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The earliest known mass extinction, the Ordovician Extinction, took place at a time when most of the life on Earth lived in its seas. Its major casualties were marine invertebrates including brachiopods, trilobites, bivalves and corals; many species from each of these groups went extinct during this time. The cause of this extinction? It’s thought that the main catalyst was the movement of the supercontinent Gondwana into Earth’s southern hemisphere, which caused sea levels to rise and fall repeatedly over a period of millions of years, eliminating habitats and species.
Towards the end of the Devonian period around 370 million years ago, a pair of major events known as the Kellwasser Event and the Hangenberg Event combined to cause an enormous loss in biodiversity. Given that it took place over a huge span of time—estimates range from 500,000 to 25 million years—it isn’t possible to point to a single cause for the Devonian extinction
The Permian-Triassic extinction killed off so much of life on Earth that it is also known as the Great Dying. Marine invertebrates were particularly hard hit by this extinction, especially trilobites, which were finally killed off entirely. But you don’t get a nickname like the Great Dying for playing favorites; almost no form of life was spared by this extinction, which caused the disappearance of more than 95 percent of marine species and upward of 70 percent of land-dwelling vertebrates.
This extinction occurred just a few millennia before the breakup of the supercontinent of Pangaea. While its causes are not definitively understood—researchers have suggested climate change, an asteroid impact, or a spate of enormous volcanic eruptions as possible culprits—its effects are indisputable
In addition to its most famous victims, the non-avian dinosaurs, the K-Pg event caused the extinction of pterosaurs and extinguished many species of early mammals and a host of amphibians, birds, reptiles, and insects. Life in the seas was also badly disrupted, with damage to the oceans causing the extinction of marine reptiles like mosasaurs and plesiosaurs, as well as of ammonites, then one of the most diverse families of animals on the planet.
The Holocene Extinction hasn’t been defined by a dramatic event like a meteor impact. Instead, it is made up of the nearly constant string of extinctions that have shaped the last 10,000 years or so as a single species—modern humans—came to dominate the Earth. Some have even suggested that the Holocene Extinction would be more aptly named the Anthropocene Extinction, after the role humans have played in this ongoing loss of biodiversity around the world.