Megalodons ate all they wanted, study shows

New research from Princeton shows that prehistoric megatooth sharks — the largest sharks that ever lived — were apex predators at the highest levels ever recorded.

Megatooth sharks get their name from their huge teeth, each of which can be larger than a human hand. The group includes Megalodon, the largest shark that ever lived, as well as several related species.

While sharks of one kind or another existed long before the dinosaurs – over 400 million years – these megatooth sharks evolved after the dinosaurs became extinct and ruled the seas until just 3 million years ago.

“We’re used to seeing the largest species — blue whales, whale sharks, even elephants and diplodocuses — as filter feeders or herbivores, not predators,” said Emma Kast, a Ph.D. graduated in geosciences who is the lead author of a new study in the current issue of scientific progress† “But Megalodon and the other megatooth sharks were really huge carnivores that ate other predators, and Meg died out just a few million years ago.”

Her advisor Danny Sigman, Princeton’s Dusenbury Professor of Geological and Geophysical Sciences, added: “If Megalodon existed in the modern ocean, it would profoundly change how humans interact with the marine environment.”

A team of Princeton researchers has now found clear evidence that Megalodon and some of its ancestors were at the very top of the prehistoric food chain — what scientists call the highest “trophic level.” In fact, their trophic signature is so high that they must have eaten other predators and predators in a complicated food web, the researchers said.

“Sea food webs are usually longer than the grass-deer-wolf food chain of terrestrial animals, because you start with such small organisms,” said Kast, now at the University of Cambridge, who wrote the first iteration of this research as a chapter in her dissertation. . “To achieve the trophic levels we measure in these megatooth sharks, we don’t need to add just one trophic level — one apex predator atop the marine food chain — we need to add several to the modern seafood web.”

Megalodon has been conservatively estimated to be 15 meters long – 50 feet – while modern great white sharks usually reach around five meters (15 feet).

To arrive at their conclusions about the prehistoric marine food web, Kast, Sigman and their colleagues used a new technique to measure the nitrogen isotopes in the sharks’ teeth. Ecologists have long known that the more nitrogen-15 an organism has, the higher its trophic level, but scientists have never before been able to measure the tiny amounts of nitrogen preserved in the enamel layer of the teeth of these extinct predators.

“We have a set of shark teeth from different time periods and we were able to trace their trophic level relative to their size,” said Zixuan (Crystal) Rao, a graduate student in Sigman’s research group and a co-author of the current paper.

One way to pack in an extra trophic level or two is cannibalism, and various evidence points to that in megatooth sharks as well as other prehistoric marine predators.

The nitrogen time machine

Without a time machine, there’s no easy way to recreate the food webs of extinct creatures; very few bones have survived with teeth marks that say, “I was chewed by a huge shark.”

Fortunately, Sigman and his team have spent decades developing other methods based on the knowledge that the nitrogen isotope levels in a creature’s cells reveal whether it’s at the top, middle, or bottom of a food chain.

“The whole direction of my research team is to look for chemically fresh, but physically protected, organic matter — including nitrogen — in organisms from the distant geological past,” Sigman said.

A few plants, algae and other species at the bottom of the food web have mastered the art of converting nitrogen from the air or water in their tissues into nitrogen. Organisms they eat then take up that nitrogen in their own bodies, and crucially, they preferentially excrete (sometimes via urine) more of the lighter isotope of nitrogen, N-14, than its heavier cousin, N- 15.

In other words, N-15 builds up over N-14 as you climb up the food chain.

Other researchers have applied this approach to creatures from the recent past — the most recent 10-15 thousand years — but until now there wasn’t enough nitrogen in older animals to measure.

Why? Soft tissue such as muscle and skin are almost never preserved. To complicate matters, sharks don’t have bones — their skeletons are made of cartilage.

But sharks have one golden ticket to the fossil record: teeth. Teeth are more easily preserved than bones because they are encased in enamel, a rock-hard material that is virtually immune to most decomposing bacteria.

“Teeth are designed to be chemically and physically resistant so they can survive in the highly chemically reactive environment of the mouth and break apart foods that may have hard parts,” explains Sigman. Plus, sharks aren’t limited to the roughly 30 pearly whites humans have. They are constantly growing and losing teeth – modern sand sharks lose a tooth on average every day of their decades-long lives – meaning each shark produces thousands of teeth in its lifetime.

“If you look in the geologic record, shark teeth are one of the most common fossils,” Sigman said. “And within the teeth, there’s a small amount of organic matter that was used to build the enamel of the teeth — and is now trapped in that enamel.”

Because shark teeth are so numerous and so well preserved, the nitrogen features in enamel provide a way to measure status in the food web, whether the tooth fell from a shark’s mouth millions of years ago or yesterday.

Even the largest tooth has only a thin shell of enamel, of which the nitrogen component is only a small trace. But Sigman’s team has developed increasingly sophisticated techniques to extract and measure these nitrogen isotope ratios, and with a little help from dental drills, cleaning chemicals and microbes that eventually convert the nitrogen from the enamel into nitrous oxide, they are now able to measure the N15. -N14 ratio in these ancient teeth to be accurately measured.

“We’re kind of like a brewery,” he said. “We grow microbes and feed them our samples. They produce nitrous oxide for us, and then we analyze the nitrous oxide they produced.”

The analysis requires a custom-built, automated nitrous oxide preparation system that extracts, purifies, concentrates and delivers the gas to a specialized stable isotope ratio mass spectrometer.

“This is a multi-decade quest I’ve been on to develop a core method to measure these traces of nitrogen,” Sigman said. From microfossils in sediments, they moved on to other types of fossils, such as corals, fish ear bones and shark teeth. “We and our collaborators then apply this to mammalian and dinosaur teeth.”

A deep dive into literature during lockdown

Early in the pandemic, while her friends were making sourdough starters and drinking Netflix, Kast flipped through the ecological literature looking for nitrogen isotope measurements of modern marine animals.

“One of the cool things Emma did was really dig into the literature — all the data that’s been published over the decades — and relate that to the fossil record,” said Michael (Mick) Griffiths, a paleoclimatologist and geochemist at the William Patterson University and a co-author on the paper.

While Kast went into quarantine at home, she painstakingly built a record of more than 20,000 marine mammals and more than 5,000 sharks. She wants to go much further. “Our tool has the potential to decipher ancient food webs; what we need now are samples,” Kast said. bases of the food web, to otoliths – bones of the inner ear – from various types of fish, to marine mammal teeth, plus shark teeth. We could do the same nitrogen isotope analysis and put together the whole story of an ancient ecosystem.”

In addition to the literature search, their database includes their own shark tooth samples. Co-author Kenshu Shimada of DePaul University associated with aquariums and museums, while co-authors Martin Becker of William Patterson University and Harry Maisch of Florida Gulf Coast University collected megatooth specimens from the seafloor.

“It’s really dangerous; Harry is a dive master and you really have to be an expert to get this one,” Griffiths said. “You can find little shark teeth on the beach, but to get the best preserved monsters you have to go to the bottom of the ocean. Marty and Harry have collected teeth everywhere.”

He added: “It’s really been a concerted effort to get the samples to put this together. Overall, working with Princeton and other regional universities is very exciting because the students are great and my colleagues there are really great to work with.”

Alliya Akhtar, a Ph.D. graduated from Princeton, is now a postdoctoral researcher in Griffiths lab.

“The work I did for my dissertation (looking at the isotopic composition of seawater) raised as many questions as it answered, and I was incredibly grateful to have the opportunity to continue working on some of these with a collaborator/mentor I respect ,” Akhtar wrote in an email. “I’m most excited about all the work that remains to be done, all the mysteries that remain to be solved!”

Reference: Kast ER, Griffiths ML, Kim SL, et al. Cenozoic megatooth sharks occupy extremely high trophic positions. Scientific advice† 2022;8(25):eabl6529. doi: 10.1126/sciaadv.abl6529

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