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Megalodon is the largest shark that’s ever existed, and according to Hollywood it’s alive and well. But according to scientists, it’s definitely extinct, and it was probably thanks to its smaller cousins, great white sharks.

Hosted by: Stefan Chin

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Sources:
Megalodon:
https://peerj.com/articles/6088/
https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0091419
https://www.ncbi.nlm.nih.gov/pubmed/23066865
https://earthsky.org/earth/supernovae-killed-off-megalodon-large-ocean-animals-extinction-pleistocene
Sevengills:
https://www.nature.com/articles/s41598-018-37576-6
http://lajamjournal.org/index.php/lajam/article/view/182
Image Sources:
https://commons.wikimedia.org/wiki/File:Megalodon_scale.svg
https://www.istockphoto.com/photo/dinosaur-fossils-megalodon-tooth-mosasaurus-tooth-spinosaurus-tooth-tyrannosaurus-rex-gm916329722-252151326
https://peerj.com/articles/6088/
https://commons.wikimedia.org/wiki/File:SN1994D.jpg
https://www.istockphoto.com/photo/the-great-white-shark-gm503988446-82899123
https://www.istockphoto.com/photo/a-broadnose-sevengill-shark-swims-in-the-aquarium-a-sevengill-shark-has-seven-pairs-gm1083283940-290615423
https://commons.wikimedia.org/wiki/File:170121-Sevengill-04_DSC0632.jpg
https://en.wikipedia.org/wiki/File:170121-Sevengill-02_DSC0627_(1).jpg
This episode of SciShow is sponsored by Skillshare { ♪INTRO }.

Megalodon is the largest shark that’s ever existed. Picture a great white, but up to three times the size!

And according to Hollywood it’s alive and well, the superstar of summer blockbuster action movies. But according to scientists, it’s definitely extinct. In fact, it’s even more extinct than they thought.

A study published last week in the journal PeerJ argues megalodon went extinct a million years earlier than previously estimated and it was probably thanks to its smaller cousins. Now there’s never been any doubt amongst scientists that megalodon is extinct. What they aren’t completely sure of is how recently it went extinct and why.

Fossils of marine vertebrates are rarer than those of marine invertebrates, and we know less about the fossilization process under sea water. So, the age of some prehistoric marine species— like megalodon—is still being debated. In 2014, researchers performed an analysis of megalodon fossils and estimated they went extinct around 2.6 million years ago.

But the new study argues that those researchers were off by about a million years. The 2014 study used a technique called optimal linear estimation analysis which is essentially a complex mathematical model to estimate when something went extinct based on observations of the species— in this case, megalodon fossils. But the model is only as good as the data you put into it.

And the authors of the new study argue that some of the fossils used as data in the 2014 study were dated incorrectly, mis-identified or were otherwise unreliable. The current study used the same optimal linear estimation technique and many of the same fossils, but they were a lot pickier about which they included. They carefully vetted each fossil, considering who collected it and when and whether it was preserved similarly to other fossils collected in the same rock layer to make sure they could be confident about its age.

For instance, some of the youngest fossils of megalodon included in the 2014 study were collected in a mining quarry in the 1920s, and the collector didn’t carefully record which rock layer they came from, which makes it really hard to estimate how old they are. The authors of the new study excluded fossils they weren’t sure about, and they paid special attention to ones collected from sites on the California and Baja California coast which can be confidently dated. And the new analysis estimates megalodon went extinct about 3.6 million years ago.

And with this when, the researchers were able to make more educated guesses as to why. Scientists thought megalodon’s extinction was linked to a supernova that occurred around the same time. That’s probably what killed off some other large marine creatures, most likely by triggering changes to the Earth’s climate.

But if megalodon actually died out a million years earlier, it couldn’t be that. So the researchers think it’s smaller kin— great white sharks—are actually to blame. Though they first appeared roughly 5-6 million years ago, around 3.6 million years ago, they were becoming a lot more common and widespread.

The idea is that great white sharks may have out-competed juvenile megalodons for food, since they were a similar size. So although megalodon may live on in our movie theaters, it’s definitely extinct, probably for longer than we thought, and maybe because of sharks still around today. Though…those sharks may not continue to dominate coastal environments much longer.

It used to be that if you wanted to see great whites, you went to the waters around Seal. Island off the coast of South Africa. That’s where people filmed those Air Jaws videos of great white sharks flying out of the water in their pursuit of seals.

But since 2015, they’ve become harder and harder to spot. In 2017 and 2018, great white sharks disappeared from regular scientific surveys for weeks, even months at a time. Where all the great whites went and why they’re gone is unclear.

But a new predator has moved in in their absence. According to a paper published last week in Scientific Reports, off the coast of South. Africa sevengill sharks are now top dog… oh, top shark!

Sevengill sharks are named, obviously, for the number of gill slits they have, which is higher than the 5 found in most other sharks. And they don’t /look/ like top predator material. Though they can be over 2 meters long, they have a sort of goofiness to them, with their rounded features, that makes them seem sluggish and non-threatening.

And their comb-like teeth just don’t inspire the same heart-stopping terror as the jagged blades in great white mouths. But the animals are known to feed on marine mammals as well as other sharks, rays, and bony fish. And the researchers have recently seen them taking out seals in South Africa when the seals’ usual predators are nowhere to be seen.

The team has been monitoring shark activity in the waters surrounding seal island since 2000 using surface baits, and for about 18 years, they’d never seen a sevengill shark. But when the great white sharks began to disappear, the sevengills started showing up. It doesn’t seem like the sevengills are outcompeting the great white sharks.

In their surveys, they saw an inverse relationship between great whites and sevengills—which may be because, well, white sharks are one of the only animals that can take out an adult sevengill shark. Most of the time, sevengills stayed about 18 kilometers away in an area with a lot of kelp that the white sharks seemed to avoid, perhaps because they’re less able to slip through the seaweed without getting wrapped up. So instead, the decline in white sharks seems to be allowing the sevengills to step in.

And no one’s sure why the great whites disappeared in the first place—or whether they can bounce back. For now, the researchers plan to continue their monitoring and see if they can determine where and why the great whites have gone. That might suggest ways to bring them back—or, if that’s even possible.

But If the sevengills are here to stay, the researchers are in a prime position to document if and how this change in top predator alters ecosystem dynamics. So they’ll learn more about shark ecology and their effects on other species—one way or another. If you’re trying to launch a new business, you might feel less like a shark and more like a seal.

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