Deciphering mass extinctions
What the planet’s past mass extinctions tell us about the future of life on Earth
The crash-landing of a 10-kilometer-wide asteroid 65 million years ago made for a very bad day for dinosaurs — or one very lucky day for mammals.
The story of the demise of dinosaurs and the ascent of mammals is a familiar one. But when it comes to mass extinction events, scientists are discovering that this simple tale is much messier: Dramatic falls of dominant creatures are by no means guaranteed, nor is the subsequent rise of other animals waiting in the shadows required. That’s not to say extinctions are necessarily random, however. The rock record has revealed some “rules” that these events seem to follow — rules that also speak to the planet’s future.
Extinction is common. The fossil record indicates that the average species lasts — from origin to extinction — a geologically short 1 million to 10 million years. And ending a species doesn’t require an asteroid the size of a small city (although that certainly helps). Extinctions largely reflect Darwin’s theory of natural selection — that those individuals better adapted to environments are more likely to carry on. When a species does fail, it is called a “background” extinction, and it is background extinctions that scientists think account for about 95 percent of all extinctions in the history of life.
The other extinctions aren’t so routine. These are mass extinctions, events that not only exterminate large numbers of species, but whole classes of animals, in a near geological instant. They include the Cretaceous-Paleogene, or K-PG, extinction (formerly known as the Cretaceous-Tertiary, or K-T, extinction), which witnessed the end of the dinosaurs 65 million years ago, and the Permian-Triassic extinction 252 million years ago in which 95 percent of marine life and 70 percent of terrestrial vertebrate life were wiped out. Scientists have identified three other major mass extinctions in this “Big Five,” likewise labeled by the geologic periods that contain them: the Late Ordovician (445 million years ago), the Late Devonian (376 million years ago) and the Triassic-Jurassic (200 million years ago).
Scientists are still working out the exact causes of these extinctions, but geological evidence points to some possibilities. Sediment deposits, fossils and isotopic evidence, for example, point to a rapid advance and retreat of glaciers during the Late Ordovician, which may have changed sea levels, climate and ocean circulation so quickly that many organisms could not adapt in time; 85 percent of marine species died.
The Permian-Triassic extinction may have been the result of gas emissions from expansive lava flows in what is today Siberia that triggered acid rain and caused a plummet in oceanic oxygen levels, making Earth inhospitable to most life forms. And in the K-PG extinction, most scientists agree that an asteroid delivered the knock-out punch that killed the dinosaurs.
When major groups are knocked off in mass extinctions, it can give other groups their big break. Today’s beaches might be littered with the shells of brachiopods — filter-feeding animals that tether to the seafloor with a stalk — rather than clams and snails, says David Bottjer, an invertebrate paleontologist at the University of Southern California in Los Angeles, had it not been for the Permian-Triassic extinction, during which more than half of all brachiopod genera disappeared. The Triassic-Jurassic extinction may have paved the way for dinosaurs, which up to that point faced a tight — if not losing — competition with a group of crocodile-like reptiles called crurotarsans, suggested Stephen Brusatte of the American Museum of Natural History in New York City and colleagues in a 2008 paper in Science. And mammals scampered at the feet of dinosaurs for more than 100 million years until the K-PG extinction opened the stage for their takeover.
For years, scientists thought such transformations — when they occurred — were precise and well-defined, that “the dinosaurs died and the mammals inherited the Earth on a silver platter,” says David Jablonski, an invertebrate paleontologist at the University of Chicago in Illinois.
But research is increasingly demonstrating that no such silver platter exists. For one, even if the Permian-Triassic extinction cleared the stage for bivalves, Bottjer notes, brachiopods almost regained their dominance later in the Triassic. And although all geologic time after the K-PG is commonly referred to as the “Age of Mammals,” says Tom Holtz, a vertebrate paleontologist at the University of Maryland in College Park, the first 10 million years of the Paleogene were an evolutionary free-for-all between mammals, birds, crocodiles and snakes — with no obvious signs at the time that mammals would ultimately rule.
Scientists have also learned that mass extinctions, while often damaging, don’t always dramatically change the layout of life. The Late Ordovician extinction, despite being the second most deadly extinction event in terms of killing off genera, left most major animal groups, like the brachiopods, intact. Additionally, some groups, such as the turtles, have maintained a solid presence throughout history, surviving events like the K-PG, Jablonski says.
The impact of mass extinction events on life is a lot more complicated than many folks realize, Jablonski says. “There are all sorts of interesting selectivities that happened that are much more complicated than, ‘Oh, the dinosaurs died.’”
Not just a matter of luck
With mass extinctions affecting various animal groups differently, it might seem like they lack any kind of recognizable pattern. One hypothesis suggests just that. In the early 1980s, now-retired University of Chicago paleontologist David Raup proposed the “Field of Bullets” model, claiming that mass extinctions kill indiscriminately. When the “shotgun” fired at the K-PG, enough bullets happened to cluster on the dinosaurs to spell their demise, while enough groups of mammals and birds beat the odds to move forward.
But other research suggests that mass extinctions, although erratic, are not so random. Beginning in the 1980s, Jablonski noted that the genera of bivalves and snails that survived the K-PG extinction had one characteristic in common: an expansive geographic range.
Thus bivalve genera like the Pycnodonte, which lived along continental shelves from the Gulf Coast to Northern Europe to Australia during the Late Cretaceous, survived the K-PG, while the bivalve genus Anthonya, found only in Japan, didn’t make it. Clearly, it pays to diversify your geographic portfolio, Jablonski says. “Each guy survives for his own idiosyncratic reasons in one place or another, so that the key to survival … is to have your eggs in many different biogeographic baskets. And that’s how you get through. You hedge your bets.” Under this logic, genera that had large geographic ranges — regardless of whether or not they were part of the dominant animal group — made it through the K-PG. Those that did not faced higher rates of extinction.
Invertebrate paleontologist Steve Stanley of the University of Hawaii at Manoa agrees with Jablonski that geographic range plays a role in extinction survival at the genus level, but adds that it can also relate to the survivorship of larger groups such as the class that includes bivalves.
The key lies in a group’s background extinction rate, which can depend on a number of factors, from a group’s ability to spread over broad geographic areas to its ability to live in a variety of ecosystems. Groups that have narrow geographic ranges or are ecologically specialized tend to experience high background extinction rates. These groups also tend to diversify quickly after mass extinction events, Stanley says, but at the same time, they also tend to suffer heavily in mass extinctions, sometimes dying out altogether.
Case in point, Stanley says: bivalves versus ammonites, an order of mollusks related to squid. The bivalves have had a relatively low background extinction rate throughout history while the ammonites had a relatively high background extinction rate.
The result? Bivalves have survived every extinction since the Late Ordovician, Stanley says, and now clams, mussels and oysters litter beaches all over the world. On the other hand, ammonites suffered huge setbacks after mass extinctions starting with the Late Devonian (despite radiating quickly afterward) until finally dying out at the K-PG.
But even with these insights, scientists can’t predict with certainty which species will and will not make it through a mass extinction. Some widespread bivalve and snail genera of the Cretaceous still died out at the K-PG, Jablonski notes.
“We can’t underestimate the role of chance,” adds Peter Harries, an invertebrate paleontologist at the University of South Florida in Tampa. “Although in general it may be true that if you have a broad enough geographic range your chances of getting through are greater, that does not guarantee it.”
And, Jablonski says, there’s no guarantee that even if an animal group survives a mass extinction, it will live on to become a regular player in the periods that follow. Take the multituberculates, a group of mammals that resembled rodents. They survived the asteroid impact only to die out 30 million years later (possibly as a result of competition with other mammals), Holtz says. And the brachiopods that survived the Permian-Triassic extinction and rebounded during the Triassic began a slow decline in the middle of the Cretaceous, Bottjer says. Although they are still around today, brachiopods are now much less abundant than they once were.
Why mass extinction survivors may not survive in the long term is a puzzle that rankles paleontologists. “I will fess up right now: We still don’t really understand how you can just get through and then not participate in what happens next,” Jablonski says. Or why certain groups, like mammals, take off. “Would you really have predicted from the small, sort of rodent-like mammals of the Late Cretaceous that you’d end up with whales in 10 million years?”
But understanding something about who survives extinction events and why is important because it gives paleontologists a good start on understanding recoveries, Jablonski says. “You’ve got to understand how that selectivity works, or you don’t know who’s in the game. Once you understand who’s in the game, then you can start asking questions about who wins and who loses in the recovery phase.”
The sixth mass extinction?
Humans are one of nature’s most successful species. But expanding our territory has come at a cost to other creatures. Some estimates show that current extinction rates are up to 1,000 times higher than they were before the arrival of humankind. Are humans the modern equivalent of massive lava outpourings or asteroid impacts? Is life headed toward the next mass extinction?
Jablonski, Stanley, Bottjer, Harries and Holtz all agree that the current situation has not reached the level of a mass extinction. But that’s not to say humans haven’t had an effect. For example, humans have decimated island birds: The first inhabitants of the Pacific Islands alone killed more than 2,000 bird species, according to some estimates, or about 15 percent of all bird species. But more widespread species are doing OK, Jablonski notes. The blue jay, for example, continues to fly across the eastern half of the United States, he says, despite massive deforestation following the arrival of Europeans.
Conserving island birds and other species that survive in isolated ecosystems will be an uphill battle for the same reason that localized bivalves like the Japan-limited Anthonya died out at the K-PG, Jablonski says. Until widespread species like blue jays start disappearing from the skies, he says, today’s extinctions will not constitute a mass one.
But even if we were in the midst of a human- or otherwise-induced mass extinction, humans probably don’t have too much to worry about, Jablonski notes. “We’ll get through. There’s just no question about it,” he says. “We’ve got the ultimate broad geographic range as a species. We’re everywhere but Antarctica. The question is going to be: What kind of quality of life will we have?”
Even if humans do cause a mass extinction, life would likely achieve new heights of diversity and complexity afterward — as it has after each of the Big Five, Jablonski says. Still, he adds, it takes millions and millions of years to regain that diversity. “However rapidly recoveries seem on the geological timescale, on human timescales, they’re grindingly, achingly slow,” he says. “And the guys who recover are not necessarily those you are particularly fond of or care about.” After all, flies have wide geographic ranges too.