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Mass Extinction: Definition, Timeline & Events

Instructor: Christopher Muscato

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

Mass extinctions are mysterious and rare events. In this lesson, we're going to briefly review the five mass extinctions in Earth's history and consider why they may have happened.

Extinction En Masse

Imagine all the plants and all the animals on Earth, from the largest whales to the smallest bacteria. According to researchers, that's only .01% of all life that has existed on Earth. The other 99.9% is extinct, gone for good. This is natural. Considering normal climate change, competition between species, and available resources, the average rate of extinction across geologic history has been about 10-25 species per year. We call this the background extinction rate, and it's perfectly normal.

However, every now and then we get moments in time that are definitely not normal, times when more species are dying out than should be expected. The worst of these events occur when more than 50% of life on Earth is lost in a relatively short time period. We call this a mass extinction.

Defining Mass Extinctions

Mass extinctions are very mysterious phenomena. They don't happen very often, and there doesn't seem to be a single cause to explain them. In fact, there are only five times in the planet's history when this occurred. We're going to look at each one, but keep in mind that none of these were absolutely instantaneous. Most mass extinctions are really a compounding of multiple large-scale extinction events all within a relatively short time period (which in geologic time could mean a few hundred thousands years), but the effects are absolutely devastating. Scientists identify mass extinctions by tracking the lifespans of species through the fossil record. When a large number of species all go extinct at the same time, we know something traumatic happened in that ancient environment.

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#1: The Ordovician-Silurian Extinction

The very first mass extinction in Earth's history was the Ordovician-Silurian Extinction. Right at the end of the Ordovician period, around 439 million years ago, something happened. Sea levels dropped and the amount of oxygen in the water declined. Why? We're not entirely sure, but increased glaciation has been one suggested factor. Since nearly all life on Earth was still aquatic at this point, this change was a big deal. About 86% of all life died, which really hurt animals like trilobites and brachiopods. (Both groups ultimately survived, but with a greatly reduced diversity of species.)

#2: The Devonian Extinction

Fast-forward 75 million years. The Earth has recovered, new species are flourishing, and then disaster strikes right around 364 million years ago, in the late Devonian period. Another global cooling trend and lowering sea levels led to the Devonian extinction, when about 75% of all species went extinct. Tropical species that lived in shallow seas were the most affected. In fact, it would take about 100 million years for coral reefs to recover from this event. Most of the first animals to make it onto dry land went extinct as well, and it would be another 10 million years before any vertebrates were able to adapt to a terrestrial habitat. It was those first vertebrates to reemerge after the Devonian extinction, however, that ended up as the ancestors of all terrestrial vertebrates on the planet today.

#3: The Permian-Triassic Extinction

The next mass extinction occurred 251 million years ago. The Permian-Triassic extinction is also known as the ''Great Dying,'' because 96% of all species on the planet died out. Think about what that means. 96% of life's diversity, of the genetic paths life could have taken, was gone. Every living thing that has existed in the last 250 million years evolved from the 4% of life that survived.

How did this happen? While there are several hypotheses, the leading explanation is that a piece of the planet's core burnt its way to the surface and exploded into massive volcanic eruptions that lasted a million years. The result would have been substantial global warming, acid rains, changing of weather patterns, and other things that would have been really bad for living organisms that required air, water, and sunlight.

#4: The Triassic-Jurassic Extinction

Most people are aware that the dinosaurs died out in a mass extinction, but did you know they may have been created by one as well? The Triassic-Jurassic extinction occurred 214-199 million years ago, again likely caused by massive volcanic eruptions (perhaps from the breakup of the supercontinent Pangaea). The exact number of species killed in this event is unknown, but very high. Most of the dominant vertebrates were killed off in this era of rapid warming. The creatures left standing were the archosaurs, ancestors of the dinosaurs who were able to become dominant thanks to the void left by other species.

Could the breaking apart of Pangaea and formation of the Atlantic Ocean have caused the T/J Extinction?
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