Sharks are usually thought of as primitive creatures, sometimes called "living fossils." But a new study of a 325-million-year-old shark fossil — the most complete of its kind — suggests modern sharks have evolved significantly from their bony ancestors.
But comparing a modern shark with a primitive one would be like comparing a modern automobile with a Ford Model T — they share some similarities, but under the hood they're completely different, Maisey told LiveScience.
The exceptionally well-preserved fossil of Ozarcus mapesae from two different lateral views. The scale bar is 10 millimeters. (Photo: F. Ippolito/AMNF)
A bony shark
The shark fossil in the study, Ozarcus mapesae, was found in Arkansas by husband and wife Royal and Jean Mapes (for whom the species is named), who donated it to the museum. The shark was about 3 feet (90 centimeters) long with very large eyes, and appears to have lived in a shallow, murky inland sea that was also home to giant squidlike creatures, Maisey said.
The researchers X-rayed the fossil, first using a CT machine and later using a synchrotron, which relies on ultra-high-energy X-rays and has become an important tool for paleontology because it doesn't destroy the fossils and provides a level of detail not possible with traditional fossil preparation. Maisey called it "the Hubble Space Telescope of paleontology."
The X-ray scans revealed the fossil had complete gill arches, the support structures for the gills that fish use to breathe. These gill arches were arranged in a serial way, more like those of bony fish than modern sharks.
The fossil's jaws were also more like those of bony fish. Most modern sharks have jaws that are attached to their skulls by flexible ligaments, whereas bony fish have jaws that are rigidly fused to their craniums.
It's not the oldest shark fossil found, but it is one of the most complete. It provides a new model for interpreting other fossils, allowing scientists to make comparisons between early jawed vertebrates and sharks, Maisey said.
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