Ordovician Extinction: 440 mya

In the Ordivician period most of the world's land formed part of the southern super-continent called Gondwana. And by the end of the ordovician period, 440 mya, this super-continent drifted away from the equater towards the south pole. Continental drift like this can play a leading role in initiating extinctions. The seas at this time were full of life and it was a period of diverse marine invertebrates. The seas and continental shelf in particular were colonised by graptolites, trilobites, brachiopods, and the conodonts (early vertebrates). A typical marine community consisted of these animals, plus red and green algae, primitive fish, cephalopods, corals, crinoids, and gastropods.

This extinction was the second largest of the five mass extinctions, eliminating about 75 percent of animal species. This glaciation of the continent Gondwana as it drifted towards the south pole lowered the sea-level on a massive scale as water became locked up in ice sheets and glaciers. So the combination of the cooling caused by the glaciation, the partial destruction of the continental shelf caused by dropping sea levels, and shallow seas draining completely meant marine life such as conodonts, trilobites, and graptolites could not adapt to their new environments and began to die off on a massive scale.

But according to Brian Thomas at the University of Kansas, fossil evidence suggests that the extinction had already began prior to the glaciation event. It was initiated by a gamma ray burst from a supernova in our vicinity (within a few thousand light years). This resulted in a depletion of the ozone layer resulting in the marine life being exposed to harmful utlra violet radiation from the Sun. And the glaciation event further increased the extinction levels.Click here for more