Whenever paleontologist Dana Ehret gives talks about the 15-meter-long prehistoric sharks known as megalodons, he likes to make a joke: “What did megalodon eat?” asks Ehret, Assistant Curator of Natural History at the New Jersey State Museum in Trenton. “Well,” he says, “whatever it wanted.”
Now, there might be evidence that’s literally true. Some megalodons (Otodus megalodon) may have been “hyper apex predators,” higher up the food chain than any ocean animal ever known, researchers report in the June 22 Science Advances. Using chemical measurements of fossilized teeth, scientists compared the diets of marine animals — from polar bears to ancient great white sharks — and found that megalodons and their direct ancestors were often predators on a level never seen before.
The finding contradicts another recent study, which found megalodons were at a similar level in the food chain as great white sharks (SN: 5/31/22). If true, the new results might change how researchers think about what drove megalodons to extinction around 3.5 million years ago.
In the latest study, researchers examined dozens of fossilized teeth for varieties of nitrogen, called isotopes, that have different numbers of neutrons. In animals, one specific nitrogen isotope tends to be more common than another. A predator absorbs both when it eats prey, so the imbalance between the isotopes grows further up the food chain.
For years, scientists have used this trend to learn about modern creatures’ diets. But researchers were almost never able to apply it to fossils millions of years old because the nitrogen levels were too low. In the new study, scientists get around this by feeding their samples to bacteria that digest the nitrogen into a chemical the team can more easily measure.
The result: Megalodon and its direct ancestors, known collectively as megatooth sharks, showed nitrogen isotope excesses sometimes greater than any known marine animal. They were on average probably two levels higher on the food chain than today’s great white sharks, which is like saying that some megalodons would have eaten a beast that ate great whites.
“I definitely thought that I’d just messed up in the lab,” says Emma Kast, a biogeochemist at the University of Cambridge. Yet on closer inspection, the data held up.
The result is “eyebrow-raising,” says Robert Boessenecker, a paleontologist at the College of Charleston in South Carolina who was not involved in the study. “Even if megalodon was eating nothing but killer whales, it would still need to be getting some of this excess nitrogen from something else,” he says, “and there’s just nothing else in the ocean today that has nitrogen isotopes that are that concentrated.”
“I don’t know how to explain it,” he says.
There are possibilities. Megalodons may have eaten predatory sperm whales, though those went extinct before the megatooth sharks. Or megalodons could have been cannibals (SN: 10/5/20).
Another complication comes from the earlier, contradictory study. Those researchers examined the same food chain — in some cases, even the same shark teeth — using a zinc isotope instead of nitrogen. They drew the opposite conclusion, finding megalodons were on a similar level as other apex predators.
The zinc method is not as established as the nitrogen method, though nitrogen isotopes have also rarely been used this way before. “It could be that we don’t have a total understanding and grasp of this technique,” says Sora Kim, a paleoecologist at the University of California, Merced who was involved in both studies. “But if [the newer study] is right, that’s crazy.”
Confirming the results would be a step toward understanding why megalodons died off. If great whites had a similar diet, it could mean that they outcompeted megalodons for food, says Ehret, who was not involved in the study. The new findings suggest that’s unlikely, but leave room for the possibility that great whites competed with — or simply ate — juvenile megalodons (SN: 1/12/21).
Measuring more shark teeth with both techniques could solve the mystery and reconcile the studies. At the same time, Kast says, there’s plenty to explore with their method for measuring nitrogen isotopes in fossils. “There’s so many animals and so many different ecosystems and time periods,” she says.
Boessenecker agrees. When it comes to the ancient oceans, he says, “I guarantee we’re going to find out some really weird stuff.”
Source: Heart - www.sciencenews.org
