Twenty million years ago, a predator with a mouth the size of a subway door and teeth bigger than a human hand roamed the seas. The megalodon, the largest shark that ever lived on Earth, could reach more than 15 meters in length, and was the scourge of the ocean for millions of years. Then he disappeared.
What exactly happened that drove this beast 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 megalodon, hunted the same kinds of animals that the much larger shark ate.
This evidence helps support the theory that competition with the great white, a predator that is still strong today, may have been a factor in pushing the megalodon out of the picture. It also underscores the idea that a predator doesn’t need to be the biggest in size to dominate an ecosystem.
Reconstructing the food webs of ancient oceans is a difficult task, said Jeremy McCormack, a geoscientist at the Max Planck Institute for Evolutionary Anthropology in Germany and author of the new paper.
We can’t see 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 to examine the molecules that make up its body. The levels of zinc isotopes in the teeth of today’s mammals correlate with their position in the food chain, as many other studies have found: the higher up the food chain an animal is, the lower the zinc isotope value it shows. Because teeth fossilize well, the team wondered if the same would happen if they examined teeth from millions of years ago.
Using teeth from more than a hundred sharks, extracted from species alive today and from those long gone, the researchers carried out tests to see if zinc levels changed as the teeth aged. They also confirmed that in sharks today, zinc isotope values ​​reflect their place in the ecosystem — sharks that eat tiny fish have higher values, for example, than sharks that eat whales and are further up the food chain.
The researchers then considered the food web outlined by the numbers of 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 we do in megalodons,” McCormack said. “It’s super interesting. They’re obviously very different in size, but that implies there’s an overlap in the species they prey on.”
He paints a picture of the huge shark gliding by, casting a shadow like a bus in its search for unfortunate fish, and in the background the great white, a comparatively diminutive form at the time, snapping up the same prey for itself.
If the great white was eating the same types of prey, then perhaps the smaller sharks were competing with the megalodon for food. In this case, they may have contributed to its eventual downfall, along with possible changes in other aspects of the ecosystem, such as the climate. It’s an idea that scientists have floated in the past, but there was no geochemical evidence to support the hypothesis, McCormack said.
As researchers look to piece together the picture of ecosystems millions of years ago — who ate what and where — a measure like the zinc isotope value could help fill in the gaps, he hopes. It’s still a new idea to use it so much later, but perhaps with more data from other creatures it can help us understand what happened so long ago, when organisms like megalodon disappeared from the fossil record.
Translated by Luiz Roberto M. Gonçalves
