A new Princeton study shows that prehistoric megatooth sharks – the largest sharks that have ever lived – were predators at the highest level ever measured.
Megatooth sharks got their name from their massive teeth, each of which may be larger than a human hand. The group includes Megalodon, the largest shark that has ever lived, as well as several related species.
While sharks of one species or another existed long before dinosaurs appeared – more than 400 million years ago – these mega-toothed sharks evolved after dinosaurs became extinct and ruled the seas just 3 million years ago.
“We used to think of the largest species – blue whales, shark whales, even elephants and broodstock – as filter feeders or herbivores, not predators,” said Emma Cast, PhD in 2019. a graduate of geological sciences who is the first author of a new study in the current issue Advances in science. “But Megalodon and other mega-toothed sharks were really huge carnivores that ate other predators, and Meg became extinct only a few million years ago.”
Her adviser, Danny Sigman, a professor of geology and geophysics at Princeton in Dusenber, added: “If Megalodon existed in the modern ocean, it would fundamentally change people’s interactions with the marine environment.”
A team of researchers from Princeton has found clear evidence that Megalodon and some of his ancestors were at the highest rung of the prehistoric food chain – what scientists call the highest “trophic level”. Indeed, their trophic trait is so high that they have probably eaten other predators and predators-predators in a complex food web, the researchers say.
“Ocean food webs tend to be longer than the grass-deer-wolf food chain in terrestrial animals because you start with such small organisms,” said Cast, who is now at Cambridge University who wrote the first iteration of this study as a section. in her dissertation. “To reach the trophic level that we measure in these mega-toothed sharks, we don’t just need to add one trophic level – one top predator at the top of the marine food chain – we need to add a few to the top of the modern marine food web.”
The length of the megalodon is conservatively estimated at 15 meters – 50 feet – while modern great white sharks typically reach about five meters (15 feet).
To draw conclusions about the prehistoric marine food web, Cast, Sigman, and colleagues used a new technique to measure nitrogen isotopes in shark teeth. Environmentalists have long known that the more nitrogen-15 in the body, the higher its trophic level, but scientists have never before been able to measure the tiny amounts of nitrogen stored in the enamel layer of the teeth of these extinct predators.
“We have a number of shark teeth from different time periods and we were able to trace their trophic level compared to their size,” said Tsisyuan (Crystal) Rao, a graduate student of Sigman’s research group and co-author of the current article. .
One way to get an extra trophic level or two is cannibalism, and some evidence suggests this in both megatooth sharks and other prehistoric marine predators.
Nitrogen time machine
Without a time machine there is no easy way to recreate the food webs of extinct creatures; there are very few bones with traces of teeth that say: “I was chewed by a huge shark.”
Fortunately, Sigman and his team have spent decades developing other methods based on the fact that the nitrogen isotope level in a creature’s cells indicates whether it is at the top, middle, or bottom of the food chain.
“The whole line of my research team is to find chemically fresh but physically protected organic substances – including nitrogen – in organisms from the distant geological past,” Sigman said.
Several plants, algae, and other species at the bottom of the food web have mastered the ability to convert nitrogen from air or water to nitrogen in their tissues. The organisms that eat them then incorporate this nitrogen into their own bodies, and, critically, they predominantly excrete (sometimes with urine) a lighter isotope of nitrogen, N-14, than its heavier relative, N-15.
In other words, N-15 accumulates compared to N-14 when you climb up the food chain.
Other researchers have used this approach to creatures from the recent past – the last 10-15 thousand years – but so far older animals have not had enough nitrogen to measure.
Why? Soft tissues, such as muscles and skin, are almost non-existent. Complicating matters, sharks have no bones – their skeletons are made of cartilage.
But sharks have one golden ticket to the fossil record: teeth. Teeth are stored more easily than bones because they are covered with enamel, a hard material that is virtually immune to most decomposing bacteria.
“Teeth are designed to be chemically and physically stable so that they can survive in a highly chemically reactive oral environment and break down food that may have solid parts,” Sigman explained. And besides, sharks aren’t limited to about the 30 pearly whites that humans have. They are constantly growing and losing teeth – modern sand sharks on average lose a tooth every day of their long life – meaning that each shark produces thousands of teeth over its lifetime.
“If you look at the geological records, one of the most common fossils is shark teeth,” Sigman said. “And in the teeth there is a small amount of organic matter that was used to create tooth enamel – and is now in that enamel.”
Because shark teeth are very rich and they are so well preserved, the nitrogen features in the enamel make it possible to measure status in the food web, regardless of whether a tooth fell out of a shark’s mouth millions of years ago or yesterday.
Even the largest tooth has only a thin enamel shell in which the nitrogen component is just a tiny trace. But Sigman’s team is developing more and more sophisticated methods for isolating and measuring these nitrogen isotope ratios, and with a little help from dentists, cleaning chemicals and microbes that eventually convert nitrogen from enamel to nitrous oxide, they are now able to accurately measure N-ratios15 N14 in these ancient teeth.
“We’re a bit like a brewery,” he said. “We grow microbes and give them our samples. They produce nitrous oxide for us, and then we analyze the nitrous oxide they produce. ”
The analysis requires a custom automated nitrous oxide preparation system that produces, purifies, concentrates and delivers gas to a specialized mass spectrometer with a stable isotope ratio.
“It was a search that lasted several decades to develop a basic method for measuring these trace amounts of nitrogen,” Sigman said. From microfossils in the sediments they moved on to other types of fossils such as corals, fish ear bones and shark teeth. “Next, we and our staff apply this to the teeth of mammals and dinosaurs.”
Immerse yourself deeply in literature during quarantine
At the very beginning of the pandemic, while her friends were preparing sourdoughs and drinking Netflix, Cast reviewed the environmental literature to look for measurements of nitrogen isotopes in modern marine animals.
“One of the cool things Emma has done is really delve into the literature – all the data that’s been published for decades – and link it to the fossil record,” said Michael (Mick) Griffiths, a paleoclimatologist and geochemist at William Patterson University. and co-author on paper.
When Cast was quarantined at home, she carefully set a record with 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; now we need samples,” Cast said. “I would like to find a museum or other archive with a picture of the ecosystem – a collection of different types of fossils from one time and place, from a hole at the base of the food web to atolites – the inner ear bone – from different species of fish to marine mammal teeth, plus shark teeth . We could do the same nitrogen isotope analysis and gather the entire history of the ancient ecosystem. ”
In addition to searching the literature, their database has their own shark tooth samples. Co-author Kenshu Shimada of the University of DePolo linked to aquariums and museums, while co-authors Martin Becker of the University of William Patterson and Harry Mache of the University of the Gulf Coast of Florida collected samples of megatooths on the seabed.
“It’s really dangerous; Harry is a master of diving, and you really need to be an expert to get it,” Griffiths said. “You can find small shark teeth on the beach, but to get the best preserved specimens, you need to go down to the ocean floor. Marty and Harry gathered their teeth from everywhere. ”