Hirnantian Stage

Geophysical

• Major climatic oscillation caused a major mass extinction event. 

• When the Hirnantian stage began, the Earth's climate was hot and sea-levels were substantially higher than today. 

• The seas were filled with a diverse fauna. 

• However, there is considerable evidence to indicate that many of these species were already in trouble when the Hirnantian began, and that overall biological diversity was already in sharp decline.

• As the climate cooled and glaciers formed during the early part of the Hirnantian, sea level dropped. 

• Estimates of the depth of this drop range from more than 50 meters (based on studies in Nevada and Utah) to more than 100 meters (from studies in Norway and the United Kingdom). 

• More recent research indicates a worldwide reduction in sea level of approximately 80 meters. 

• This drop dried up and exposed the extensive shallow-water continental shelves that existed throughout the world at that time, causing the extinction of large numbers of species who depended on this shallow water environment. 

• Those species that survived were greatly reduced in numbers.

• The overall result was a major reduction in the biodiversity of the world's oceans.

• During this ice-house period (as scientists sometimes call periods of global cold weather and glaciation), surviving species began to adapt.

• Cold environment species replaced the warm environment species that had thrived during the previous hot period (hot-house to scientists).

• Just as the species were adapting, however, the climate changed again. 

• During the latter part of the Hirnantian, temperatures rose, the glaciers melted, and sea level rose to the same or perhaps an even higher level than before the glaciation. 

• The exposed shelves were flooded, resulting in additional extinctions among the fauna that had survived the first extinction event.