What many of us may know about mass extinction events probably concerns the disappearance of the dinosaurs. We know that the dinosaurs became extinct at some point in the planet's long history. However, what many of us may not know is that there are other mass extinction events that have also occurred.
Mass extinctions occur when an unusually high number of species on Earth go extinct within a short, specific time frame. While some mass extinctions occur at smaller scales, there are what scientists call the Big Five mass extinctions in which a vast number of species went extinct.
The Ordovician-Silurian extinction occurred between 445 and 415 million years ago and is the second-largest mass extinction in history. 86% of life on Earth died out. Massive glaciations occurred, which turned much of Earth's water into ice. Most of the life on Earth during the Ordovician were marine life, so animals such as brachiopods, conodonts, graptolites, and trilobites were the hardest-hit.
Scientists also suggest that oxygen depletion in the world's oceans may have released toxic chemicals into the water. High lead, arsenic, and iron levels—which are still harmful to humans and animals today—may have caused deformities in marine animals.
The Devonian extinction began about 360 million years ago and may have gone on for 20 million years. It may have been a series of extinctions that lasted 100,000 to 300,000 years instead of a single event.
75% of the Earth's species perished in this extinction event. However, life forms living in shallow parts of the oceans had it the worst. Reefs, for example, almost completely disappeared. Like other mass extinction events, we aren't sure what caused these species to die off. There may have been a drop in global temperatures, reduced carbon dioxide in the air, or an asteroid impact.
The Permian extinction occurred about 250 million years ago. It is the deadliest of all mass extinction events, and nearly all life on Earth died out. About 96% of Earth's species went extinct during this event. Thus, all life still on Earth today descended from the small remnants that survived the mass extinction.
Scientists still aren't sure what killed nearly every living thing on the planet. An asteroid or comet may have caused the die-off, but so far there have been no craters to attest to this theory. Volcanism in Russia's Siberian Traps may have also been the culprit.
The Triassic-Jurassic extinction occurred about 200 million years ago. It wiped out about 20% of the world's marine animals, and about 80% total of the world's species. What many don't know is that this mass extinction event is what gave rise to the age of the dinosaurs. As other dominant life forms died out, the dinosaurs were able to fill the ecological niches left behind.
Theories on what caused this extinction event include climate change, flood basalt eruptions, and an asteroid impact. However, recent findings show that continuous volcanic eruptions may have caused the extinction event.
The Cretaceous-Tertiary extinction occurred about 65 million years ago. This is perhaps the most well-known of all extinction events, as it was during this time that the dinosaurs went extinct. Of course, this extinction event didn't just single out the dinosaurs. Other species, including ammonites and flowering plant species, went extinct during this period as well.
The prevalent theory on what caused this event is an asteroid or comet impact. Like with other mass extinction events, however, there's no hard evidence that an asteroid hit the Earth during this time. However, a number of other causes are also possible, including volcanic activity and a changing global climate.
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