Ancient Fish Head Suggests Sharks Aren't as Primitive as we Thought

A newly discovered species of ancient shark-like fish may cause biology textbooks that suggest modern sharks are primitive to be rewritten.

Writing in the journal Nature, a team of scientists describe the 325-million-year-old species, named Ozarcus mapesae, noting that its physiology is quite shark-like, which suggests that modern sharks are, in evolutionary terms, quite advanced.

"Sharks are traditionally thought to be one of the most primitive surviving jawed vertebrates. 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," said lead study author Alan Pradel, a postdoctoral researcher at the American Museum of Natural History. "But we've found that's not the case. The modern shark condition is very specialized, very derived, and not primitive."

The fossil specimen was collected in Arkansas, where an ocean basin was once home to a diverse marine ecosystem, the researchers said in a statement.

Skeletons of sharks and other cartilaginous fishes are often not preserved well, usually pressed into flattened fragments that make it difficult to determine their shape. However the fossilized head of O. mapesae was remarkably well-preserved.

Like all fishes, sharks' heads are segmented into jaws and a series of arches that provide support to the jaws and gills. These arch structures are thought to have given rise to jaws early in the tree of life, the researchers said.

"This beautiful fossil offers one of the first complete looks at all of the gill arches and associated structures in an early shark. There are other shark fossils like this in existence, but this is the oldest one in which you can see everything," said study author John Maisey, paleontology curator at the American Museum of Natural History. "There's enough depth in this fossil to allow us to scan it and digitally dissect out the cartilage skeleton."

After scanning the fossil with high-resolution X-rays, the researchers were able to get an intimately detailed view of the shape and organization of the ancient creature.

"We discovered that the arrangement of the arches is not like anything you'd see in a modern shark or shark-like fish," said Pradel. "Instead, the arrangement is fundamentally the same as bony fishes."

The apparent evolution of these jaw structures in sharks is not unexpected, the researchers said, but it may have significant impacts on future evolutionary studies of this group.

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