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  • 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 megalodonis 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 speciesin 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 sharksare 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.

  • Thoughthose 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 dogoh, 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 sevengillswhich

  • 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 placeor 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 backor, 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 speciesone 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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  • { ♪OUTRO }

This episode of SciShow is sponsored by Skillshare

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伟大的白人可能已经消灭了360万年前的巨齿龙(Great Whites May Have Taken Out Megalodon 3.6 Million Years Ago | SciShow News)

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    joey joey 發佈於 2021 年 05 月 13 日
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