Almost everything paleontologists know about ancient sharks comes from their teeth. That’s because the animals had skeletons made of cartilage, which does not fossilize as easily as bone. So researchers were surprised to find several shark skulls and an almost complete skeleton of 360-million-year-old primitive shark in the Atlas Mountains of Morocco.
The fossils, described in the journal Proceedings of the Royal Society B, come from two species of sharks in the genus Phoebodus, which went extinct during the Caroboniferous Period about 299 to 359 million years ago, leaving behind no known ancestral species. Bob Yirka at Phys.org reports that prior to the discovery, Phoebodus sharks were only known from three teeth.
These fossils survived because the area where the animals died was a shallow sea basin. Their bodies were covered in sediment and limited water circulation and low oxygen levels allowed them to fossilize without being destroyed by predators or broken down by bacteria.
Still, the fossils were fragile, so the team chose to examine them using a CT scan instead of chipping them out of the rock. The imaging reveals a very strange, un-shark-like creature. Yirka reports Phoebodus had a long, thin body along with a flat skull and jaw. The creature looks much more like a giant eel than a typical modern shark.
But it does resemble an atypical modern shark—the elusive frilled shark. That species is found in deep water around the world, but is little studied. Until 2004 when the creature was first video recorded, it was only known from being pulled up in fishing nets.
Tim Vernimen at National Geographic reports that the three-cusped teeth of the ancient species and the frilled shark are similar and can offer clues to how the ancient species hunted.
“The frilled shark is a specialized predator, with the ability to suddenly burst forward to catch its prey,” David Ebert at the Pacific Shark Research Center, who has studied frilled sharks, but was not involved in the new study, says. “The inward-pointing teeth then help to make sure the prey can only go one way: into its throat. Maybe Phoebodus did something similar.”
While most modern sharks use their teeth to rip prey to pieces before gobbling them up, the frilled shark—and perhaps Phoebodus—use their unique teeth to capture prey and swallow them whole, study coauthor Christian Klug of the University of Zurich tells Vernimen .
Because data on frilled sharks is almost as elusive as fossils of Phoebodus, the team also examined the jaws and teeth of the alligator gar, a species of North American fish dating back 100 million years that has a surprisingly similar mouth to the ancient shark. The gar hunts in open water, and its long jaw and flat head allows it to snap at a fishing coming from almost any direction.
It’s possible that Phoebodus developed its unique shape hundreds of millions of years earlier to hunt in the same manner. “When a certain structure or strategy is effective, there is a tendency for it to show up time and time again—both in living creatures and in the fossil record,” Justin Lemberg, gar researcher at the University of Chicago who was not involved in the study, tells Vernimmen. “While a lot has changed since Phoebodus swam the Devonian oceans, the physics of feeding in water have not.”
This isn’t the only rare shark fossil rewriting what we know about ancient sharks. Last month, researchers from the University of Chicago made a CT scan of a 2-foot-long, 335-million-year-old shark found in Scotland in 1837. They found that the early shark was a suction feeder, using mouth parts in a manner similar to modern day nurse-sharks and carp.
Modern imaging techniques are showing researchers that ancient sharks had diverse feeding patterns, similar to modern sharks.
“The quantity of data that is emerging from studies such as this is staggering,” paleontologist John Maisey of the American Museum of Natural History, not involved in the study, tells Vernimmen. “We are experiencing a renaissance of anatomy.”