Shelf Life 12: Six Extinctions In Six Minutes

"Extinction is a way of life, but there have been mass extinction events where a whole array of species get wiped out." 

-Michael Novacek, Provost of Science

Six (Mass) Extinctions in 440 Million Years

All things must pass. But the idea that a species could go extinct is a relatively new one, first proposed by anatomist Georges Cuvier in a presentation in Paris in 1796 in a lecture on the extinction of the mastodon, then thought by some to still be roaming the ill-explored western reaches of North America. 

Mastadon fossil on display in Museum gallery.
Discovered in 1845 in Newburgh, New York, the Warren mastodon now resides in the Museum's Hall of Advanced Mammals.
Denis Finnin/© AMNH

Cuvier’s suggestion that life on Earth was not static, and that species could disappear, was groundbreaking. Studying the collections of the National Museum of Natural History in Paris and records from other collections around the world, he soon identified several species whose like we would never see again, including the mosasaur, the cave bear, and the Irish elk.

Painting of a knight elk in a field with the sun shining through the cloudy sky above.
Fossil skeleton of Irish elk on display in Museum in Hall of Advanced Mammals.Il

Buoyed by the research of scientists like Charles Lyell and Charles Darwin, the idea that species developed gradually, over time, gained acceptance in the scientific community. For generations, it was dogma that extinctions happened slowly, too. The idea that species could be wiped out in a fell swoop, even one with catastrophic consequences, wasn’t given much credence. 

That changed in the late 1980s and early 90s, with the Alvarez hypothesis, which stated that a huge comet or asteroid impact was responsible for the sudden disappearance of non-avian dinosaurs and many other forms of life 66 million years ago. Proposed by physicist Luis Alvarez and his son, geologist Walter Alvarez, the hypothesis took time to gain acceptance, but buoyed by evidence like the Barringer Crater pictured below, an extraterrestrial impact is now the most widely accepted explanation for the Cretaceous-Paleogene (K-Pg) extinction.

Overhead shot of a large meteor crater.
Credit: Shane Torgerson

That acceptance also opened the door for further study of geological and fossil records, which led researchers to a surprising conclusion: While the K-Pg extinction event was a very bad day for life on Earth, it was by no means the only one on record. Researchers now think that the K-Pg was just the latest of five major extinction events—and that we’re currently in the middle of a sixth mass extinction, one caused not by a volcano or asteroid impact, but by humans.

Each event had a different impetus. Some took place over the span of millions of years while others were extremely sudden. What they have in common, though, is that they reshaped the face of life on Earth by wiping out a significant portion of it. 

About 445 Million Years Ago: Ordovician Extinction

Twelve trilobite fossil specimens in clusters.
Trilobites were once one of the most common lifeforms on the planet, but many species were wiped out during the Ordovician extinction. 

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. The onset of a late Ordovician ice age and changes in water chemistry may also have been factors in this extinction. 

About 370 Million Years Ago: Late Devonian Extinction

Illustration of a tiktaalik, a long fish with front fins that allow the tiktaalik to prop themselves up and walk partially on land.
Tiktaalik, considered a transitional species between fishes and the first legged animals, developed during the Devonian Period. 
© National Science Foundation

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, though some suggest that the amazing spread of plant life on land during this time may have changed the environment in ways that made life harder, and eventually impossible, for the species that died out. 

Model of a large armored fish with open mouth and sharp teeth hanging from the ceiling.
Dunkleosteus terrelli is one of the species of armored fish called placoderms that went extinct at the end of the Devonian Period. 
Denis Finnin/© AMNH

The brunt of this extinction was borne by marine invertebrates. As in the Ordovician Extinction, many species of corals, trilobites, and brachiopods vanished. Corals in particular were so hard hit that they were nearly wiped out, and didn’t recover until the Mesozoic Era, nearly 120 million years later. Not all vertebrate species were spared, however; the early bony fishes known as placoderms met their end in this extinction.

252 Million Years Ago: Permian-Triassic 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.

Museum diorama depicting the ocean floor in the Permian period, including brachiopods, bryozoans, sponges and microbial mats in a reef scene.
The Permian Period, depicted in this diorama at the Museum, ended in an extinction known as The Great Dying.
Denis Finnin/© AMNH

So many species were wiped out by this mass extinction it took more than 10 million years to recover from the huge blow to global biodiversity. This extinction is thought to be the result of a gradual change in climate, followed by a sudden catastrophe. Causes including volcanic eruptions, asteroid impacts, and a sudden release of greenhouse gasses from the seafloor have been proposed, but the mechanism behind the Great Dying remains a mystery. 

201 Million Years Ago: Triassic-Jurassic Extinction

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. 

Colorful illustration depicting modern countries in the formation of the supercontinent Pangea.
The Triassic-Jurassic extinction event took place just a few thousand years prior to the breakup of the supercontinent Pangea. 
Wikimedia Commons/Massimo Pietrobon

More than a third of marine species vanished, as did most large amphibians of the time, as well as many species related to crocodiles and dinosaurs. 

66 Million Years Ago: Cretaceous-Paleogene Extinction 

The most recent mass extinction event is also likely the best understood of the Big Five.

Tyrannosaurus rex fossil on display in Museum gallery.
Tyrannosaurus rex was among the many species of dinosaurs that went extinct as a result of the Cretaceous-Paleogene extinction event.
Craig Chesek/© AMNH

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. 

In all, scientists estimate that 75 percent of species living at the time of the K-Pg extinction were wiped out.

Now: The Holocene Extinction

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. 

“Many of the past mass extinction events are mysterious in some ways because we really don’t know the cause,” says Michael Novacek, the Museum’s provost of science and a curator in the Division of Paleontology. "But we have a good idea of what the cause of the current changes are, this century and the centuries before: it’s human activity.”

Humans have contributed to factors like climate change and the introduction of invasive species, which are leading to even more extinctions as animal habitats disappear or are disrupted by new species.  “Some biologists think that the current rate of species loss is probably a thousand times what the normal rate is,” says Novacek.

Many of the species going extinct are doing so before they are even identified. In light of this, researching new species for a fuller understanding of the world’s biodiversity grows ever more urgent for institutions like the American Museum of Natural History. These records, Novacek says, are vital to our knowledge of the world around us.

“The collections in the Museum here and other museums are really a record of life," Dr. Novacek says. “They’re very important for not only telling us what went extinct, but what survived.”