COMBINED CARBON AND OXYGEN STABLE ISOTOPE EXAMINATION OF WESTERN INTERIOR SEAWAY AMMONITES MAY GIVE INSIGHT INTO THEIR HABITAT AND BIOLOGY

Ammonites are common and abundant fossils, which has led to increased use as paleothermometers using oxygen isotopes (δ¹⁸O), but much is still unknown regarding these organisms' ecology and biology. Carbon isotopes (δ¹³C) are simultaneously collected with δ¹⁸O in the carbonate shells of these organisms, and while δ¹³C is a complicated proxy to interpret, interrogation of δ¹³C of these organisms may yield insights regarding ammonite biology (e.g., metabolism) and environment.

This study explores the implications of paired δ¹³C and δ¹⁸O values from 113 ammonites of various taxonomic groups (scaphitids, baculitids, and heteromorphs) from the Western Interior Seaway. The specimens span the last 25 million years of the Cretaceous, including 45 Western Interior Ammonite Taxon Range Zones (AZ). To make comparisons between groups of similar ages, we created 12 “Compiled Zones” (CZ) by combining 2-4 AZs in each. Only one CZ (late Campanian) has all 3 morphotypes. We then combined scaphitids with other heteromorphs to compare against baculitids, resulting in 5 CZs in which morphotype groups can be compared.

In 4/5 zones, heteromorph ammonites have more positive δ¹³C values than the baculitids. If this δ¹³C difference is driven by metabolic rate, this would indicate that heteromorphs have lower metabolic rates than baculitids. Other explanations may be environmental preferences reflecting the DIC of the habitat i.e., stratification of the water column with more positive δ¹³C_DIC near the surface, or nearshore proximity with more negative δ¹³C_DIC. Additionally, 3/5 CZs demonstrate heteromorphs have more negative δ¹⁸O values than the baculitids, wherein one of the CZs has identical δ¹⁸O and only 1/5 CZs indicates heteromorphs being more positive than baculitids. Baculites δ¹⁸O being more positive would indicate colder temperatures (or higher δ¹⁸O_SW) such that they lived deeper in the water column, which is consistent with the δ¹³C being more negative than heteromorphs with δ¹³C_DIC stratification.

Due to the complexity of δ¹³C as a proxy, future work with this study will include examination of stable isotopes of multiple specimens within single carbonate concretions, thus removing the confounding factors of spatial and temporal variability in the present dataset. Additionally, we will be adding longer ontogenetic serial sampling to consider the implications of metabolic rate.