Twenty million years ago, a predator with a mouth like a subway door and teeth the size of your palm roamed the seas. The megalodon, the largest shark ever to live on Earth, could grow more than 50 feet long, and it was the scourge of the ocean for millions of years. Then it disappeared. The megalodon was no more.

Exactly what happened to push this beast of a shark to extinction is a topic of much debate among scientists. Now a paper published Tuesday in the journal Nature Communications suggests that great white sharks, which coexisted with the megalodon, preyed upon the same kinds of animals the much larger shark ate. This evidence helps to support the theory that competition with the great white, a predator that is still going strong today, might have been one factor that took the megalodon out of the picture. It also highlights the idea that a predator doesn’t have to be the biggest to eventually dominate an ecosystem.

Reconstructing the food chains of long-ago oceans is a difficult task, said Jeremy McCormack, a geoscientist at the Max Planck Institute for Evolutionary Anthropology in Germany and an author of the new paper. You can’t watch extinct animals feeding or set up a camera to spy on how they lived.

But there are other methods. One option for deducing what an animal ate is examining the molecules that make up its body. Zinc isotope levels in the teeth of present-day mammals correlate with where they fall in the food chain, many other studies have found: The higher up the food chain an animal is, the lower the zinc isotope values they show. Because teeth fossilize well, the team wondered whether the same would hold true if they looked at teeth from millions of years ago.

Using teeth from more than a hundred sharks, drawing from species alive today and those long gone, the researchers ran tests to see whether zinc levels changed as teeth weathered. They also confirmed that in present-day sharks, zinc isotope values reflect their place in the ecosystem — sharks that eat tiny fish have higher values, for instance, than sharks that eat whales and are higher in the food chain.

The researchers then considered the food web sketched by the numbers from ancient teeth. The results showed intriguing patterns.

“We have the same range of zinc isotope values in great white sharks, in the same locality, as the megalodon,” Dr. McCormack said. “It’s super interesting. They are obviously very different in size, but that implies that they have an overlap in their prey species.”

It paints a picture of the massive shark gliding along, casting a shadow like a bus in its pursuit of hapless fish, and in the background, the great white, a comparatively diminutive shape at the time, snapping up the same prey for itself.

If the great white was eating the same kinds of prey, then perhaps the smaller sharks competed with the megalodon for food. If so, they might have contributed to its eventual downfall, alongside potential changes in other aspects of the ecosystem, like climate. It’s an idea that scientists have floated in the past, but there was no geochemical evidence in support of the hypothesis, Dr. McCormack said.

As researchers seek to piece together what ecosystems were like millions of years ago — who ate whom, and where — a measurement like zinc isotope value can help fill in the blanks, he hopes. It’s still a new idea to use it this far back in time, but perhaps with more data from other creatures, it may eventually help us understand what happened so long ago, when organisms like the megalodon wink out in the fossil record.

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