450 Million B.C.T. - The Ordovician Extinction -- The First Major Extinction Event -- Began

Around 450 million years ago, the first great extinction of life on Earth began. 

The Ordovician–Silurian extinction event, or quite commonly the Ordovician extinction, was the first major extinction event.  Overall, the Ordovician extinction was the second-largest of the five major extinction events in Earth's history in terms of percentage of genera that went extinct and it was the second largest in scope and in the overall loss of life. Between about 450 to 440 million years ago, two bursts of extinction, separated by one million years, appear to have happened. This was the second biggest extinction of marine life, ranking only behind the Permian extinction that occurred 251 million years ago. At the time, all known animal life was confined to the seas and oceans. More than 60% of marine invertebrates died including two-thirds of all brachiopod and bryozoan families. Brachiopods, bivalves, echinoderms, bryozoans and corals were particularly affected. The immediate cause of extinction appears to have been the movement of the supercontinent Gondwana into the south polar region. This led to global cooling, glaciation and consequent sea level fall. The falling sea level disrupted or eliminated habitats along the continental shelves. Evidence for the glaciation was found through deposits in the Sahara Desert. A combination of lowering of sea level and glacially-driven cooling were likely the driving agents for the Ordovician mass extinction.

The Ordovician extinction occurred between 450 to 440 million years ago, during one of the most significant diversifications of life in Earth history. This extinction event marks the boundary between the Ordovician and following Silurian period. During this extinction event there were several marked changes in biologically responsive carbon and oxygen isotopes. This complexity may indicate several distinct closely spaced events, or particular phases within one event.

At the time, most complex multicellular organisms lived in the sea, and around 100 marine families became extinct, covering about 49% of faunal genera (a more reliable estimate than species). The brachiopods and bryozoans were decimated, along with many of the trilobite, conodont and graptolite families.

Statistical analysis of marine losses at this time suggests that the decrease in diversity was mainly caused by a sharp increase in extinctions, rather than a decrease in speciation.

The extinction pulses that comprise the Ordovician extinction event appear to correspond to the beginning and end of the most severe ice age of the Phanerozoic Eon, -- the most severe ice age of our current Eon.  The Ordovician ice age marked the end of a longer cooling trend in the Hirnantian faunal stage towards the end of the Ordovician, which had more typically experienced greenhouse conditions.

The event was preceded by a fall in atmospheric carbon dioxide, which selectively affected the shallow seas where most organisms lived. As the southern supercontinent Gondwana drifted over the South Pole, ice caps formed on it. The strata have been detected in late Ordovician rock strata of North Africa and then-adjacent northeastern South America, which were south-polar locations at the time. Glaciation locks up water from the world-ocean, and the interglacials free it, causing sea levels repeatedly to drop and rise.  The vast shallow intra-continental Ordovician seas withdrew, which eliminated many ecological niches, then returned, carrying diminished founder populations lacking many whole families of organisms. Then they withdrew again with the next pulse of glaciation, eliminating biological diversity at each change. In the North African strata, five pulses of glaciation from seismic sections have been found.

The glaciations incurred a shift in the location of bottom-water formation, shifting from low latitudes, characteristic of greenhouse conditions, to high latitudes, characteristic of icehouse conditions, which was accompanied by increased deep-ocean currents and oxygenation of the bottom-water. An opportunistic fauna briefly thrived there, before anoxic conditions returned. The breakdown in the oceanic circulation patterns brought up nutrients from the abyssal waters. Surviving species were those that coped with the changed conditions and filled the ecological niches left by the extinctions.

The end of the second phase of the Ordovician extinction event occurred when melting glaciers caused the sea level to rise and stabilize once more. The rebound of life's diversity with the sustained re-flooding of continental shelves at the onset of the Silurian Period saw increased biodiversity within the surviving orders.