BIG COMEBACK: RAPID AND RESILIENT NICHE OCCUPATION IN EARLY TRIASSIC AMMONOIDS

Recovery of pelagic faunas after the end-Permian mass extinction, during the Early Triassic, has been a topic of intense interest. A new study largely using Canadian Geological Survey collections by T. Tozer to assess ammonoid ecological niche loading at sub-zone stratigraphic resolution demonstrates ecological dynamics consistent with recognized patterns in Early Triassic ammonoid diversification, biogeography, and disparity. We use Westermann Morphospace (Paleobiology 38:3) to sort shells into four mutually exclusive morphotypes (oxycone, serpenticone, platy/planorbicone, sphaerocone). In ammonoids’ modern relatives, locomotion scales directly with metabolism, so ammonoid shell shape is a good indication of ecological roll and physiological susceptibility to environmental perturbation. Only oxycones likely supported fast, directed swimming. In this study, within-zone comparisons of Early Triassic type specimens (Tozer 1994) to intraspecies variation (Survey collections; n>100) yield no significant changes in morphotype representation. Through 14 boreal subzones, ecological niche loading is remarkably stable, despite continual genus and species turnover. Though represented by the fewest species, oxycones are present throughout all but late Griesbachian zones. Species diversity within each morphotype significantly tracks overall species diversity, but does not significantly correlate to diversity within any single clade. During the Smithian, lower latitudes (collections from South China, Brayard and Bucher 2008) contain significantly more species of oxycones. This difference in oxycone species richness relates to the latitudinal faunal gradient and the presence of equatorial oxygen minimum zones, a dynamic habitat for modern cephalopods. We interpret temperature as the critical stress during Early Triassic ammonoid diversity crashes because these extinctions are not morphologically selective, and modern cephalopods of high and low metabolisms share similar thermal stress thresholds. Finally, after the end-Triassic mass extinction, earliest Jurassic ammonoids were dominated by serpenticones, which provides significant contrasts to rapid Early Triassic niche occupation.