About 445 million years ago, about 85 percent of marine species were brutally wiped off the face of the earth. But what caused the loss of all this life?

Most will know that the dinosaurs went extinct during a great mass extinction event. But did you know that the Earth has also experienced other mass extinctions? A total of five – known as the ‘big five’ – have passed in review, with at least three quarters of all species living on Earth facing extinction. In this article we focus on the first mass extinction that the earth went through: the so-called Late Ordovician mass extinction. Because what exactly was the basis for this?

Cause of mass extinctions

Scientists have long been interested in discovering the cause of mass extinctions. That’s because understanding the environmental conditions that led to the extinction of most species in the past could help us prevent a similar event from happening again in the future. In addition, many scientists believe that we are currently in the midst of a sixth mass extinction. And so studying this phenomenon is more important than ever.

Late Ordovician Mass Extinction

The Late Ordovician mass extinction occurred about 445 million years ago. During this mass extinction, about 85 percent of marine species, most of which lived in shallow waters off the coast of continents, disappeared from the face of the earth. And that while before that, at the time of the Ordovician period, the seas were full of biodiversity. “If you went snorkelling in an Ordovician sea, you would have seen some familiar animals, such as mussels, snails and sponges,” said study researcher Seth Finnegan. “But you would also have come across many other species that are now very limited in diversity or completely extinct, such as trilobites, armpods and sea lilies.”

Fossils from the Ordovician period, found on the Canadian island of Anticosti. Image: André Desrochers, University of Ottawa

But then something catastrophic happens. Unlike the mass extinction where the dinosaurs died out quite suddenly, the Late Ordovician mass extinction took place over a fairly long period of time. It is possible that this mass extinction even extended over two million years. But what caused the loss of all this life?

Question mark

We know that the climate was cooling back then. But could that have caused the loss of as much as 85 percent of marine life? “Several geologically more recent mass extinctions are known to have been caused by volcanically-driven rapid warming and the resulting loss of oxygen in the oceans,” Finnegan said. Scientias.nl. “But what is particularly striking about the Late Ordovician mass extinction is that the combination of environmental changes associated with the extinction are unusual and difficult to understand.”

Lack of oxygen

One of the main discussions surrounding the Late Ordovician mass extinction is whether lack of oxygen in seawater caused the mass extinction. To investigate that question, the researchers measured the concentration of iodine in carbonate rocks from the Late Ordovician period. The concentration of this element in carbonate rocks serves as an important indicator of changes in the oceanic oxygen level.

Shallow waters

It leads to an interesting discovery. “In our paper, we show that it does indeed appear to be true that the amount of dissolved oxygen in the deep ocean decreased,” Finnegan said. “However, we found no evidence of a decrease in oxygen in shallow waters. And it was here that most of the animal species that died out lived. It means that a decrease in oxygen probably cannot explain the mass extinction.”

Oxygen starvation in the deep ocean
Although it appears that shallow waters were not affected by oxygen deprivation, the phenomenon did extend into the deep ocean around the same time. The researchers attribute this to the circulation of seawater through the global oceans. An important point to keep in mind is that ocean circulation is a very important part of a climate system. The cooling of the climate may have changed ocean circulation, stopping the flow of oxygen-rich water from the shallow seas to the deeper ocean. This means that climate cooling can also lead to lower oxygen levels; at least, in certain parts of the ocean.

But if the mass extinction was not set in motion by lack of oxygen, what was the underlying cause? While the researchers can’t provide a definitive answer, they suspect that colder conditions during the late Ordovician, combined with additional factors, were likely responsible for the demise of numerous animal species.

Climate cooling

“So the causative agent may well have been the climate cooling itself,” emphasizes Finnegan. “We see evidence of very substantial climate cooling and ice sheet expansion, with a large associated sea level drop. This cooling and drop in sea level may have been considerably more extensive than during more recent ice ages. This means that habitat loss may have been responsible for some of the extinction. However, the question is whether so much extinction (85 percent of marine species, ed.) can be attributed to habitat loss.”

Will we ever know?

The question is whether we will ever fully unravel the mystery of the Late Ordovician mass extinction. “A consequence of the drop in sea levels is that marine sedimentary rocks have become quite scarce,” Finnegan said. “So to understand what caused most of the extinctions, we need to study those areas in detail, but also use models to try to understand what happened in the parts of the world for which we don’t have well-preserved sedimentary data. It is a very challenging problem. But I’m optimistic that we as a community are starting to get to grips with it.”

What the researchers in their studies at least have shown is that one of the most frequently advanced explanations – oxygen starvation in the oceans – probably cannot explain the Late Ordovician mass extinction. Nevertheless, according to researcher Zunli Lu, we should certainly not take our oxygen-rich world for granted, he says in an interview with Scientias.nl. “Much is still unknown about previous mass extinctions. We are far from fully understanding these events. It would be prudent not to further drastically disrupt the ocean environment and climate system until human society is able to better predict and mitigate extensive changes in the Earth system,” he concludes.