March 31. End of the Ordovician mass extinction

It’s the end of March, and the end of the Ordovician Period. If you’ve been following along, you know that this time, 443 million years ago, is marked by a huge mass extinction – by some estimates, second only to the Great Dying at the end of the Permian in terms of its decimation of life on earth.

More than 100 families of marine animals went extinct. Families are groups of genera which are groups of species – so this was a LOT of diverse fauna. More than half the species of bryozoans, which had just appeared early in the Ordovician, were destroyed. Trilobites, brachiopods, and nautiloids were severely impacted. Something like 60% of all marine species died.

Ordovician life

As near as we can tell, it seems that the extinction event was worse among animals that lived in shallow tropical seas, which supports the idea that the glaciation going on at the same time was an important factor. Cooler conditions, cooler water, fewer tropical animals. Also, when water is tied up in ice on land, sea levels drop, so the shallow seas that predominated over much of North America retreated. There were many fewer niches for life to occupy. And in fact there are two phases to the extinction, which seem to be related to these two different but related causes.

We talked about some of the factors that may have led to the glacial period. The position of the supercontinent of Gondwana over the south pole, the possibility that life on land was removing CO2 from the atmosphere, reducing greenhouse conditions, and the cooling effect of some of the greatest volcanic episodes known in earth history.

There isn’t much doubt that whatever caused the glaciation, the consequences for life were pretty dire, and generally, it’s the cooler conditions and lower sea level that probably did the killing of marine life. All of the phyla of life survived and came back after the glacial period, in the Silurian that starts tomorrow.

I think it’s important to note that this is the only glacial period that is closely connected with a mass extinction, unless the modern glacial period has been a factor in the fairly recent destruction of a great many species. The jury is still out on whether or not it was climate change or human activity that eliminated things like mastodons, giant sloths, and saber-toothed tigers.

The end Ordovician glacial epoch was relatively short-lived, spanning just a few million years, and the extinction event at 443.4 million years ago is very much a sharp spike in terms of species loss. I think it’s fairly well accepted that cooling and loss of shallow-water niches were the ultimate primary causes of the extinction, and those things were caused by the glaciation. There are lots of potential factors that might have caused the glaciation, as we discussed March 28.

A recent look at gamma-ray bursts in the Milky Way Galaxy suggests the possibility that such an event might have been a factor in at least one extinction in the past 500 or so million years, so it is a possible cause for the Ordovician extinction. 

Tomorrow, the Silurian begins.

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March 31, 1850, is the birthday of Charles Doolittle Walcott, in New York Mills, New York. Walcott was the geologist who discovered and described the Burgess Shale in British Columbia back in 1909. You can hear more about Walcott in the podcast for February 12.

—Richard I. Gibson

Photo by Ryan Somma under Attribution-ShareAlike 2.0 Generic (CC BY-SA 2.0)