THE BEACHES WERE deserted near Santa Cruz, California, on December 23, 2007. Temperatures were cool for Cali standards, and the wind whipped unpleasantly across the sandy expanse.
But that didn't stop palaeontologist Robert Boessenecker from avidly trolling the chilly shores. A senior at Montana State University at the time, Boessenecker was on the hunt for fossils, and he soon found his prize: a dark greenish blue tooth the size of his hand—“about as big as they come,” he says—peeking out of a cliff.
This rare find came from the ancient Otodus megalodon, the largest shark to ever glide through Earth's oceans. Though movies continue to stoke the conspiracy that these nearly 18-metre-long beasts still lurk in the inky depths, the megalodon is unquestionably extinct. Boessenecker's find on that wintery day instead kicked off his decade-long quest to figure out exactly when these behemoths made their earthly exit.
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Now, he finally has his answer: The megalodon vanished some 3.6 million years ago, around a million years earlier than previous estimates. The new date, published February 12 in the journal PeerJ, coincides with the rise of the modern great white shark, hinting that this shakeup in the marine hierarchy may have spelled doom for the mighty “meg.”
Hunting the megalodon
To solve the cold case, Boessenecker, now an adjunct lecturer at the College of Charleston, and his colleagues started searching for more traces of the megalodon in California, creating a compendium of West Coast finds. They quickly expanded their search to other regions, to take a broader look at this ancient disappearing act.
The samples in their growing list were not only ones the researchers found themselves, but also fossils from the published literature and freshly scrutinised discoveries in museum collections. Several times throughout this hunt, the team tried to publish their work. And though reviews were generally positive, Boesenecker says, there always seemed to be a reason for rejection, including concerns about the paper's length (the final version is 47 pages, not counting appendices).
Still, they weren't the only ones hunting for clues. In 2014, Catalina Pimiento of the University of Florida and Christopher Clements of the University of Zurich published their analysis of the megalodon's extinction using available records. That team concluded that these creatures could have lingered in the oceans until sometime around 2.6 million years ago, a mere half million years or so before our ancient human relatives Homo erectus took their first wobbly steps.
Deadly detective work
For the latest study, Boessenecker and his colleagues combined their sleuthing with the 2014 study's data into one mega-catalogue. But they were suspicious of some of the more recently described teeth and vertebrae. Some samples were broken or chemically altered by the element phosphorus—evidence that they hadn't stayed put through the millennia and could be feigning a younger age. Others seemed to have iffy origins, making it impossible to accurately position them in time. Still others needed date readjustments that took more recent analyses of those samples into account.
Though movies continue to stoke the conspiracy that these nearly 18-metre-long beasts still lurk in the inky depths, the megalodon is unquestionably extinct. Scientists are now trying to figure out both when and why.
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Boessenecker estimates that they excluded 10 to 15 percent of the samples that lacked the necessary exactness in space and time. And as they pored through the remaining records one by one, a pattern started to emerge.
“It wasn’t quite Woodward and Bernstein sitting in the Library of Congress in All the President's Men,” Boessenecker says. “But it’s a lot of boring but classic detective work.” In the end, the results were clear: It's likely the megalodon was gone by 3.6 million years ago, with margins of error that mean the date could be as young as 3.2 million years ago and possibly as old as 4.1 million years ago.
“This is much more believable and robustly supported by the data,” says Tom Deméré, curator of paleontology at the San Diego Natural History Museum, who has worked with Boessenecker before and who was a reviewer of Pimiento and Clement's 2014 study.
“I think it is great that more research has been done on this species,” Pimiento says. She agrees with the exclusion of some of the samples, particularly ones whose physical positions shifted over time, or ones that had uncertain origins. But she doesn't necessarily agree with excluding samples that have a wide range of possible ages.
“You may get larger uncertainty around your most likely extinction time, but you wouldn’t be disregarding valuable information,” she says in an email.
Getting a more accurate read on the megalodon's demise is vital for understanding the forces driving the disappearance of these mega-beasts. Past theories were often tied to a wider marine extinction that happened as the curtains closed on the Pliocene epoch roughly 2.6 million years ago.
Before this change, the waters were full of relative oddballs: toothless walruses, aquatic sloths, dwarf baleen whales, and more. Some 36 percent of early marine genera didn't make it to the next epoch, Pimiento and her colleagues estimate in a 2017 study. Afterwards, a more modern menagerie began to appear.
Some astronomers have suggested the die-out was tied to a supernova that enveloped the planet in harmful muon radiation. But Pimiento and Boessenecker both emphasise that the extinction event could have been a much slower-moving affair, and many factors were likely at play. For one, Earth was going through a mighty flux during this time frame, Pimiento says. The world was cooling, so glaciers were expanding and sea levels were dropping, transforming available ocean space.
“For some species it may have been mainly climate,” she says. “For others, the extinction of those that became extinct first. For others, the evolution of new species after some had gone extinct (via competition). For others, all of these combined.”
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Shark vs. shark?
And based on the new study, Boessenecker thinks something else may have wiped out the megalodon. Intriguingly, the new dates coincide with the global spread of the creature's smaller but still fierce relative, the great white shark, Carcharodon carcharias, which made its global debut by some four million years ago. Though there were other changes during this window that could have affected the megalodon, they were largely local shakeups.
“Nothing else is that cosmopolitan,” Boessenecker says.
University of Maryland's Bretton Kent, who studies ancient sharks, praises the study's thoroughness, but suggests another possible culprit: the modern tiger shark, Galeocerdo cuvier, which once lingered in the same marine neighbourhoods as juvenile megalodon. Still, he adds that he's not convinced competition between species could cause such a decline.
It's tough to say exactly what happened in the ancient seas, and more study is needed to truly close the case, Boessenecker says.
“One of the fascinating things about paleontology is we get to explore this history of life,” Deméré adds. “The more people that are looking, and the more closely sampled this archive of Earth’s history is, the more we learn.”