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 to ever live on Earth, could reach more than 15 meters in length and was the scourge of the ocean for millions of years. Then it disappeared. The megalodon was no more.
Exactly what happened to force this beast of a shark into extinction is a subject of much debate among scientists. Now, a paper published Tuesday in the journal Nature Communications suggests that great white sharks, which lived alongside the megalodon, preyed on the same species of animals as the much larger sharks. This evidence helps support the theory that competition with the great white, a predator still going strong today, could be a factor that has pushed the megalodon out of the picture. It also emphasizes the idea that a predator doesn’t have to be the largest to ultimately dominate an ecosystem.
Reconstructing the food chains of oceans from long ago 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 eat or set up a camera to spy on how they lived.
But there are other methods. One way to deduce what an animal has eaten is to examine the molecules that make up its body. Zinc isotope levels in the teeth of contemporary mammals correlate with where they fall in the food chain, many other studies have shown: The higher up the food chain an animal is, the lower the zinc isotope values they exhibit. Because teeth fossilize well, the team wondered if the same would apply if they looked at teeth from millions of years ago.
Using teeth from more than a hundred sharks, drawing on species alive today that are long gone, the researchers conducted tests to see if zinc levels changed as teeth weathered. They also confirmed that in today’s sharks, zinc isotope levels reflect their place in the ecosystem — sharks that eat small fish, for example, have higher values than sharks that eat whales and are higher up the food chain.
The researchers then considered the food web outlined by the numbers of ancient teeth. The results revealed intriguing patterns.
“We have the same range of zinc isotope values in great white sharks, in the same place, as the megalodon,” said Dr. McCormack. “It’s super interesting. They are of course very different in size, but that implies that they overlap in their prey species.”
It paints a picture of the huge shark gliding past, casting a shadow like a bus in its pursuit of unlucky fish, and in the background the great white, then a relatively small form, picking up the same prey for itself.
If the great white ate the same kind of prey, then perhaps the smaller sharks were competing with the megalodon for food. If so, they may have contributed to its eventual demise, in addition to possible changes to other aspects of the ecosystem, such as climate. It’s an idea that scientists have floated in the past, but there was no geochemical evidence to support the hypothesis, said Dr. McCormack.
As researchers try to figure out what ecosystems were like millions of years ago — who ate who and where — a measurement like zinc isotope value could help fill in the blanks, he hopes. It’s still a novel idea to use it that 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 winked in the fossil record. .
