#TeachNatGeo: The Big Six Extinctions (one is happening right now)

We’re in the middle of one of the biggest mass extinctions the world has ever seen.

There have been five previous major mass extinctions, when more than 75% of all species on Earth vanished. Think about that for a second . . . three-quarters of life on this planet died. Five times.

This list includes pictures of iconic species that went extinct during the five major extinction events. Fossils of these species are found in abundance all over the world . . . until all of a sudden, they disappear.

1. The Ordovician-Silurian Extinction

444 million years ago
86% of the Earth’s species lost

These trilobite fossils were found near Chengjian, China.
Photo by James L. Amos, National Geographic

The Ordovician-Silurian extinction involved massive glaciations that locked up much of the world’s water as ice and caused sea levels to drop precipitously. The event took its hardest toll on marine organisms such as shelled brachiopods, eel-like conodonts, and the trilobites.

2. Late Devonian

375 million years ago
75% of Earth’s species lost

Conodonts

Conodonts are extinct chordates that resemble eels. All that remains are these tooth-like fossils.
Photograph courtesy USGS.

Plants began spreading beyond the wetlands during the Devonian period. The new life burgeoning on land, including Earth’s first forests, apparently escaped the worst effects of the mass extinction that ended the Devonian. The main victims were marine creatures, such as conodonts , with up to 70% of species wiped out. Reef-building communities almost completely disappeared. Theories put forward to explain this extinction include global cooling due to the re-glaciation of Gondwana, or reduced atmospheric levels of the greenhouse gas carbon dioxide due to the foresting of the continents. A major asteroid impact has also been suggested.

3. End Permian

251 million years ago
96% (!!!) of Earth’s species lost

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A diver swims besides corals near Tahiti.
Photo by George F. Mobley, National Geographic

The Permian-Triassic extinction event about 250 million years ago was the deadliest: 96% of all species perished. In other words, all of the life that exists today can be traced back to the 4% of species that survived. Many scientists think an asteroid or comet triggered the massive die-off, but no crater has been found. Various forms of volcanic activity have also been suggested.

4. End Triassic

200 million years ago
80% of Earth’s species lost

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Therapsids were animals related to mammals. This short-snouted, boar-sized therapsid, Tiarajudens eccentricus, had big teeth. Illustration by Maurico Anton, National Geographic

Massive floods of lava from the Central Atlantic magmatic province (what is today the North Atlantic Ocean, eastern North America, northern South America, and northwestern Africa) may explain the Triassic-Jurassic extinction.An asteroid impact is another possible cause of the extinction, though a telltale crater has yet to be found. About 20% of all marine families went extinct, as well as most mammal-like creatures, many large amphibians, and all non-dinosaur archosaurs.

5. Cretaceous–Paleogene

66 million years ago
76% of Earth’s species lost

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The Chicxulub asteroid impact may have heralded the Cretaceous–Paleogene extinction event.
Illustration by Franco Tempesta, National Geographic

This is the one everyone knows about. The Chicxulub asteroid, a rock more than six miles wide, slammed into the Gulf of Mexico, shooting rocks and sediment into the atmosphere, blocking radiation from the sun and plunging Earth into an impact winter. Plants couldn’t photosynthesize, so there was nothing to eat. Everyone knows the non-avian dinosaurs died, but three-quarters of all plant and animals species were lost in this event. A thin layer of sediment rich in the metal iridium, abundant in asteroids, can be around the world in the geologic record.

6. “The Anthropocene Extinction”

RIGHT NOW

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Canyon tree frogs, like this one in Zion National Park, Utah, are not endangered. For now.
Photo by Robert Sisson, National Geographic

Many paleontologists argue that we’re currently experiencing the highest rate of species extinctions since the loss of the dinosaurs. It’s too early to say for sure, but the signs are all around us: frogs are dying at alarming rates around the world, climate change is eliminating habitats of countless species, and the decline of pollinators will have far-reaching consequences. The culprit? Without question, humans. We can wait to find out if the “Anthropocene Extinction” turns out to be in the ranks of the “Big Five Extinctions”—possibly eliminating ourselves in the process—or we can take the more hopeful approach, and change our ways.

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