GLOBAL MACROEVOLUTIONARY RATES FOR BRACHIOPODS DRIVEN BY FATE OF STENOTOPIC GENERA DURING THE LATE PALEOZOIC ICE AGE

The usual temporal pattern of extinction and rebound that characterized the Paleozoic history of marine life was interrupted in late Paleozoic time by a conspicuous interval of sluggish diversification and low rates of origination and extinction. This unique interval coincided with a period of extensive glaciation termed the late Paleozoic ice age (LPIA). Following a second-order mass extinction at the onset of the LPIA in late Mississippian time, the global fauna rebounded only weakly, and rates of origination and extinction for all major groups of marine organisms fell and remained low until glaciation waned in middle Permian time.

The causal biogeographic changes underlying these global evolutionary patterns were investigated using a database of stratigraphic and geographic occurrences of brachiopod genera compiled from the primary literature. Latitudinal distribution patterns reveal that the tropical brachiopod fauna suffered great losses of diversity relative to the fauna at higher latitudes during the onset of the LPIA, and rediversified strongly as the LPIA waned. These latitudinal patterns were created by the fate of genera with narrow latitudinal ranges (stenotopic genera), whose proportional diversity was highest at the equator and declined toward the poles, and who suffered the highest rates of extinction at the onset of the LPIA in late Visean and early Serpukhovian time. When glaciation waned in late Sakmarian and Artinskian time, stenotopic genera rebounded abruptly via high origination rates and restored the global fauna to its pre-LPIA configuration.

As geographic range size is correlated with stratigraphic duration, the global brachiopod fauna entered a new macroevolutionary state when the LPIA began, characterized by broadly-adapted genera with long stratigraphic durations at all latitudes. The major effects of the LPIA were felt in the tropics, which experienced the most drastic change in composition. The long average durations of the constituent genera during the LPIA caused rates of origination and extinction to fall and remain low, and diversification to remain sluggish, while the climatic regime of the LPIA persisted.