Late Ordovician Mass Extinction: Earth, fire and ice

The Late Ordovician Mass Extinction was the earliest of the 'big' five extinction events and the earliest to affect the trajectory of metazoan life. Two phases have been identified near the start of the Hirnantian period and in the middle. It was a massive taxonomic extinction, a weak phylogenetic extinction and a relatively benign ecological extinction. A rapid cooling, triggering a major ice age that reduced the temperature of surface waters, prompted a drop in sea level of some 100 m and introduced toxic bottom waters onto the shelves. These symptoms of more fundamental planetary processes have been associated with a range of factors with an underlying driver identified as volcanicity. Volcanic eruptions, and other products, may have extended back in time to at least the Sandbian and early Katian, suggesting the extinctions were more protracted and influential than hitherto documented. The Late Ordovician mass extinction was the first of the big five Phanerozoic extinctions and the first to affect animal life in oceans. The extinctions removed about 75% of animal species and were coincident with a major ice age. The search for an ultimate cause has proved difficult for both the extinctions and ice age, but accumulating evidence points to a possible volcanic trigger.