A family of small sharks—some of which had spiky heads—cruised the ancient seas for far longer than scientists had suspected, surviving to about 120 million years ago. Their surprising survival suggests that deep oceans sheltered predators during past mass extinctions.
Some 252 million years ago, roughly 90 percent of the planet's marine species perished in the Permian-Triassic mass extinction event.
Tied to everything from volcanic eruptions to ocean oxygen depletion to severe climate change, the Great Dying represents the most severe challenge to life's survival seen in the fossil record. (See "The Permian Extinction—When Life Nearly Came to an End.")
Among the victims were thought to be a wide variety of early cladodont sharks. These sharks—which brandished long, sharp, T-shaped teeth—are today seen as vanished, unsuccessful cousins of modern broad-toothed sharks.
However, an international team led by Switzerland's Guillaume Guinot of the Natural History Museum in Geneva, reports in the journal Nature Communications that the sharks survived the Great Dying, lasting at least another 170 million years—well into the age of dinosaurs.
"Something we didn't ever expect—[it's] so amazing to find these ancient sharks," Guinot says. "We were looking for modern sharks, to be honest."
Ancient Sharks Survived
The team reports that teeth from three new species of cladodont sharks—one of which is unlike any kind seen before—turned up in limestone fossil beds in southern France.
The 170-million-year survival of the sharks was a revelation to researchers not involved in the new study.
"It would be the equivalent to finding a living population of the descendants of T. rex [today]," says ancient shark expert and independent researcher John-Paul Hodnett.
Nevertheless, Hodnett says, "I have to agree with their conclusions. They have genuine cladodont shark teeth."
"If they had just survived the mass extinction and then died out, that would have been no great surprise," paleontologist Mike Benton, of the University of Bristol, U.K., said by email.
"However, survival by a 100 [million years] into the Mesozoic is interesting—even if they were presumably quite rare during that long span of time."
A Toothsome Tale
The sharks were small ones—likely only about 8 inches (20 centimeters) long—and their teeth were tiny, about a millimeter across.
Even so, they were true predators, likely dining on the ancestors of today's squids and small fish in ancient oceans, Guinot says.
One of the shark species discovered by the team sported teeth similar to a more ancient shark called the falcatus shark, whose males sported a flat, sword-shaped fin above their heads, thought to have been used for sexual displays.
A second species belonged to a family of sharks with two fins on their backs, similar to modern Port Jackson sharks found today off Australia's coasts.
And the third sported teeth with neat cutting edges never before seen on this kind of ancient shark, but often seen on modern ones. (See pictures of today's sharks.)
Finally, the teeth also possessed an enamel structure similar to the teeth of modern sharks as well—another surprise, Hodnett says.
The results "strongly suggest that this trait evolved more than once" in sharks, he says, pointing to how evolution sometimes finds different avenues to the same toothy ends.
Ancient Escape Act
The limestone beds where the shark teeth turned up were deep ocean floors some 120 million years ago, study author Guinot says.
That suggests that although ocean oxygen depletion was at least one likely cause of the extinctions seen during the Great Dying, some isolated seas served as refuges for the sharks and their prey.
The small cladodont sharks probably were flexible enough in their diet to survive in the deep ocean or on the coasts.
"These were not top, top predators—they would have eaten lots of things," Guinot says.
"They probably ate other sharks as well."
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