New research from Princeton shows that prehistoric megatooth sharks – the largest sharks that ever lived – were apex predators at the highest level ever measured.
Megatooth sharks get their name from their massive teeth, which can each be bigger than a human hand. The group includes Megalodon, the largest shark that ever lived, as well as several related species.
While sharks of one type or another existed long before the dinosaurs – for more than 400 million years – these megatooth sharks evolved after the extinction of the dinosaurs and ruled the seas until there. only 3 million years ago.
“We are used to thinking of the larger species – blue whales, whale sharks, even elephants and diplodocus – as filter feeders or herbivores, not predators,” said 2019 PhD student Emma Kast. geoscience graduate who is the first author of a new study in the current issue of Scientists progress. “But Megalodon and the other megatooth sharks were really huge carnivores that ate other predators, and Meg went extinct only a few million years ago.”
His adviser Danny Sigman, Dusenbury Professor of Geological and Geophysical Sciences at Princeton, added: “If Megalodon existed in the modern ocean, it would completely change the way 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 highest rung of the prehistoric food chain – what scientists call the highest “trophic level”. Indeed, their trophic signature is so high that they must have eaten other predators and predators of predators in a complicated food web, the researchers say.
“Ocean food webs tend to be longer than the grass-deer-wolf food chain of land 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 of his thesis. “To reach the trophic levels that we measure in these megatooth sharks, we don’t just need to add one trophic level – one predator at the top of the marine food chain – we need to add several at the top of modern marine food. . the Web.”
The megalodon has been conservatively estimated to be 15 meters long – 50 feet – while modern great white sharks are typically over about five meters (15 feet).
To reach their conclusions about the prehistoric marine food web, Kast, Sigman and their colleagues used a new technique to measure nitrogen isotopes in shark teeth. Ecologists have long known that the more nitrogen-15 an organism contains, the higher its trophic level, but scientists have never been able to measure the minute amounts of nitrogen stored in the tooth enamel layer. of these extinct predators.
“We have a series of shark teeth from different time periods, and we were able to trace their trophic level against their size,” said Zixuan (Crystal) Rao, a graduate student in Sigman’s research group and co-author of the paper. current item. .
One way to tuck in an extra trophic level or two is through cannibalism, and multiple lines of evidence show this in both megatooth sharks and 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 up by a huge shark.”
Fortunately, Sigman and his team have spent decades developing other methods, based on the knowledge that 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 search 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 knack of turning nitrogen from the air or water into nitrogen in their tissues. The organisms that eat them then incorporate this nitrogen into their own bodies and, importantly, 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, relative to N-14, as you move up the food chain.
Other researchers have used this approach on creatures from the recent past – the last 10 to 15,000 years – but there hasn’t been enough nitrogen in older animals to measure, until now.
Why? Soft tissues like muscle and skin are almost never preserved. To complicate matters, sharks have no bones – their skeletons are made of cartilage.
But sharks have a golden ticket in the fossil record: teeth. Teeth are more easily preserved than bones because they are coated with enamel, a rock-hard material that is virtually impervious to most decaying bacteria.
“Teeth are designed to be chemically and physically resistant so they can survive the highly chemically reactive environment of the mouth and break down foods that may have hard parts,” Sigman explained. And besides, sharks are not limited to the thirty or so pearly whites that humans possess. They are constantly growing and losing teeth – modern sand sharks lose a tooth every day of their decades of life, on average – meaning each shark produces thousands of teeth over its lifetime.
“When you look in the geological record, one of the most abundant types of fossils are shark teeth,” Sigman said. “And in teeth, there’s a tiny amount of organic material that was used to build tooth enamel – and is now trapped in that enamel.”
Since shark teeth are so abundant and so well preserved, the nitrogen signatures in the enamel provide a way to gauge the state of the food web, whether the tooth fell from a shark’s mouth ages ago. millions of years ago or yesterday.
Even the largest tooth has only a thin enamel envelope, of which the nitrogen component is only a tiny trace. But Sigman’s team has developed increasingly refined techniques for extracting and measuring these nitrogen isotope ratios, and with a little help from dentist drills, cleaning chemicals and microbes that ultimately convert the enamel nitrogen to nitrous oxide, they are now able to accurately measure the N15-N14 ratio in these older teeth.
“We’re a bit like a brewery,” he says. “We grow microbes and give them our samples. They produce nitrous oxide for us, and then we analyze the nitrous oxide they produced.
The analysis requires an automated, customized nitrous oxide preparation system that extracts, purifies, concentrates, and delivers the gas to a specialized stable isotope ratio mass spectrometer.
“It’s been a decades-long quest that I’ve been part of, to develop a basic method to measure 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. “Then we and our collaborators apply that to mammalian teeth and dinosaur teeth.”
A dive into literature during confinement
Early in the pandemic, while his friends were whipping up sourdough starters and gorging on Netflix, Kast scoured the ecological literature for measurements of nitrogen isotopes of modern marine animals.
“One of the cool things Emma did was dig into the literature — all the data that’s been published over the decades — and connect it to the fossil record,” said paleoclimatologist Michael (Mick) Griffiths. and geochemist at William Patterson University and a co-author on the paper.
While Kast was in quarantine at home, she painstakingly put together a record with over 20,000 marine mammals and over 5,000 sharks. She wants to go much further. “Our tool has the potential to decode ancient food webs; what we need now are samples,” Kast said. “I would like to find a museum or other archive with a snapshot of an ecosystem – a collection of different types of fossils from one time and place, from forams near the very base of the food web, to otoliths – inner ear bones – from different types of fish, to marine mammal teeth, as well as shark teeth We could do the same nitrogen isotope analysis and piece together the whole story of an ancient ecosystem.
In addition to desk research, their database includes their own shark tooth samples. Co-author Kenshu Shimada of DePaul University connected with aquariums and museums, while co-authors Martin Becker of William Patterson University and Harry Maisch of Florida Gulf Coast University collected specimens of megadents on the seabed.
“It’s really dangerous; Harry is a dive master, and you really have to be an expert to get them,” Griffiths said. “You can find small shark teeth on the beach, but to get the best-preserved samples you have to go down to the bottom of the ocean. Marty and Harry have been picking up teeth from everywhere.
He added, “It was a real collaborative effort to get the samples to bring it all together. In general, collaborating with Princeton and other regional universities is really exciting because the students are amazing and my colleagues there have been really great to work with.
Alliya Akhtar, a 2021 Ph.D. graduate of Princeton, is now a postdoctoral researcher in Griffiths’ lab.
“The work I did for my thesis (examining the isotopic composition of seawater) posed as many questions as it answered, and I was extremely grateful for the opportunity to continue working on some of them with a collaborator/mentor that I respect,” Akhtar wrote in an email. “I’m very 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 occupied extremely high trophic positions. Science Adv. 2022;8(25):eabl6529. doi: 10.1126/sciadv.abl6529
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