Cephalopod ecology inferred from high-resolution ion microprobe analyses of stable isotopes Benjamin Linzmeier
Modern cephalopods, including squid, octopus, and Nautilus exploit diverse niches throughout the oceans. Cephalopods also have an abundant fossil record spanning ~450 million years. Presumably, extinct groups also filled diverse ecological roles, however their ecology can only be inferred through indirect observations. Ammonoids are a particularly perplexing group of shelled fossil cephalopods because no soft tissue preservation has been discovered and they diverged from all extant cephalopods by ~400 million years ago. By using analogy to modern cephalopods, fossil stratigraphic occurrence, and functional morphology, cephalopod paleontologists have proposed that ammonoids have planktic egg development and active swimming in adults. To test these hypotheses in this thesis, I employ secondary ion mass spectrometry (SIMS) that samples the stable isotope composition of ~10 µm diameter pits and allows analysis of portions of shell that are too small to sample with conventional methods. As a case study to determine if behavior was recorded in a depth-migrating Nautilus, daily growth bands were subsampled by SIMS. Within 1 cm of shell, across ~35 growth bands, a range of oxygen isotope ratios corresponding to ~10 °C was found, demonstrating shell precipitation across a range of temperatures during depth migration behavior. In the shells of adult Cretaceous ammonites from Antarctica and the Mississippi embayment, similar oxygen isotope variability was observed, suggesting they were able swimmers through a temperature stratified water column. Analysis of the pre- and post- hatching shell of Hoploscaphites ammonites from the Fox Hills Formation of South Dakota indicate benthic eggs and planktic hatchlings. Stable isotope transitions are also present at about one whorl of growth and likely show unique habitat change in individuals. This dissertation provides the methodological and interpretive framework for further high spatial resolution isotope analyses of fossil and modern cephalopod material.