Stable isotope data (δ18O, δ13C) of the ammonite genus Simbirskites — implications for habitat reconstructions of extinct cephalopods

Abstract

The accurate habitats of the extinct ammonites and belemnites are largely unknown. Most ammonites are thought to have had a pelagic lifestyle, while belemnites are often considered to have dwelled in deeper, colder waters. Stable isotope analysis (δ18O, δ13C) provides a useful method to reconstruct the habitats of these two groups of extinct cephalopods, but is at the same time limited by various unknowns, including the isotopic composition of the past seawater, ‘vital’ effects and diagenesis.In this paper, stable isotope data from 12 ammonite shells (Simbirskites spp.) from the upper Hauterivian of the Lower Saxony Basin (northwest Germany) are presented. Ontogenetic isotope profiles have been obtained from six of these shells in order to reconstruct habitat changes during their lifetime. These new ammonite data are combined with published stable isotope data from belemnites (Hibolithes jaculoides) and TEX86 palaeotemperature-estimates from the same section.The combination of isochronous ammonite and belemnite stable isotope data shows that the resulting values of both groups go along with each other. Calculated palaeotemperatures of the two groups differ by 5–10°C, with belemnites always showing the colder temperatures. All juvenile shells show an increase of the δ13Camm to more positive values throughout their ontogeny; they are stable in an adult shell-fragment. The incorporation of carbon isotopes in the ammonite shell therefore correlates with ontogeny and is influenced by metabolic effects, probably linked to the shell growth rate. The isotope profiles further suggest vertical habitat changes during the ontogeny of the ammonites, indicating that the δ13Camm values might record a combination of environmental and metabolic signals. Taking into account studies on oxygen isotope fractionation of recent aragonitic and calcitic mollusc shells, a similar pelagic habitat is proposed here for both, ammonites and belemnites.