About 385 million years ago in the Devonian age, a strange, two-and-a-half-foot-long shark died and drifted to the bottom of the sea, over what is now the Rhine River valley. Gladbachus adentatus, named after the German city near where it was found, wasn’t the only one of its kind, but it’s the only surviving fossil of the species we have today.
The evolutionary descendants of Gladbachus died out, but new analysis of the fossil is helping build out the rest of the shark family tree. And based on its odd assortment of features from both sharks and bony fishes, scientists now think we can push back the date for humanity’s last common ancestor with great whites to 440 million years ago.
“Sharks aren’t as primitive as generally assumed,” said Michael Coates, a paleontologist at the University of Chicago who led the new study of Gladbachus. “It’s a bit of a textbook cliché that the sharks you see swimming around today look much like their earliest ancestors. But that's partly because people are uncertain of what our last common ancestors looked like.”
Part of the problem is that the fossil record for sharks is thin. Because their cartilage skeletons don’t fossilize very well, there are very few specimens that have survived that long; discovery of a well-preserved, intact new one can add significant insight into the origins of these ancient fishes.
Gladbachus, now housed at Cambridge University in the United Kingdom, was first described in 2001. The fossil, compressed flat over time, comprises much of the shark’s jaws, teeth, skull, gill arches and scales.
It’s encased in a slab of resin with only the top portions visible, so Coates and his colleagues had it CT-scanned at the University of Texas at Austin. Later, they took smaller samples to be scanned at the Advanced Photon Source at Argonne National Laboratory. The detailed images gave the researchers a full view from all sides of the specimen, along with a detailed look at the microscopic makeup of its cartilage, teeth and scales.
They discovered that Gladbachus has a grab-bag of features from both placoderms—an early, jawed type of fish that had armored plates on its head and thorax—and acanthodians, a group of small, spiny fishes that have characteristics of both sharks and bony fishes.