Ammonite suture complexity as a paleoclimate indicator comparable to δ18O stable isotope ratios

The exact genesis and function of ammonite sutures have long been sought as a simple answer and debated fiercely for decades. While possible functions of the highly iterative septal fluting, particularly that seen in the Jurassic and Cretaceous, may be complex, multifaceted, and mutually inclusive, the true mechanism for the formation of ammonite sutures almost certainly contraindicates all other possible methods of generation, and as a result, has been particularly unsettled in paleobiological literature. Reasons for ammonite suture complexity have been discussed both formally and informally in the ammonoid community and include capillary maximization of cameral fluid, age-related wrinkling of the rear mantle, branching of soft tissues in the rear mantle that compare with dendronotid sea slugs, survival of bite force or increased water pressure as per the Westermann morphospace, and the famous “tie points” and “viscous fingering” models of the mid-twentieth century. Here, we present evidence of another impetus for high complexity in ammonite sutures: cold seawater. We examine the strengths of ammonite suture complexity as a standalone or auxiliary index of sea-surface temperature and paleoclimate data that follows the same patterns as δ¹⁸O ratios, particularly in cool conditions, such that ammonites whose isotopes indicate a cooler climate tend to have high fractal complexity in the adult (and sometimes juvenile) stages. Coldwater ammonite faunas exclusively exhibit septa with fractal dimensions in the highest possible ranges (Df  > 1.6), such as the transition from temperate to cold conditions between the Campanian and Maastrichtian Stages on the Antarctic peninsula. Ammonite sutures also follow clear patterns of increasing fractal complexity descending thermoclines in a single locality, such as the Western Interior Seaway. Although it is impossible to standardize overall sea-surface temperature (SST) through stable isotope ratios, which require context-specific isotope examination, our results suggest that septal organogenesis of Late Mesozoic ammonites is deeply linked to climate-driven physiological inputs such that ammonites may be a functional tool for understanding Cretaceous ocean temperatures. Ammonite sutures simplify paleoclimate analyses in the Cretaceous because their fractal complexity seems to always follow a temperature-driven pattern, helping to reduce the noise around δ¹⁸O from locations with complicated isotopic contexts, such as the Western Interior Seaway. We intend to continue this work to further develop and understand the full implications of fractal analysis of ammonite sutures as a novel tool for reconstructing Cretaceous paleoclimate.