The Ordovician-Silurian Mass Extinction (OSME) was the first "Big Five" events. It happened between 450 million years to 440 million year ago which can be separated to two global extinction events by one million years (during Hirnantian Age of Ordovician Period and the subsequent Rhuddanian Age of the Silurian Period). It was known as the second largest extinction event of marine life just below the Permian-Triassic Extinction because of the tremendous number of marine species lost. Besides, it has been ranked as the third largest extinction in the "Big Five" because of the percent genera lost.
(The Ordovician-Silurian Mass Extinction diagram)
In the Ordovician-Silurian Mass Extinction, an estimated 85% of species became extinct, 61% of genera and 12-24% of families (Brenchley,et al. 2001).
In details, about 60-75% of genera in the mainly benthic groups (eg. branchiopods, echinoderms, corals, trilobites and bivalves) went extinct., about 85% of the pelagic graptolites and 89% of the conodonts went extinct simultaneously. In all, more than 100 families of marine invertebrates died out in this extinction event.
(What species lived in the sea in Ordovician period）
As mentioned in part one, the OSME can be divided into two phases. Some groups like brachiopods and trilobites had the similar levels of extinction in these two phases, while the extinction of graptolites groups took place mostly in phase one. 
(Figure shows the diversity changes in the OSME)
Why did it happen?
To put it simple, the most possible culprit that caused the OSME was the change of climate which could break the balance of environment. This was also the reason why the extinction rate was particularly high around the tropical areas in the two phases mass extinction as the long-established environment stability there leaded to the evolution of poor adaptive species.
(Figure presents the Late-Ordovician-Early Silurian stratigraphy, isotope stratigraphy, environmental change and survival faunas)
The stable isotopic composition of marine carbon indicate the global environmental changes that
- the oxygen isotope excursion suggests the growth of ice caps and drop of seawater temperature
- the carbon isotope excursion indicates the onset of major changes in carbon cycle
To explain in details, the main cause of the two phases mass extinction was a brief glacial interval (Sheehan,2001).
The primary phase occurred at the beginning of the glaciation (Katian/Hirnantian stage) coincided with a rapid growth of south polar ice sheets on Gondwana. Together with the expansion ice sheets, there were a substantial cooling of the tropical oceans, a major perturbation of global carbon cycle and a large sea-level decline which drained the wide cratonic seaways. This situation made it difficult for all the shallow marine life (most of the marine life at that time were the shallow type) to live on Earth.
After the first phase extinction when the survival faunas can adapt to the new ecologic condition, the second phase occurred. The glaciation ended unexpectedly which leaded to a rise in sea-level and temperature, and also resulted in a stagnation of oceanic circulation. The sudden change in the world made some life difficult to survive which was the second phase extinction.
Those fall and rise sea-level, changes in the oceanic structure, nutrient fluxes,etc. which were all related to climatic change leaded to the consequence of the Ordovician-Silurian Mass Extinction.
Reference:
P.J.Brenchley,et al. (2001) Do all mass extinctions represent an ecological crisis? Evidence from the Late Ordovician. Department of Earth Sciences, University of Liverpool, Liverpool, UK.
P.M.Sheehan (2001) The Late Ordovician Mass Extinction. Department of Geology, Milwaukee Public Museum, Milwaukee, Wisconsin 53233.
