About 19 million years ago something terrible happened to sharks.
Fossils collected in Pacific Ocean sediments reveal an unknown and dramatic shark extinction event, during which predator populations plummeted by as much as 90 percent, researchers reported on June 4th. And scientists don’t know what may have caused the death.
“It’s a big mystery,” says Elizabeth Sibert, a paleobiologist and oceanographer at Yale University. “Sharks have been around for 400 million years. They went through hell and back. And yet, this event wiped out (up to) 90 percent of them. "
Sharks suffered losses of 30 to 40 percent after the asteroid attack that killed all non-bird dinosaurs 66 million years ago (SN: 8/2/18). But after that, sharks enjoyed approximately 45 million years of peaceful ocean domination, going through even major climate disruptions such as the Paleocene-Eocene Thermal Maximum, an episode about 56 million years ago marked by a sudden rise in global carbon dioxide and high temperatures – without many problems (SN: 5/7/15).
Now, the clues found in the fine red clay sediments beneath two vast regions of the Pacific add a surprising new chapter to the history of sharks.
Sibert and Leah Rubin, then undergraduate students at Atlantic College in Bar Harbor, Maine, sifted through fish teeth and shark scales buried in sediment cores collected during pre-North Pacific and South Pacific research expeditions.
“The project arose from a desire to better understand the natural variability of the bottom of these fossils,” says Sibert. The bodies of sharks are mostly made of cartilage, which does not tend to fossilize. But its skin is covered with small scales or dermal denticles, each about the width of a human hair follicle. These scales are an excellent record of the abundance of past sharks: like shark teeth, the scales are made of the mineral bioapatite, which is easily preserved in sediments. “And we’ll find several hundred more denticles compared to a tooth,” Sibert says.
Sibert and L.D. Rubin / Science 2021
The researchers did not expect to see anything particularly surprising. From 66 million years ago to about 19 million years ago, the ratio of fish teeth to shark scales in sediments has remained constant at about 5 to 1. But abruptly – the team estimates within 100,000 years, and possibly even faster – that ratio has changed dramatically, to 100 fish teeth for every 1 shark scale.
The sudden disappearance of shark scales coincided with a change in the abundance of shark scale forms, which give some clues about changes in biodiversity. Most modern sharks have linear striations in their scales, which can increase swimming efficiency. But some sharks lack these striations; instead, scales come in a variety of geometric shapes. By analyzing the change in the abundances of the various forms before and after 19 million years ago, the researchers estimated a loss of shark biodiversity of between 70 and 90 percent. The extinction event was "selective," says Rubin, now a marine scientist at New York State University College of Environmental Science and Forestry in Syracuse. After the event, the geometric scales "almost disappeared and never reappeared in the diversity they (before) did."
There is no obvious climatic event that could explain such a huge change in shark population, Sibert says. "Nineteen million years ago is not known as a formative time in Earth history." Solving the mystery of death is at the top of a long list of questions he hopes to answer. Other questions include a better understanding of how different denticles may relate to shark lineages and what impact the sudden loss of so many large predators on other ocean dwellers could have.
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It is a question with modern implications, as the same issue of Science is written in a commentary by paleobiologist Catalina Pimiento of the University of Zurich and paleobiologist Nicholas Pyenson of the Smithsonian National Museum of Natural History in Washington, D.C. In the last 50 years alone, the abundance of sharks in the oceans has decreased dramatically by more than 70 percent as a result of overfishing and warming of the oceans. The loss of sharks – and other major marine predators, such as whales – from the oceans has "profound, complex and irreversible ecological consequences," the researchers write.
In fact, one way to look at the study is as a warning tale about the limits of modern conservation, says Catherine Macdonald, a marine conservation biologist at the University of Miami who did not participate in this study. "Our power to act to protect what's left doesn't include the ability to completely reverse or undo the effects of the huge environmental changes we've already made."
Major marine predator populations may be important indicators of these changes and revealing how the ocean ecosystem has responded to its loss in the past could help researchers anticipate what may happen in the near future, Sibert says. "Sharks try to tell us something," he adds, "and I can't wait to find out what it's all about."