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Ordovician-Silurian Mass Extinction: Causes, Evidence & Species

Instructor: Christopher Muscato

Chris has a master's degree in history and teaches at the University of Northern Colorado.

What can cause over half of the life on Earth to die? In this lesson, we'll check out the Ordovician-Silurian mass extinction, and examine the theories that seek to explain this traumatic event.

The Ordovician-Silurian Mass Extinction

There are some contests in which it's definitely better to finish in second place instead of first. Mass extinctions would be a good example. In all of Earth's history, there have been five events where more than 50% of life died out, which is a big deal. Around 443 million years ago, at the end of the Ordovician period, a major mass extinction occurred. In fact, the Ordovician-Silurian extinction was the second worst mass extinction in the planet's history. Just because it came in second, however, don't imagine that it wasn't that serious. The loss of life in this event wasn't far behind the leading mass extinction. Not that it's a competition.

What Happened at the End of the Ordovician

Let's set the stage here. It's the end of the Ordovician period and life is flourishing. Jawless fish are the latest in evolutionary advancements and sophisticated vertebrates in a world mostly filled with invertebrates. At this time, nearly all life was still in the ocean, and a lot of it was concentrated near the tropics, where waters were shallow and warm. Much of this was a result of the interaction between landmasses at the time. The smaller continent of Laurentia was right at the equator. Near it was the supercontinent Gondwana, which contained what are now South America, Africa, India, Australia, and Antarctica. Together, these land masses created a lot of coastline and shallow, coastal seas.

The warm Ordovician waters were paradise for creatures like orthoceras
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Then, something happened. The global temperatures cooled and the sea levels fell. Considering that much of the plant and animal diversity of the time had adapted to shallow, warm waters, this was devastating. These species could not survive in colder, deeper oceans and many died out. In fact, about 85% of all species on Earth went extinct. Many of these were kinds of corals, trilobites, and similar marine organisms. It was so bad that over 100 families went completely extinct, including about 1/3 of all families in the phylum Brachiopoda (animals that created many of the seashells of the ancient world).

Brachiopod fossils from the Ordovician period
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Causes

In reality, the Ordovician-Silurian extinction was not a single event. It was a combination of multiple extinction events that compounded over a relatively short time period into a massive loss of genetic diversity on Earth. We see two main phases in these extinctions, one that happened as global cooling began and one that occurred as sea levels dropped. Those seem to be the main causes of the Ordovician-Silurian extinction, but why did they occur? There have been several attempts to explain this mass extinction throughout the years. Here are some of the leading theories.

Volcanic Activity

When the rates of volcanic activity spike, chemicals are pumped into the air that can block out the Sun, disrupt weather patterns, and alter global temperatures. Those chemicals then settle into the ocean over time. This has happened at other points in Earth's history, and some believe that increased volcanism was responsible for the Ordovician-Silurian extinction as well. The main evidence for this is in trace amounts of mercury and organic materials in late Ordovician rocks, but the theory is not accepted by everyone.

Gamma Burst

There's also another theory which is a little more out there…literally. Some scientists think that a hypernova explosion somewhere in our galaxy could have flooded the Earth with a burst of gamma radiation. Since there weren't many land plants to create lots of oxygen, the ozone would have been thinner and lots of that radiation may have gotten through. This could have led to radiation poisoning of many species, as well as generated the global cooling trend. It's an interesting theory, but not one that's heavily supported by actual data.

Continental Drift

We may never know for certain what caused the Ordovician-Silurian extinction, but there is one theory that is much more widely accepted than the others. It begins with that supercontinent of Gondwana. Throughout the Ordovician, Gondwana drifted steadily to the south. Eventually, it reached the bottom of the planet and crossed over the pole. The theory is that when this happened, the presence of such a large landmass at the axis of the Earth began altering weather patterns, winds, and even ocean currents.

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