their mostly uncoiled morphology allows easy access to shell sampling. However, this uncoiled structure may fragment more easily during post-mortem transport, and in most fossils only a portion of the shell is preserved, and full Baculites specimens are rare. Several long (20-40 cm) Baculites fragments from the Western Interior Seaway and Mississippi Embayment were analyzed sequentially with bulk stable isotope geochemical techniques (δ¹³C, δ¹⁸O). The microstructure of aragonite samples was analyzed using an SEM and the preservation index of Cochran et al. (2010). Those fossils that were well-preserved generally produced isotopic patterns in both δ¹³C and δ¹⁸O that were fit with sinusoidal curves, with a period consistent with a seasonal cycle of 30-35 cm long. Due to the limited lengths of Baculites sampled specimens, the interpretation of these patterns as seasonal is necessarily tentative, and because of their mobility, both geographically and vertically within the water column, it is difficult to assign clear climatological or behavioral interpretations to these isotopic shifts. Further results from sclerochronological sampling of Baculites aptychi will be presented at the conference. While their small size limits temporal resolution, they provide an opportunity to sample the full lifespan of the organism.