Mystery event caused shark populations to crash 19 million years ago

Fossil records show that roughly 90 percent of the planet's open-ocean sharks inexplicably vanished.

Sharks are some of nature’s greatest survivors. For more than 400 million years, the marine predators have plied Earth’s waters, from shallow reefs to the heart of the open ocean. Sharks are older than the oldest fossil forest. They’ve made it through at least four mass extinctions.

And yet, 19 million years ago, something mysteriously dealt open-ocean sharks a huge blow—one from which they’ve never recovered.

Records of this extinction, detailed for the first time in the journal Science, come in the form of shark scales, called denticles, found in seafloor samples from the Pacific Ocean. Based on the shapes and abundance of denticles in the samples, the researchers estimate that the planet’s open-ocean shark populations suddenly and inexplicably fell by more than 90 percent. By contrast, during the extinction event that killed off the non-avian dinosaurs 66 million years ago, sharks suffered losses of roughly 30 percent.

“The sharks are screaming, Something really big happened here,” says lead study author Elizabeth Sibert, a paleobiologist and oceanographer at Yale University. “Sharks have a 400-million-year evolutionary history; they’ve been around a long time; they’ve seen a lot. And there’s something that could knock out 90 percent of them?”

Nobody knows what triggered the extinction, but whatever occurred, it must have happened in roughly 100,000 years, the geological blink of an eye. Bizarrely, the extinction doesn’t align with any known major shift in Earth’s climate or any major evolutionary change among other open-ocean predators.

That said, the decline in sharks may have given other marine animals their chance to shine. Several million years after the extinction, groups including tuna, seabirds, whales, and migratory sharks diversified, locking in the kind of open-ocean ecosystem we see today.

Researchers also caution that the findings may bode ill for modern shark populations. Since 1970, the total number of oceanic sharks and rays has fallen by 71 percent, according to a recent study published in Nature, a decline brought on by overfishing. If open-ocean sharks could take an existential wallop 19 million years ago and never fully recover, what will future oceans look like because of human activity?

“In a way, it does show clearly how fragile these top predators—these charismatic animals—are to any sort of sudden environmental change,” says shark paleobiologist Mohamad Bazzi, a Ph.D. candidate at Uppsala University in Sweden who wasn’t involved with the study. “It has tremendous implications today.”

Shark library

Sibert first stumbled across hints of this mystery extinction several years ago, while she was trying to understand the broad patterns of how fish—including sharks—lived in the open ocean over the last 85 million years of Earth’s history.

To uncover these big-picture trends, Sibert sought out one of Earth’s most important libraries: deep-sea sediment cores that scientists have been drilling since 1968. In essence, the seafloor acts as a history book the size of a planet. Chemical and fossil clues in each sediment layer inscribe the history of how Earth has changed over time, as well as how life responded. For example, these records have been critical to reconstructing past changes in Earth’s climate.

Sibert’s focus was on a more obscure component of the cores: “ichthyoliths,” tiny fish-derived fossils that include fish teeth and shark denticles, which the animals constantly shed and replace in life. By tracking the types and overall amounts of these fossils through the cores’ many layers, Sibert hoped to track changes in ocean ecosystems over long periods of Earth’s recent history.

To ensure that she got a good sense of global patterns, as opposed to local or regional ones, Sibert relied on two sediment cores that had been drilled from the Pacific Ocean’s subtropical gyres, vast swirling currents that have remained stable for tens of millions of years, and where any given spot on the seafloor might contain denticles and teeth from animals that lived hundreds to thousands of miles away. This kind of science requires the investment of generations. The main core Sibert focused on had been collected in the South Pacific in 1983, before Sibert was born.

When Sibert counted the shark scales and fish teeth, she found that the open ocean shifted gears several times over the past 85 million years. Up until the dinosaur-killing extinction 66 million years ago, sediments contained about one shark denticle for every fish tooth. By a couple million years later, the proportion of shark denticles had fallen by half.

By roughly 56 million years ago, the ratio had stabilized to one shark denticle for every five fish teeth. That ratio stayed rock-solid for the next 40 million years or so—until 19 million years ago, when she could find only one denticle for every 100 fish teeth.

“There’s no way to unsee it,” she says.

The devil’s in the denticles

Sibert published these observations in the journal Proceedings of the Royal Society B in 2016, but there was still a lot she didn’t know. Did the collapse hit all types of sharks equally? Or did some types of scales—and, by extension, types of sharks—go completely extinct 19 million years ago?

To find out, Sibert teamed up with a dedicated student, Leah Rubin, who was then an undergraduate student at the College of the Atlantic in Bar Harbor, Maine. After looking at nearly 600 pictures of modern shark, skate, and ray skin, and nearly 1,300 fossils, Rubin figured out how to classify the sediments’ fossil denticles based on features such as their shapes and ridges.

“They’re too small to be seen with the naked eye to any detail … You can’t really get a feeling for how gorgeous and intricate they are,” says Rubin, who is now starting a Ph.D. at SUNY ESF in Syracuse, New York.

Once Rubin and Sibert classified the denticles, their results were shocking. Samples less than 19 million years old had just 30 percent the denticle types that older sediments had. Something, somehow, had wiped out many, if not most, of the Pacific’s open-ocean shark species.

What’s more, the extinction appears to have hit some groups harder than others. So-called geometric denticles, which tend to belong to slower-swimming sharks among modern species, collapsed at 19 million years, while other scale types persisted.

Spotty records

The discovery will no doubt spark renewed interest in this time period, known in scientific circles as the Early Miocene sub-epoch. Existing climate records from the time suggest that Earth’s climate was stable then, but it’s also poorly understood.

According to Sibert, of the 683 deep-sea sediment cores that go deep enough to cover the Early Miocene, more than 80 percent are missing sediments from that time period, for reasons that remain unclear. But between the clear fossil evidence and Earth’s spotty record-keeping, it’s entirely possible that some kind of short-term climate event struck Earth 19 million years ago.

“Some aspects of [ancient Earth’s] study are still so young that you can make enormous discoveries about things that happened relatively recently,” says James Rae, a climate scientist at the University of St. Andrews in Scotland who wasn’t involved with the study.

In the 1980s, researchers noticed that deep-sea sediments showed that marine plankton went through a major extinction about 55 million years ago. Later evidence revealed that at that time, carbon dioxide levels rose rapidly, causing temperatures to rise and Earth’s oceans to acidify.

Geologists now closely study this period, which is called the Paleocene-Eocene Thermal Maximum, to learn more about how Earth might respond to human-caused climate change. Perhaps future scientists will study the Miocene shark extinction in the same way—but only more data will solve the mystery.

“There’s gotta be something there,” Sibert says. “We just don’t know what it is yet.”

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