A massive deterioration in ecosystems throughout the world occurred during the Frasnian-Famennian (Late Devonian) extinction event. Marine ecosystems were more severely affected than terrestrial ecosystems; reefal ecosystems disappeared entirely. Regional analysis of the Appalachian marine fauna of the eastern United States reveals that at least 70% of the macroinvertebrate species vanished during the extinction event. Though the net loss in species diversity was catastrophic, it was not instantaneous. The major period of extinction was spread over at least seven m.y. Preliminary evidence suggests that the ecosystem deterioration may have been driven by a period of global cooling. As there exists no firm evidence for Late Devonian glaciation, such a cooling trend was not due to ice buildup. Climatic cooling, due to a global period of “darkness” precipitated by asteroidal impact, remains a distinct possibility.