The fossilized head of a shark-like species that lived an estimated 325 million years ago and was recently unearthed in Arkansas reveals the physical characteristics of modern sharks have undergone many more evolutionary changes than previously thought.

"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 Alan Pradel, a postdoctoral researcher at the American Museum of Natural History and the lead author of a study based on the new research and recently published in the journal Nature.

"The modern shark condition is very specialized, very derived, and not primitive," he asserted.

The research was based on a very well-preserved shark fossil collected in Arkansas, where evidence shows a giant ocean basin once served as home for a wide array of marine life. The skull of the species, named Ozarcus mapesae, was one of 540,000 fossils donated to the museum by Ohio University .

Working with scientists at France's European Synchrotron, one of the world's most powerful X-ray emitters, Pradel captured high-resolution images of the specimen for a detailed view of individual arch shapes and structural organization of the prehistoric creature.

The heads of all fish, including sharks, are segmented into the jaws and then a series of arches that support the jaw as well as gills. In fact, it's suspected the arches led to the evolutionary development of jaws.

Because shark skeletons are made of cartilage and not bone, their fragile fossils are usually found in flattened fragments, leaving it close to impossible for researchers to determine the shape of the internal structures.

But the fossilized Ozarcus mapesae in question was preserved in a nearly three-dimensional state.

"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 fossil, said John Maisey, a curator in the Museum's Division of Paleontology and one of the other authors of the study, "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 ... There's enough depth in this fossil to allow us to scan it and digitally dissect out the cartilage skeleton."

Ultimately, said Maisey, the new work, when viewed within the context of other discoveries in the study of early jawed vertebrates, may provide greater insights into the understanding of Earth's evolutionary lines as a whole.

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