The big. The mighty. The Meg.
When you think of Megalodon – which is the generalized name we having given megasharks like the 3.5-metre-long Otodus auriculatus to the 15-metre-long Otodus megalodon – you think of a behemoth ruler of our ancient oceans. With fossilized teeth exceeding seven inches (17.8 centimeters), there is nothing ‘tiny’ about these predators. Thus, one shouldn’t be surprised that they were top dogs of our oceans.
However, a new study highlights how unique their reign was.
“It is very likely that megatooth sharks were at a higher trophic level than any other marine predator,” says co-author Zixuan Rao, a graduate student from The Sigman Laboratory at Princeton University. Led by biogeochemist Dr. Emma Kast, from the University of Cambridge, a team of researchers discovered that Megalodon and other megatooth sharks may have eaten not only other predators but each other, meaning they occupied an unusually high position in the ancient oceanic food web.
3D render of an extinct Megalodon shark in the seas of the Cenozoic Era
Having long been extinct, scientists couldn’t observe their interactions with other animals in real-time like they often do for modern sharks. Instead, they relied on clues left behind by the sharks on their teeth.
The colleagues analysed nitrogen ratios in enameloid – found in many fish and acts like an enamel-like tissue covering the teeth – from five extinct species of megatooth sharks. They also carried out stable isotope analysis on the teeth, focusing specifically on measuring the nitrogen isotope ratio. Based on the idea that “you are what you eat,” stable isotope ratios vary within and among food webs and are incorporated into an animal’s tissue through its diet. One of the most important applications of nitrogen stable isotopes is its ability to determine the trophic level of a species. Nitrogen has two isotopes found in the tissues of animals, N-14 and N-15, but because of the way this element is processed and excreted the latter builds up the higher in the food chain you go. Ultimately, if another predator eats the first predator, it will incorporate even more N-15 into its tissues and so on and so forth. Scientists can estimate how high up in a food web an ancient animal would have been by using the ratio of N-15 to N-14 in fossils.
The results from the teeth analysis suggest not only that megatooth sharks were at the top of food webs, but that they ate other predators near the top of the food web too. “We’re used to thinking of the largest species – blue whales, whale sharks, even elephants and diplodocuses – as filter feeders or herbivores, not predators,” says Kast. “But Megalodon and the other megatooth sharks were genuinely enormous carnivores that ate other predators, and Meg went extinct only a few million years ago.”
Harry Maisch of Florida Gulf Coast University, whose hand is holding this Megalodon tooth, gathered … [+]
Which predators could the Meg have dined on? Well, short of us getting into a time machine and seeing it for ourselves, we don’t know since the nitrogen isotope ratios in the marine mammals of today weren’t high enough to account for the high N-15 levels found in these fossils. “We’ve never seen nitrogen ratio values this high until this project. We expected high values, but not this high,” says Rao.
One way that extra trophic level or two could be explained? Cannibalism. Not a far-fetched idea since some modern-day sharks are known to partake in this, and evidence has pointed that megatooth sharks and other prehistoric marine predators also were cannibalistic. “To reach the trophic levels we’re measuring in these megatooth sharks, we don’t just need to add one trophic level — one apex predator on top of the marine food chain — we need to add several onto the top the modern marine food web,” explains Kast.
The team suggests that understanding the evolution and behaviour of these megasharks through the remnants they left behind can help us to understand how past climate events impacted the marine ecosystems they called home. “Looking at the past is the key to the future, if we can understand how the climate impacted ecosystems in the past, it can help guide us to protect life in the future,” says Rao.
The project idea came to life as Kast quarantined at home during COVID-19, painstakingly building up a record with more than 25,000 marine individuals. Now, the researchers want to turn their chemical analysis system on to teeth from other animals, including mammals and dinosaurs. “Our tool has the potential to decode ancient food webs; what we need now is samples,” said Kast. “I’d love to find a museum or other archive with a snapshot of an ecosystem — a collection of different kinds of fossils from one time and place, from forams near the very base of the food web, to otoliths — inner ear bones — from different kinds of fish, to teeth from marine mammals, plus shark teeth. We could do the same nitrogen isotope analysis and put together the whole story of an ancient ecosystem.”
