The Late Devonian extinction event: evidence for abrupt ecosystem collapse

The Late Devonian extinction event was not geologically “instantaneous,” in that extinctions during the epoch are not concentrated into a single sharp pulse at the end of the Frasnian. Extinction rates are elevated for a period of at least 2 to 4 m.y. during the middle and late phases of the Frasnian, with maximum rates occurring generally 2 m.y. before the terminal Frasnian. Neither was the Late Devonian biotic crisis a “gradual” event. In the analysis of the evolution of ecosystems, it is misleading to consider the pattern of extinction rates alone. Frasnian marine ecosystems flourished during the same time interval characterized by elevated extinction rates because origination rates of new species are higher, per time interval, than corresponding extinction rates. This pattern of relative origination/extinction rates abruptly reversed during the latest Frasnian—precipitating a rapid loss of species diversity. Within limits of current stratigraphic correlation, the ecosystem collapse appears to have occurred simultaneously in such widespread geographic regions as New York State (U.S.A.) and the southern Urals (U.S.S.R.). In viewing the Late Devonian event from an ecological perspective, the most important question is not “What triggered the elevated extinction rates?”, but rather “What was the inhibiting factor that caused the cessation of new species originations?”