THE FIRST GREAT EXTINCTION. THE ORDOVICIAN-SILURIAN EXTINCTION 455-430 Ma

Extinction rate: 85% of species.

Affected lifeforms: almost a half of the living genera, which were then almost all marine life. Drastic reduction of the number of trilobites, cephalopods, brachiopods, graptolites and echinoderms.

Main causes and consequences: global cooling and volcanism induced changes in sea level, darkening of the atmosphere, reduction of photosynthesis and oxygen levels, decrease of bioproductivity, anoxia and ocean acidification.