Beginning 455 MYA, the mass extinction event ending the Ordovician (485–443 MYA) was incited by 2 glaciations with global impact separated by a million years. The 1st glaciation came on the heels of an extraordinarily warm world during the early and middle Ordovician; this despite the Sun basking Earth with 5% less radiation than today.
There must have been a 10% greater concentration of greenhouse gases for such warmth. How that came about, whether by volcanism or a different carbon cycle, is not known. An elevated level of greenhouse gas during hothouse meant low atmospheric oxygen levels: something which organisms had adapted to by the Late Ordovician.
The toll of the Ordovician–Silurian extinction event was: 12% of marine families, encompassing 96% of marine species, at a time when most macroscopic life lived in the seas. More than 60% of marine invertebrates died. In terms of diversity loss, this was the 3rd-worst extinction in the history of life.
During the Ordovician Gondwana shifted south, into the polar region. This, along with the freshening impact of plants’ arrival on land, led to global cooling. The cooling initially delivered anoxic (low oxygen) bottom waters, especially beneath regions of high sea productivity, such as those zones fed by nutrient-upwelling currents. This anoxia accelerated extinction.
The onset of glaciation altered the carbon cycle, with the atmospheric oxygen level rising. This was after much extinction had already transpired.
Glaciation lowered sea levels by 70–100 meters. The drop devastated habitats on the continental shelves.
After the 1st glaciation, the planet reeled from frigid icehouse to hothouse in a half-million years. A strong thermocline developed, returning the deep ocean to its anoxic state.
Sea levels rose rapidly, blanketing shallow marine habitats with anoxic waters. The benthic faunas on the continental margins that had managed to survive the freeze succumbed from the warming.
The species that survived tended to be small and simple. Reducing body size shortens the time to reproductive maturity. This evolutionary strategy provides more opportunities for adaptation from one generation to the next.
The end-Ordovician extinction pulses favored life forms with greater tolerances to changing conditions. This adaptive stratagem would replay in succeeding extinction events.