The Ordovician Mass Extinction Killed 85 Percent Of Life On Earth In A Totally Unique Way

When you think about a prehistoric mass extinction event, chances are your mind goes to the K-Pg boundary, and the day that fateful asteroid smashed into Earth with enough force to wipe out some three-fourths of all plant and animal life, putting an end to the age of dinosaur supremacy forever.

It’s the best known of the six mass extinctions the planet has seen – or is seeing – through its long history. But that’s just good PR. Long before that – like, almost 400 million years before – the world was already testing out its appetite for apocalypse with a little-known event called the Late Ordovician mass extinction.

Life in the Ordovician

Go back far enough in time – past the Ice Age; past the fall and rise of the dinosaurs; past even the development of reptiles and amphibians – and the Earth starts looking very strange. 

“It was a very, very different world” during the Ordovician period, says Richard Twitchett, Research Leader in the department of Earth Sciences at the UK’s Natural History Museum. “The climate was very different – it was a very warm period, and CO₂ levels were very high.”

Look around, and you won’t see any animals you recognize. Neither will you see any trees, or flowers; no grasses, ferns, or almost any other kind of vegetation. “If the land was green, it was green with algae, at most – we just have no evidence that there was anything like we would think of as a plant community,” Twitchett tells IFLScience. “And therefore, of course, we don't have an animal community, since there's nothing to eat.”

It's a description that seems at odds with paleogeology textbooks’ account of the Ordovician. The 45-million-year-long period is characterized there as a time of flourishing biodiversity. So, where was all this life?

Take a peek under the ocean, and you’ll find it: a wealth of weird and wonderful creatures, changing and diversifying at a rate rarely seen before. It’s notable enough in the fossil record to have its own name: the Great Ordovician Biodiversification Event, or GOBE – and for good reason: “it's just this time that, you know, evolution is just doing a lot of experimentation,” Twitchett tells IFLScience. “There were animals trying to exploit different lifestyles […] but it's also diversification within body plans that are obviously becoming more successful for these warm Paleozoic seas.” 

Like the first finches to arrive on the Galapagos Islands, these weird sea creatures basically had the entire ecosystem at their disposal – and boy, did they exploit it. Body plans got refined and complex; animals we still see today, like starfish, sea urchins, corals, and so on, made their debut during this period. Waters that had previously been home to just a few types of deposit feeders suddenly found themselves dominated by suspension feeders at every depth – brachiopods on the seafloor, corals and bryozoans just above, and crinoids near the surface.

“It’s not just species diversity, but it's also ecological diversity,” says Twitchett. “So, yeah, the Ordovician is a time of huge increasing biodiversity across the planet.”

Death in the Ordovician

All good things, the saying goes, must come to an end – and for the Ordovician era, that end was staggered and confusing. No end-Cretaceous team-up of volcanoes and asteroids for this time period; instead, the “late Ordovician mass extinction” should really be thought of as a series of mini-extinctions, followed by a big boss wipeout at the end.

“Quite often now, it’s resolved into two separate events,” Twitchett tells IFLScience: first, the Earth cooled down – and then it warmed back up again.

That makes the Ordovician mass extinction kind of an oddball, actually. “All those later mass extinctions in the Big Five – yes, there's a climate change component to many of them, but it's always warming,” Twitchett says. “It's not cooling.”

The end of the Ordovician, in contrast, was kicked off by the Earth’s thermostat firmly flipping to “cold” – and much like our own current mass extinction situation, it may have been the contemporary species’ success that eventually caused their downfall.

“There has been a hypothesis that it was the evolution of some of the plants on land that might have caused the drawdown of CO₂,” Twitchett explains. “And that triggered the glaciation at the end of the Ordovician.”

Not many plants had made it out of the ocean – but those few mosses and worts that had eked out a life on land by the end of the period would have had a big effect on the planet. Their very presence on the rocks would be enough to weather and erode them, creating soil and enhancing the absorption of CO₂ from the atmosphere. 

An alternative – or maybe complementary – explanation for the Earth’s sudden cooling may have been a simple coincidence of geology. The world at this point had no controversy over the number of continents sitting atop it: there was one; it was called Gondwana, and it was around this time that it was being pushed into a new position over the South Pole. 

“It’s totally possible that actually some of the cooling might have been triggered by [that],” Twitchett tells IFLScience. “Even if it only triggers a slight increase each year – say in snowfall or in ice formation – you get that albedo effect, with white reflective surfaces on the planet reflecting more of the sunlight and the heat energy back. You can see how that positive feedback loop could develop.”

And just as soon as the planet had finished reeling from that, everything started warming up again. But this time, not only did the species that survived the ice now have to grapple with the heat, they were also suffocating – as evidence from the fossil record shows the oceans underwent a massive de-oxygenation at this point.

Why precisely that was is, once again, still a mystery. Perhaps it was algal blooms, thriving in the warmer weather, that drew out the oxygen from the water; maybe it was the result of increased volcanism, the lava spewing oxygen-hungry minerals into the oceans. 

The reason that we're familiar with the end of Cretaceous extinction is the death of the terrestrial, big, scary dinosaur, right? It's not because of the collapse of the planktonic foraminifera, which was just as important at that time.

Richard Twitchet

Either way, the end result was the same: a global die-off of some 60 percent of genera, and nearly 85 percent of species. In terms of pure mortality, it stands almost alone in the history books – the only event that beats it is the end-Permian mass extinction that would occur some 200 million years later.

The unheard extinction

The term “mass extinction” comes with certain expectations. Huge die-offs; the loss of entire clades or genuses, perhaps; a fundamental reshaping of global ecosystems at large, that kind of thing. The end of the Ordovician certainly meets the first two of those criteria – but by the final metric, it was… kind of underwhelming, actually.

“If we're just looking at raw numbers – taxa that became extinct, whether we think about families or genera – it’s sitting up there as the second most important [mass extinction] in terms of biodiverse loss,” Twitchett tells IFLScience. “A lot of species, genera, and organisms seemingly became extinct at the time.”

“But if you look at the ecological impacts – the sort of, long-term ecological impact of the events,” he adds, “it's the least important.”

When it comes to mass extinctions, Twitchett explains, the devil is in the details. Take the end-Cretaceous extinction, he says: “If you actually count up globally, the species that went extinct – both in the marine and the land – that's actually the lowest ranked of those five extinctions.” But in terms of actual impact, it was hitting well above its weight: “Entire terrestrial dinosaur-dominated ecosystems vanished, because of the species that were wiped out,” Twitchett points out.

The end of the Ordovician, he says, was almost the exact opposite: “There's a lot of things that disappear, but that long-term impact was actually very, very little.” All major groups lost important species, but none disappeared entirely; ecological niches remained filled, and despite the loss of manifold individual species, “the world basically came back to pretty much the same very quickly,” Twitchett explains.

It’s another way in which the Ordovician mass extinction sits apart from the others in the Big Five. “Clearly biodiversity and species loss happened,” Twitchett says, “but not to such an extent that you've actually lost any of these key functional groups that were evolving at the time – and are still around today.”

Perhaps it’s that nuance which, despite its uniqueness of conditions and extreme mortality, the Ordovician mass extinction is also arguably the least well-known one. But, to be honest, there may be a simpler reason it’s so unheard of: it doesn’t really affect us.

“The reason that we're familiar with the end of Cretaceous extinction is the death of the terrestrial, big, scary dinosaur, right?” Twitchett says. “It's not because of the collapse of the planktonic foraminifera, which was just as important at that time.” 

“So, yeah, I think that partly explains why the Ordovician is less well-known,” he tells IFLScience. “Yes, because it's so far back in time – but also because there's not this terrestrial component to it, that maybe the general public is just more likely relate to.”