AMMONOID MACROEVOLUTIONARY PATTERNS TRACK EPISODES OF CHAOTIC CARBON CYCLING DURING THE EARLY MESOZOIC

Episodes of mass extinction are the largest known events of biodiversity loss in the geologic record, and can be used to test hypotheses of biodiversity-ecosystem stability. We present the first correlation between ammonoid macroevolutionary patterns and paleoenvironmental change as represented by variations in carbonate and bulk organic carbon isotopic values for the Late Permian through Early Jurassic, focusing on the end-Permian and Triassic-Jurassic mass extinctions. Genus-level ammonoid diversity from a single biogeographic realm shows that nearly all taxa disappeared coincident with major carbon isotopic shifts to lighter values. In both cases, however, major diversity loss had already occurred at the end of the penultimate stage so that clades had little standing diversity to lose at mass extinction levels. The immediate post-extinction intervals following these two major mass extinctions were characterized by one (Hettangian) or more (Griesbachian to middle Anisian) chaotic oscillations of carbon cycling and were composed of ammonoid faunas characterized by higher proportions of passively floating, non-swimming morphotypes than before or after these events. Overall diversity was highest during intervals of stable carbon isotope values. We also present new isotopic data for the upper Norian Stage that demonstrates for the first time a negative carbon isotope excursion coincident with the major ammonoid extinction episode previously identified from this stratigraphic interval.