TERRESTRIAL-MARINE LINKS IN THE FRASNIAN-FAMENNIAN EXTINCTION EVENT

The Frasnian/Famennian Extinction Event affected both marine and terrestrial ecosystems, as demonstrated by the diversity record of marine invertebrates, land-plant spores, and macrofloras. Marine extinctions associated with this event appear linked to widespread anoxia in shelf environments, which began in the Givetian and peaked with the Hangenberg event at the Devonian/Carboniferous boundary. Data from the Paleobotany Database Project and other sources indicate that land-plant macrofloras experienced a diversity minimum in the Late Frasnian through the mid-Famennian. Spore diversity dropped in the Late Frasnian and remained low into the Early Famennian. A variety of causes have been proposed to explain the Frasnian/Famennian event including climate change (both global warming and cooling) and increased influx of terrigenous sediments to continental shelves caused by the appearance and spread of deeply rooted forest ecosystems. Yet few of these causal hypotheses have been evaluated in light of the evidence suggesting a linked marine and terrestrial diversity crisis at the Frasnian-Famennian boundary. Terrestrial and marine biogeographic patterns in the Late Devonian and earliest Carboniferous indicate cosmopolitan floras and faunas, despite evidence of continental glaciation. This cosmopolitan biogeographic pattern differs radically from the highly provincial pattern associated with continental glaciation in the Permo-Carboniferous and Late Tertiary, both ice-house intervals. It suggests that Late Devonian glaciation had a very different effect on global climate, perhaps due to relatively high levels of atmospheric CO2 during this interval. The appearance and spread of deeply rooted forests in the late Givetian through Famennian may have triggered widespread anoxia on marine shelves due to the influx of terrigenous sediments. However, this mechanism can not explain diversity loss among terrestrial land plants.