Ancient Fossil Reveals Sharks are Not Living Fossils: Jaw Evolution in Fish

Sharks may not be the living fossils that scientists once thought they were. An ancient fossil has revealed that the living sharks of today are far more advanced in evolutionary terms, despite having retained their basic shark-like attributes over millions of years.

The fossil in question is from a 325-year-old shark-like species. This fossil actually suggests that early cartilaginous and bony fish may have more to tell us about the early evolution of jawed vertebrates than most modern sharks.

"Sharks are traditionally thought to be one of the most primitive surviving jawed vertebrates," said Alan Pradel, one of the researchers, in a news release. "And most textbooks in schools today say that the internal jaw structures of modern sharks should look very similar to those in primitive shark-like fishes. But we've found that's not the case. The modern shark condition is very specialized, very derived, and not primitive."

The heads of all fish, including sharks, are segmented into the jaws and a series of arches that support the jaws and gills. Yet because shark skeletons are made of cartilage, their fossils are very fragile and are usually flattened fragments. This makes it difficult to study these internal structures. Yet this latest fossil provided a rare opportunity to do otherwise.

"This beautiful fossil offers one of the first complete looks at all of the gill arches and associated structures in an early shark," said John Maisey, one of the researchers, in a news release. "There are other shark fossils like this in existence, but this is the oldest one in which you can see everything."

The latest fossil reveals that the arrangement of arches in the ancient creatures is not like anything in a modern shark. Instead, the arrangement is the same as in bony fishes. This means that over time, the structures must have evolved to be more like those found in modern sharks.

"Bony fishes might have more to tell us about our first jawed ancestors than do living sharks," said Maisey in a news release.

The findings are published in the journal Nature.