Science knows very little about shark evolution. This is partly because “cartilage is a funny tissue,” says John Maisey, a paleontologist at the American Museum of Natural History in New York City. Shark bodies are largely made of this firm, white connective substance—which does not fossilize well. For hundreds of years scientists have only been able to guess that sharks probably had some bony fish ancestors. But now, using a CT scanner to evaluate the only known fossil of an ancient fish called Doliodus problematicus, Maisey and his colleagues may have found a crucial missing piece in the shark origin puzzle.

“Shark skeletons are among the rarest [finds],” Maisey explains. The 400-million-year-old Doliodus skeleton was discovered in the mid-1990s in the Canadian province of New Brunswick. Yet it was not until 2014 that advanced CT technology allowed Maisey to see that the specimen had sharklike jaws and tooth arrangements. Earlier this year he and his colleagues reported in American Museum Novitates that it also had a row of spines along its back and pelvic fins that match a much older and well-studied class of extinct bony fishes called acanthodians. In other words, Doliodus bears features of both the older bony fishes and modern sharks.

“This is a significant discovery,” says Michael Coates, an evolutionary biologist at the University of Chicago, who was not involved in the study. The findings support the idea that acanthodians “represent a missing chunk of early shark evolution.” Thanks to Maisey's find, researchers will now have to go back and study acanthodians in a whole new light.