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Ancient shark-like fossil challenges old belief that sharks have not evolved much

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Scientists have often viewed sharks as primitive animals that have not evolved much through the years. However, newly discovered fossil evidence shows that modern sharks may not be as primitive as previously thought.

Previously, schools of thought speculated that sharks chanced upon the right type of physical characteristics that allowed it to dominate the seas as apex predators very early in its evolutionary history. Since early sharks could be considered as evolutionarily "fit," experts believe that they didn't change much over the course of a few hundred million years. However, a few bones from a shark-like fossil seems to indicate that the current theory on shark evolution may need to be revised.

"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 American Museum of Natural History postdoc researcher Alan Pradel. "But we've found that's not the case. The modern shark condition is very specialized, very derived, and not primitive."

The fossil evidence consists of a small number of bones estimated to be around 325 million years old. The ancient fossil comes from an ancient shark like animal called Ozarcus mapesae, named after the Ozark Mountains in Arkansas where the fossil was found. The fossil was relatively well preserved considering that ancient shark fossils are quite rare. The scientists found that the fossil showed the arrangement of the gill supporting structures in O. mapesae.

 A team of researchers from the American Museum of Natural History analyzed these bone fragments and according to the team's findings, the ancient Paleozoic shark may have had gills that could be said to be similar to similar structures found in modern bony fish such as tuna. The presence of these branchial arches that can also be seen in the gills of other ancient fishes referred to as acanthodians and placoderms. The team published its findings in the online journal Nature.

"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 the museum's division of paleontology curator John Maisey. "There's enough depth in this fossil to allow us to scan it and digitally dissect out the cartilage skeleton."

The fossil can also shed light on the evolution of the vertebrate jaw as well as specialized ear bones such as those found in humans. The team also concluded that further research into the role of these special gill support structures to jaw evolution may require further study of modern fishes in comparison to modern sharks.

"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 presence of these supporting structures also means that sharks may have evolved much more than previously thought. Modern sharks posses a totally different set of gill supports that evolved after the time of O. mapesae.

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