EXTINCTION PATTERNS OF WESTERN LAURUSSIAN TROPICAL BENTHIC FAUNAS SUGGEST INTENSIFICATION OF LATIDUINAL TEMPERATURE GRADIENT AND RAPID CLIMATE CHANGE AS MAJOR CAUSE OF LATE FRASNIAN KELLWASSER EXTINCTIONS

Late Frasnian and early Famennian oxygen isotope records derived from conodont apatites from subtropical sites reported by Joachimski et al. record relatively rapid Sea Surface Temperature (SST) cooling events associated with the late and latest Frasnian stepped Kellwasser global extinction bioevents. Faunal records of benthic shelly faunas spanning the late Frasnian-early Famennian (Late Devonian) interval from four sites situated in southern hemisphere subtropical and near equatorial tropical paleolatitudes in western Laurussia document the step-wise Kellwasser extinction bioevents. Prior to the Lower Kellwasser event alpha diversity, subtropical and near equatorial platform brachiopod and coral faunas are similar. The most significant collapse of brachiopod alpha diversity (up to 70% and higher) and near total extinction of colonial rugosan corals is observed following the Lower Kellwasser bioevent in the subtropical sites. Resumed warming of the tropics, reflected by apatite-derived SST calculations, is evidenced by higher alpha brachiopod diversity in near equatorial sites as a consequence of higher species survivorship and recruitment of taxa via migrations from eastern Laurussian carbonate and possibly Siberian platforms. Frasnian rugosan diversity peaked in reef associations in the near equatorial Kakisa Platform (NWT) following the Lower Kellwasser bioevent, whereas brachiopod and rugose coral alpha diversity remained low in subtropical sites. This pattern appears to reflect a climatic intensification of the latitudinal sea surface temperature gradient within the tropical ocean in the intervening interval between the Lower and Upper Kellwasser events perhaps with lower temperatures reducing the rate of brachiopod and colonial rugosan recruitment in the subtropical sites versus warmer equatorial sites. Rapid and steady decline of SST in the very latest Frasnian appears to have contributed to the final decline of the Frasnian platform shelly faunas and stromatoporoid-coral reef ecosystem during the Upper Kellwasser bioevent across the tropics of western Laurussia.