A NEW STATE OF THE MARINE ECOSYSTEM DURING THE LATE PALEOZOIC GLACIAL INTERVAL

A long interval of late Paleozoic time was marked by persistently low rates of turnover for marine animal genera. This interval coincided precisely with the existence of massive glaciers in Gondwanaland. Its inception is marked by a global disconformity at the Mississippian-Pennsyvanian boundary, which reflects an expansion of continental glaciers that coincided with a second-order mass extinction of marine life. The distinctive interval ended during the Sakmarian Age of the Early Permian during a major eustatic sea level rise. Mean rate of generic origination and extinction for every major taxon of Sepkoski’s Paleozoic and Modern faunas was lower for the interval of massive glaciation than for Paleozoic intervals characterized by background turnover rates and for the two Mississippian intervals that immediately preceded the glacial expansion.

The Cenozoic onset of the massive glaciation in Antarctica was also associated with a second-order mass extinction, in late Eocene time, but failed to produce a conspicuous decline in turnover rates for marine genera; mean rates of origination and extinction for major taxa were not significantly lower for the Oligocene-Pliocene interval than for middle Eocene time. Not only were rates of evolutionary turnover universally low during the late Paleozoic interval of massive glaciation, but the rate of recovery of global diversity from the terminal Mississippian crisis was sluggish, in contrast to the rapid recoveries that followed other Paleozoic mass extinctions. These traits for the glacial interval are well known for faunas of high southern latitudes, which also became less cosmopolitan in distribution. What is remarkable is that they also characterize faunas of the tropics and of extratropical northern regions. Thus, it appears that the inception of the late Paleozoic glacial interval had a unique effect, abruptly shifting the marine ecosystem to a new global state that persisted for tens of millions of years.