BIGGER ON THE INSIDE: SOUTH POLE MAASTRICHTIAN AMMONITES AND BERGMANN’S RULE

Ammonite sutures usually show stasis at either the genus or family level for the duration of their temporal range. However, Late Cretaceous ammonites show specific physiological changes affecting body size, heterochrony, and metabolism when climate becomes sufficiently extreme. During the Maastrichtian Stage, genera associated with the South Pole region are also those with the highest values of sutural complexity, despite some of their closest relatives in other biomes having vastly less complex suture patterns. I compared the extremely high mean fractal dimensions and whorl diameters of these ammonites with the same traits in ammonites from other climates during approximately the same time period; as well as O-18 and C-13 isotope ratios. There is a strong correlation between exceptionally high septal complexity and cold-water paleoenvironment in mid-latitudes for several desmocerids, pachydiscids, and diplomoceratids, including global cooling associated with the European “Puzosia Event." This suggests that ammonite septa respond to Bergmann's rule. Though folded many times, the septa are larger overall, and as sutural complexity is in a near-perfect inverse relationship with septal thickness, ammonites appear to grow larger once they have reached maximum septal thinness. Analysis of the exceptionally high sutural complexity of Antarctic ammonites from the Zinsmeister Collection (PRI) compared lower complexity and average size in warm-water ammonite faunas of the Maastrichtian Stage from the Western Interior Seaway, Izumi Group, and European localities indicate that high complexity as well as large shell size have a direct relationship with seawater temperature.