IMPLICATIONS FOR AMMONITE ECOLOGY AND PHYLOGENY FROM CARBON ISOTOPES

Ammonites have been studied extensively, but due to a lack of modern analogues, it has been difficult to assess possible paleoecological differences between morphological groups. It has been proposed that morphological variations across ammonite families reflect active and passive swimming styles whichmay have affectedmetabolic rates. Functional morphology principles imply that hydrodynamic morphologies, such as planaspiral ammonites, were active swimmers, and less hydrodynamic morphologies, such as baculitids, were more passive. Recent studies have proposed that the carbon isotopic composition of ammonite shells may be influenced by metabolic differences between mollusks. If this is true, then morphological variations between ammonites may result in shells with different isotopic values.

To explore this proposition, we compared the stable isotopic ratios of fossilized shell material from planaspiral, scaphitid, and baculitid ammonites as well as contemporaneous benthic mollusks. The physiology of benthic mollusks is generally better understood than that of ammonites, and they serve as a comparison due to their immobile nature relative to free swimming ammonites. Specimens were sourced from the Western Interior Seaway of North America and are Cretaceous (~80-70 Ma) in age. Sample collection for stable isotopic analysis was performed using a computerized micromill or hand drill and analyzed with an isotope ratio mass spectrometer. We observean overlap in the δ¹³C values of baculitid and planaspiral ammonites, which may be explained by two interpretations. Metabolic rates between the two genera may be similar, implying that our understanding of the ecology of these groups may be incorrect. Alternatively, δ¹³C variation in ammonites may not reflect differing metabolic rates but rather another physiological factor.Notable similarities in oxygen isotopic (δ¹⁸O)values between planaspiral and baculitid ammonites further indicate a shared position in the water columnduring life. Additional data, from scaphitid ammonites and benthic mollusks, will provide a broader framework for comparison.