Great whites may have doomed the biggest shark that ever lived, fossil teeth reveal

A tooth belonging to an extinct Otodus megalodon (L) and a tooth of a modern great white shark (left) are shown. (MPI for Evolutionary Anthropology/CNN)

Megalodon, the giant shark that lived more than 23 million years ago and was inspiration for the movie "The Meg," was almost four times bigger than the great white shark that cruises our oceans today.

However, the two shark species, which once coexisted, likely hunted some of the same prey. This competition could potentially have been one reason why the 65-foot-long (20-metre-long) megalodon went extinct, a new study has suggested.

To arrive at this finding, the researchers involved in the study used a new technique. They analyzed dietary signatures contained in the teeth of 13 extinct shark species and 20 modern sharks to understand where they fit in the food chain -- also known as their trophic level.

"Megalodon is typically portrayed as a super-sized, monstrous shark in novels and films, but the reality is that we still know very little about the extinct shark," said study author Kenshu Shimada, a professor of paleobiology at DePaul University in Chicago and research associate at the Sternberg Museum of Natural History in Kansas.

"Our new study shows that the dietary range of the early Pliocene great white shark is very similar to that of megalodon, indicating that our data do not contradict the competition hypothesis," he said via email.

The researchers were able glean this information by looking at the presence of different isotopes, or variants, of the chemical element zinc preserved in the sharks' tooth enamel.

Zinc is essential for living organisms and plays a crucial role in bone development. The ratio of heavy and light zinc isotopes in the teeth preserves a record of the kind of animal matter that the sharks ate.

"Zinc isotopes can be used as ecological indicators because of the ratio of these two different isotopes changes as you move up the food chain," said coauthor Michael Griffiths, a geochemist and professor in the department of environmental science at William Paterson University in New Jersey.

For example, if megalodon ate great white sharks, its higher position in the food chain would be reflected in the isotopic record. But the study found the two species had some overlap, suggesting they shared similar prey items. However, the authors cautioned that they cannot rule out megalodon preying on great whites, given that its isotope values, and especially a close relative of megalodon, called Chubutensis megalodon, had values lower than any modern and fossil marine vertebrate measured.

Feeding at the same trophic level does not necessarily imply direct competition between megalodon and great white sharks for the same prey, as both species could have specialized in different prey. However, at least some overlap in food items between both species was likely, the study said.

"Like great whites today, they likely fed on large fish. The smaller sized great whites likely did not require as much food as megalodon did, so they would have had the competitive advantage if they were feeding on similar prey items," Griffiths said via email.

The research was published in the journal Nature Communications on Tuesday.

This study was the first time that zinc isotopes related to diet have been proven to preserve in fossilized shark teeth.

A similar technique, using nitrogen isotopes to study the dietary signatures for other groups of animals, is well established, the study said. However, nitrogen in teeth dentine doesn't preserve well enough to study animals that went extinct millions of years ago.

The technique using zinc isotopes could be applied to other extinct animals to understand their diet and ecology.