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If there are animals that can boast of being ‘ancient’ on the face of the Earth, those are the sharks. They have, in fact, been patrolling the oceans of our planet for more than 400 million years, in which they have lived since long before the first dinosaurs. They have survived several mass extinctions and are still thriving in our seas today. And despite the fact that there are many different species, sharks, in essence, have barely changed in all this time. If we came across a specimen, let’s say from 300 million years ago, we would have no problem recognizing it, since today’s sharks still retain most of their original characteristics. Something that the vast majority of other living beings cannot say.
But among all the sharks that have ever existed, there is undoubtedly one that powerfully draws our attention. Its about megalodonthe largest fish known to science and the largest predator that has ever existed on our planet.
Present in our oceans until about three million years ago, its exact size is still a matter of debate, although there are good reasons to think that the megalodon could have been between 15 and 18 meters long, more than three times than current great white sharks. Its jaws opened in such a way that a human being could have easily stood up in its huge gaping mouth.
At the top of the food chain
And now, from the hand of a team of paleontologists from Princeton University, we have received new information about this true king of the seas. The megalodon was unrivaled, and it ate what it wanted, including other marine predators and other megalodons. The results of this study, published in Science Advances, indicate that this ancient shark was a predator unlike any other in Earth’s history.
“We are used to thinking of the largest species (blue whales, whale sharks, elephants and diplodocus) as filter feeders or herbivores, not predators – says the geoscientist Emma Kast, from the University of Cambridge and first author of the article. But the megalodon and other mega toothed sharks were really huge carnivores that ate other predators, and ‘Meg’ went extinct only a few million years ago.”
“If megalodon existed in modern oceans,” he says. Danny Sigmana professor of geological and geophysical sciences at Princeton and lead author of the study, would completely change how humans interact with the marine environment.”
Kast and Sigman found compelling evidence that the megalodon and its ancestors occupied the top rung (or trophic level) of the prehistoric food chain. In fact, its ‘trophic signature’ is so high that researchers are convinced the megalodon must have eaten other predators, and predators of predators, in a really complicated food web. There is even evidence of cannibalism in both mega toothed sharks and other prehistoric marine predators.
The long oceanic chain
“The food chain in the ocean -explains Kast- tends to be longer than that of land animals (grass-deer-wolf), because you start with very small organisms. To reach the trophic levels we’re measuring in these megatoothed sharks, we don’t just need to add one trophic level, that is, a single predator at the top of the marine food chain. In the modern marine food web, we need to add several.”
To reach these conclusions, the researchers used a new technique to measure nitrogen isotopes in fossilized megalodon teeth. The general rule is that the more nitrogen-15 there is in an organism, the higher its trophic level. But this is the first time the tiny amounts of nitrogen preserved in these prehistoric teeth have been measured.
“We have a number of shark teeth from different time periods,” he says. Zixuan C. Raoco-author of the study – and we were able to track their trophic level compared to their size.”
The Nitrogen Isotopes Method
Prehistoric food webs can sometimes be measured through bite marks on fossilized bones, but such evidence is scant for extinct sharks. The novel nitrogen isotope method, however, helps paint a picture of an ancient world of fish eating other fish. “My research team’s goal is to search for chemically fresh, but physically protected organic matter, including nitrogen, in organisms from the distant geological past,” says Sigman.
Organisms at the bottom of the food web, such as plants and algae, absorb nitrogen from the air or water. When other species eat them, the predatory species incorporates that nitrogen into their own bodies. And while it’s true that some of that nitrogen later returns to the sea in feces, more of the lighter isotope of nitrogen, N-14, is excreted (sometimes in the urine) than the heavier N-15.
Therefore, N-15 accumulates in the body relative to N-14 as it moves up the trophic level.
However, there is a problem. While it is relatively easy to measure nitrogen levels in animals up to 15,000 years old, the lack of well-preserved soft tissues in older species has made measuring nitrogen nearly impossible. Until now. Fortunately, researchers have realized that they can turn to another type of remains: teeth.
In fact, teeth are much more easily preserved because they are coated with a rock-hard enamel that acts as a barrier to decaying bacteria. And sharks have many teeth that are constantly being replaced.
“Teeth,” Sigman explains, “are designed to be chemically and physically resistant, to be able to survive in the chemically reactive environment of the mouth and to break down foods that may have hard parts. When you look at the geological record, one of the most abundant types of fossils is shark teeth. And inside the teeth, there is a small amount of organic matter that was used to build the enamel that is now trapped within that enamel.”
Sigman and his collaborators have perfected a method to measure small amounts of nitrogen in a thin layer of enamel on the teeth of sharks extinct millions of years ago and thus obtain its isotope ratios. Thus, after drilling the teeth, cleaning chemicals and microbes convert the nitrogen in the enamel into nitrous oxide, making it possible to read the ratio. A laborious method that Sigman has taken a whole decade to fine-tune, but which now allows even minute amounts of nitrogen to be measured.
Researchers began using this technique with microfossils found in sediments. From there, they moved on to fossil corals, fish ear bones, and finally shark teeth. “Now,” says Sigman, “both we and our collaborators are applying this to mammalian and dinosaur teeth.”
«Our tool -says Kast- has the potential to decipher ancient food webs; what we need now are samples. 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 one place, from foraminifera near the very bottom of the food chain to otoliths (inner ear bones) from different types of fish, even teeth of marine mammals, as well as shark teeth. We could do the same nitrogen isotope analysis and piece together the whole story of an ancient ecosystem.”
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