Friday, February 20, 2015

Ordovician-Silurian Mass Extinction

The third largest extinction in Earth's history, the Ordovician-Silurian mass extinction had two peak dying times separated by hundreds of thousands of years. During the Ordovician, most life was in the sea, so it was sea creatures such as trilobites, brachiopods and graptolites that were drastically reduced in number. In all, some 85% of sea life was wiped out. An ice age has been blamed for the extinctions - a huge ice sheet in the southern hemisphere caused climate change and a fall in sea level, and messed with the chemistry of the oceans.

A diorama portraying the seas of the Ordovician Period (from the Exhibit Museum, University of Michigan)

 

When did it happen?

 

Around 445-440 million years ago, possibly in 2 phases

What went extinct?

26% of all marine families, 60% of all genera, an estimated 82-88% of all species.
Although no major groups were completely lost, many sea creatures suffered substantial losses. These included nautiloids (cousins of today's squid and cuttlefish), which were the top marine predators of the time, as well as brachiopods, corals, bryozoans, echinoderms, graptolites and eel-like conodonts. Over 90% of trilobite species disappeared.

Nautiloids are a large and diverse group of marine cephalopods (Mollusca) belonging to the subclass Nautiloidea that began in the Late Cambrian

Causes

At the end of the Ordovician Period, the world entered an intense ice age, possibly brought about by the location of the supercontinent Gondwana over the southern pole.

The formation of large ice sheets meant sea levels fell dramatically, perhaps by as much as 70-100m. This particularly affected the corals and bryozoans that were living in shallow inland seas, which drained of water. Global cooling spelt disaster for warm-adapted species that had nowhere to migrate to.
Then, after about 1 million years, the glacial conditions ended rapidly. Sea levels rose, water low in oxygen blanketed the shallow marine habitats, and deep ocean waters stagnated, delivering a second blow to the marine life that had managed to survive.

 

 


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