Sharks have looked more or less the same for hundreds of millions of years. But a newly discovered fossil suggests that under the hood, a modern shark is very different from its ancient ancestors.
The finding, published in the journal Nature, strongly implies that sharks are not the "living fossils" many paleontologists once thought they were. "They have evolved through time to improve upon the basic model," says John Maisey, a paleontologist at the American Museum of Natural History who helped identify the fossil.
This newly discovered creature dates back 325 million years. It's no megalodon. It was probably just 2 or 3 feet long and its teeth were tiny, "although there are rows of teeth in the mouth, so it would certainly give you a painful nip," Maisey says.
The fossil of this beastie is equally modest: It looks like an ordinary brown rock. But in recent years, paleontologists have begun using tools like CT scanners to look inside fossils. When Maisey scanned the fossilized head of the new shark, he got a shock. Inside, "it's not like the anatomy of a modern shark at all," he says.
The skeleton supporting this ancient shark's gills is completely different from a modern shark's. In fact, the gill skeleton looks much more like that of an average modern-day fish.
The finding turns old ideas about sharks on their head, says Michael Coates, an evolutionary biologist at the University of Chicago. Previously, many scientists had believed that shark gills were an ancient system that predated modern fish. But this new work suggests that modern-day fish may actually be the ones with the ancient gill structures. Shark gills are the gills that evolved.
"That bucks a trend that's been in the literature for years and years that sharks are somehow primitive living fossils," Coates says.
Why did sharks change the structure of their gills? Maisey suggests it might be to help them sprint after prey. Or to open their jaws more widely, so they could snap up bigger things, like swimmers.
Whatever the reason, sharks have been changing. Per Ahlberg at Uppsala University in Sweden says the new work is an important reminder that so-called living fossils like sharks and crocodiles aren't fossils at all. They are constantly adapting to the world around them. "We have to be very, very careful with the idea of living fossils," Ahlberg says.
This ancient little shark shows that evolution is always at work.
AUDIE CORNISH, HOST:
Long before there were humans, there were sharks. Shark fossils go back hundreds of millions of years. Researchers had thought that sharks hadn't evolved much since then. But a newly discovered fossil described today in the journal Nature suggests otherwise.
NPR's Geoff Brumfiel has the story of a little fish with big implications.
GEOFF BRUMFIEL, BYLINE: Three-hundred-twenty-five million years ago, the world was very different. A lot of the action was still in the oceans. And if people had been around back then, many of the fish wouldn't be familiar. But there is one you could recognize: the shark.
John Maisey is a paleontologist with the American Museum of Natural History.
JOHN MAISEY: If you were back then fishing and you fished out a shark, you'd recognize it as pretty much as what it was.
BRUMFIEL: Sharks appeared to be pretty much the same as they are today, or so Maisey thought until he began studying a fossil of a newly discovered ancient shark. Now, this wasn't some huge dino-shark. It was probably just two or three feet long, not much of a threat.
MAISEY: This thing would nibble your toes, really tiny little teeth. Although there were rows of teeth in the mouth so, you know, it would certainly give you a painful nip.
BRUMFIEL: On the face of it, the fossil looked like an ordinary brown rock.
MAISEY: If I had one in my hand and held it up and showed you, I'd say, well, here's the head and here's the eye and here's the jaws and this is where the gills are, and you'd look at me as if I was nuts.
BRUMFIEL: But in recent years, paleontologists have begun using tools like CT scanners to look inside fossils. When Maisey scanned the fossilized head of the new shark, he got a shark shock. Because inside...
MAISEY: In fact, it's not like the anatomy of a modern shark at all.
BRUMFIEL: The bones supporting this ancient shark's gills are completely different from a modern shark. So that means the gills of modern sharks aren't ancient. They must have evolved over time, maybe to help the shark sprint after prey. Or to open its jaws more widely so it could snap up bigger things, like swimmers.
(SOUNDBITE OF MUSIC)
BRUMFIEL: Whatever the reason this shows sharks have been changing.
MAISEY: They have evolved through time to sort of improve upon the basic model, as it were.
BRUMFIEL: Sounds like the difference between a Model T and a Formula 1 car or something...
MAISEY: It really is. You know, a Model T has a steering wheel and four wheels and that's about it. But under the hood, yeah, it's all different.
BRUMFIEL: Per Ahlberg, at the Uppsala University in Sweden, says this new work upends old ideas about sharks.
PER AHLBERG: There's been this very, very deeply established idea of sharks as being primitive, as being unchanging.
BRUMFIEL: It's an important reminder that animals we think of as living fossils, like sharks, have evolved.
AHLBERG: We have to be very, very careful with the idea of living fossils. And it's something that researchers in this field have more and more moved away from.
BRUMFIEL: This ancient little shark shows that evolution is always at work.
Geoff Brumfiel, NPR News.
(SOUNDBITE OF MUSIC)
ROBERT SIEGEL, HOST:
You are listening to ALL THINGS CONSIDERED from NPR News. Transcript provided by NPR, Copyright NPR.