Nov 2, 2021 8:00 AM

Climate-Driven Extinction Made Mammals' Teeth Less Weird

Fossils show how species diversity—and dental diversity—suddenly collapsed 30 million years ago, suggesting a link between climate, diet, and survival.

Dorien de Vries always asks for permission before flying across the world to touch someone else’s teeth. Some of the owners are anxious. Their teeth are fragile—irreplaceable. But de Vries, a paleontologist, sets their minds at ease. She knows how to be extra careful. “It's exactly the same as dentists’,” she says of the gooey paste she uses to capture the tooth topography. “It sets really quickly and you can peel it off.” She casts the molds and then 3D-scans the replica teeth into digital immortality.

Well, maybe not exactly like a dentist. The teeth De Vries is working with are up to 56 million years old—they once belonged to the mammals of the late Eocene, Oligocene, and Miocene Epochs and are now preserved in museum and university collections.

De Vries, currently a postdoctoral researcher at the University of Salford, in the United Kingdom, has traveled to Paris, Kenya, and around the United States to inspect these molars, hard evidence of why their former owners survived a climate-driven extinction—or why they didn’t. In new results published in October in Communications Biology, a team led by de Vries and Erik Seiffert, a paleontologist with the University of Southern California, shows that a mass extinction swept across Africa and Arabia roughly 30 million years ago. They used fossils from five groups of mammals, and their results suggest two-thirds of those species got wiped out. “It is huge,” says de Vries. “It's a lot of diversity loss.”

Scientists have known that the climate cooled in the transition from the Eocene to Oligocene 34 million years ago, leading to extinctions in different parts of the globe. But because less attention has been paid to the region, the question of whether the mammals of Africa and Arabia were similarly wiped out has been a long-running mystery. “The record has been so crappy,” says Samantha Hopkins, a paleontologist at the University of Oregon who was not involved in the study.

“Africa has always been a big question mark,” says Seiffert, who is de Vries’ former advisor. The two of them thought this was a problem teeth could solve. Since mammals’ teeth are so hard, they’re the most likely body part to fossilize and survive to the present day. They are also, as Seiffert puts it, “really amazingly diverse. From an elephant to a cow to a human, the teeth are really dramatically different.” Because they are so different, they are excellent evolutionary signatures, offering clues to how and when a species lives or dies out. Over the past 20 years, aided by improving computer algorithms that make sense of high-resolution 3D data, like the dental scans de Vries made, teeth have become a better and better tool for teasing out evolutionary trees.

“The really exciting thing about this study is that they've taken an unusual—and really creative—solution to understand the time period that we just don't have a whole lot of fossil records of right now,” says Hopkins. Their clever use of tooth records shows how the global climate shaped survival in the past—and offers important lessons about what might make animals vulnerable to extinction today.

Earth was warmer 34 million years ago—and wonkier. The Pangea supercontinent had split up. The dinosaurs were long gone. But Antarctica contained glacier-free forests. The other continents looked like squished and smeared versions of their present selves. Mammals were everywhere—especially primates and rodents. “From New York to Los Angeles, up into Canada, bopping around in the trees all over the place,” says Seiffert of North America’s primates. “But when this climatic event happened 34 million years ago, they all disappeared.”

Some scientists believe that atmospheric carbon dioxide levels dropped past a critical threshold, causing average air temperatures to drop and Antarctica to freeze. More sunlight reflecting off of more ice made temperatures plummet further. The transition from the Eocene to Oligocene has been described as a transition from “greenhouse” to “ice house.”

Then, in Africa, things got even worse. Around 31 million years ago, volcanoes near the equator, in present-day Ethiopia, exploded with devastating toxic fumes and unrelenting floods of molten basalt.

North American, European, and Asian fossil records are pretty well-established during the 11 million-year stretch before and after these events. Scientists could count up the fossils showing which animals existed before the climate cooled, and which after, and figure out what went missing. But, Seiffert says, “during this time period, the fossil record in Africa is actually really patchy.” That discrepancy bugged him, so his team has tried to parse the relationships between whatever fossil records they do have.

For their study, Seiffert and de Vries focused on a family tree that goes back 76 million years, when primates and rodents diverged. Specifically, they studied the teeth of two suborders of rodents (hystricognath and anomaluroid) and two suborders of primates (strepsirrhine and anthropoid). These clades gave rise to existing species like capybara, scaly-tailed flying squirrels, lemurs—and us.

The researchers resolved to reconstruct the phylogeny—or family tree of evolutionary relationships—of these groups from 56 million to 15 million years ago. Using teeth as a guide for “who is who,” they drew branches between lineages extending from fossils found in the late Eocene to their progeny that survived into the Miocene, around 20 million years ago. When they finished, a stark gap emerged: The lineages from the Miocene descended from an oddly small fraction of earlier mammals. The researchers found that 63 percent of lineages that existed in the late Eocene never made it past the next epoch. About 30 million years ago, they concluded, these species must have vanished, thanks to their changing environment. “There's really no other explanation,” Seiffert says. “They must have gone extinct.”

Lineage diversity gave the team a big picture view of how many species were lost to a changing climate, but not of how different those species might have been from one another—in other words, how much anatomic diversity went extinct, too. For example, says de Vries, imagine a scenario in which two kinds of bird go extinct. Those two species could be very similar, or they could be wildly different in terms of body types, genetics, or ecological niches. “If you have a hummingbird and a flamingo, that is very different than if you had a pigeon and a dove,” she says.

And for the extinction of biodiversity among mammals, the teeth kept the record. A wide variety of tooth shapes narrowed down to a few. During her PhD at Stony Brook University, de Vries had digitized about half of the 329 fossilized teeth used for the study, which represent 134 species. After taking molds from the teeth, she used a micro-CT scanner to analyze the casts. This technology can achieve higher-resolution images than the normal CT scans you’d get in a hospital and let her record objective metrics of how teeth varied between lineages, and over time. She could study every bump, peak, and valley, measuring curviness or sharpness.

These metrics could quantify how complex an animal’s chewing apparatus was. Tooth shape suggests a lot about an animal’s diet. Fruit eaters have rounded bumps, or cusps, that can burst through berries. Leaf eaters tend to have tall, sharp crests that rip through tough plant cell walls. Super-flat molars, on the other hand, are prime topography for crushing seeds. Yet that specialization can be a problem. For example, while flat-toothed primates can munch the hell out of some seeds, they’re lousy with leaves. And if the climate changes in such a way that the stuff an animal has evolved to chew becomes scarce, they’ll go hungry.

Based on the team’s analysis of 3D scans, many lineages with specialized teeth died off during this slow mass extinction. One Apidium monkey species sticks out to Seiffert. Human upper molars typically have four cusps. This species had nine. “We have never seen that kind of really weird tooth type again—in the fossil record or among living species,” he says. Seiffert supposes that this monkey ate fruits and seeds. But it no longer exists.

It’s an “almost unbelievably tight” connection between the die-off of species and the die-off of tooth diversity, says de Vries. The extinction didn’t axe any particular branch of these mammals’ family trees, and no single tooth shape or diet disappeared. But the specialists, whose tooth shape restricted their diets the most, were more likely to die out, and the generalists tended to be less vulnerable. It's a not-entirely-surprising warning for modern-day species like lemurs in Madagascar that only eat bamboo. If climate change wipes out that bamboo, these unique creatures are out of luck.

For the mammals that did survive, the results also suggest dietary changes once the global cooling began—likely a consequence of them migrating toward the equator for a warmer climate. (Seiffert notes that some areas may have not cooled much, but rather just become more arid and less hospitable to forests.) For example, a subset of anomaluroid rodents (the ancestors of some scaly-tailed squirrels) evolved sharper crests suited to leafy diets and tree sap, possibly to avoid competition with the hystricognaths, the capybara and guinea pig ancestors that lived on the ground.

But it was as more species funneled toward the equator that Ethiopia’s volcanoes blew. Seiffert sees the climate-volcano combo as a “one-two punch” that unlucky species couldn’t escape. Plots showing the diversity of both the phylogenetic tree and tooth shapes show two distinct drops, around 34 million and 31 million years ago, reflecting these back-to-back disasters.

Seeing that “double drop” was surprising, says Alistair Evans, an evolutionary biologist at Monash University in Australia who specializes in tooth analyses but was not involved in the work. “Nobody thought that that would be there,” Evans says.

But, he adds, the team’s data clearly shows the large-scale effect of volcanism and global cooling because diversity collapses twice. You can even see indicators of when mammals moved into trees, shown by how their teeth adapted to bite leaves, he says. “It reaffirms my confidence that we can actually tease apart these patterns in paleontological time—in deep time,” he continues.

The same type of analysis could help biologists understand different periods, like how mammals recovered after the Cretaceous–Paleogene extinction that killed 75 percent of all species. Here, mammalian teeth would perhaps instead show a boom in diversity as species evolved to fill environmental voids.

In December, Seiffert is heading to Kenya to study more evidence from this particular stretch of time. If he's lucky, he'll find teeth that will help him draw new branches on the mammalian evolutionary tree. Maybe pointy teeth. Or round ones. Or flat ones. Or ones simply unlike any he’s ever seen.