We’ve talked about mass extinction many times, and indeed we’ve covered four out of the famous “Big Five” that so shaped the history of life on Earth. This time, we’re completing the set, discussing the earliest of the five, exploring the setting, causes, and results of the Ordovician Mass Extinction.
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The Ordovician Period
The Cambrian Period is famous for the incredible radiation of animal life that kick-started the Paleozoic Era and the entire Phanerozoic Eon. But immediately after the Cambrian came the Ordovician with it’s own explosion: the Great Ordovician Biodiversification Event.
This event saw the rise of ecosystem structures that would dominate the rest of the Paleozoic Era. The oceans filled with new forms of brachiopods, trilobites, corals, predatory cephalopods, and much more. The backdrop for this radiation of life was a warm and wet world, with plenty of habitable space in both deep oceans and shallow continental seaways.
This Ordovician radiation ended around 445 million years ago at the close of the Ordovician Period with a mass extinction. As we’ve discussed in previous episodes, mass extinctions are events in which lots of life disappears in a geologically short period of time. The Ordovician is the first of the famous “Big Five,” which include the most devastating extinction events in Earth history (we’ve done episodes on all five now! 1, 2, 3, 4, and now this).
This event was felt worldwide. Ecosystems were hit particularly hard in the deep sea and in the shallow continental seaways. All groups of life experienced major losses, and it was particularly bad for many of the Cambrian forms like early brachiopods and trilobites. In total, it’s estimated that around 85% of marine species went extinct at this time.
While the tempo and timing of the extinction are still under study, with new evidence coming to light all the time, the prevailing understanding of this event is that it happened in two pulses on either end of the Hirnantian, the final stage of the Ordovician Period. These two pulses line up suspiciously closely with the leading suspect in this mass murder mystery…
An Early Paleozoic Ice Age
The start of the Hirnantian stage saw the onset of a major glaciation centered on the supercontinent Gondwana, which at that time sat over the South Pole. The formation of an enormous southern ice sheet was accompanied by a series of global changes: cooling global temperatures forced many marine species out of higher latitude habitats; falling sea levels shifted shorelines and drained continental seas; and changes to ocean circulation rearranged nutrients and oxygen levels, wreaking havoc with deep-sea environments.
As the Ordovician drew to a close, the glacial period abated: ice retreated, sea levels rose, and ocean circulation changed again. This seems to have dramatically impacted the ecosystems that had evolved and adapted to glacial conditions during the Hirnantian. The beginning and ending of this Ordovician Ice Age seem to be the best contenders for the two pulses of this mass extinction.
The glaciation is thought to have been kicked off by dropping CO2 levels, but exactly how that happened is still under study. It might be that intense weathering of newly-forming mountain chains pulled CO2 out of the atmosphere, or that shifting continents and changing oceans may have led to blooms of life that trapped CO2 in their bodies as they died and became buried beneath the sea.
There’s also evidence for intense volcanic activity toward the end of the Ordovician, which could have impacted life directly as part of the extinction event, or could have been related to geologic changes that impacted the start or end of the glacial period.
Intense as this extinction was, it didn’t rearrange global ecosystems quite as dramatically as other mass extinctions did. While it took several million years for life to recover, and indeed Paleozoic life never again reached the levels of diversity seen during the peak of the Ordovician radiation, the life that returned was largely similar to what had come before. Dominant organisms like brachiopods and nautiloids regained their dominant positions, while reefs and continental seaway ecosystems returned with similar communities of life as before.
One major lasting effect of this extinction is that many Cambrian animal communities, which had already been in competition with newer Ordovician groups, never recovered from this event. Ordovician-style communities persisted until the Permian extinction 200 million years later.
Get to know the Ordovician Period at UCMP Berkeley