USING ISOTOPIC COMPOSITION TO DETERMINE GROWTH AND ECOLOGY IN BACULITE AND SCAPHITE AMMONITES

Ammonites have constituted a primary data source for the fields of evolution, paleoceanography, biostratigraphy, and paleoecology for more than a century, yet their ecologies remain poorly understood. The upper Maastrichtian Owl Creek Formation exposed in Tippah County, Mississippi, contains a diverse assemblage of well preserved ammonites, bivalves, and gastropods, and therefore is an excellent site to examine the ecology of two ammonite taxa, the Baculitidae (Baculites sp., Baculites sp. C, Eubaculites latecarinatus, and Eubaculites carinatus) and the Scaphitidae (Discoscaphites iris, Discoscaphites minardi, and Discoscaphites sp.). A previous isotopic study indicated that Owl Creek baculites and scaphites lived near the seafloor, but the range of measured isotopic values suggested the possibility that these taxa could have migrated to deeper (cooler) water at times. Here, we test the migration hypothesis by comparing ontogenetic isotopic trends in the shells of co-occurring ammonites, bivalves, and gastropods. Over eight hundred isotopic measurements through ontogeny were taken from 35 individuals distributed amongst these four groups. These new data were combined with the previous isotopic work for a total dataset of over one thousand measurements. We found δ¹⁸O values to be generally similar among baculites, scaphites, Nucula (Bivalvia), and various gastropods, and that there are no consistent δ¹⁸O trends through ontogeny in baculites or scaphites. Thus, there seems no need to invoke regular migration for either ammonite group to colder offshore regions. Additionally, the within-shell δ¹⁸O variability is low in scaphites and baculites, intermediate among gastropods, and high in Nucula, indicating that temperature variability must have occurred on timescales that were long relative to the rate of shell formation for ammonites but short relative to that the lifespan of many gastropods and most Nucula. This interpretation implies rapid growth among baculites and scaphites, a conclusion consistent with lower δ¹³C values in scaphites and baculites compared to other molluscs.