Arkansas is hundreds of miles from the Gulf of Mexico, but it’s home to countless sharks. They aren’t swimming in any body of water, however. A trove of the fossilized predator’s remains are embedded within the Fayetteville Shale—a roughly 350-million-year-old geological formation in the state’s northwestern corner. Because a shark’s cartilage skeleton decomposes so quickly, they usually only leave teeth behind when they die. But examples embedded in the Fayetteville Shale are different. Instead of only showcasing their chompers, this geology contains extremely rare, three-dimensional skeletal impressions.

“The fossils provide a glimpse into shark anatomy unparalleled for this period of time anywhere in the world,” Cal Poly Humboldt paleontologist Allison Bronson said in a recent university profile.

The situation is so unique that the area has earned a fitting nickname: Sharkansas. Paleontologists have long wondered how these remarkable preservations formed. According to a study published by Humboldt’s team in the journal Geobios, it has everything to do with conditions on the seafloor hundreds of millions of years ago.

Bronson and her colleagues reached these conclusions after employing a range of technologies to examine both the preserved shark skeletons and the rocks themselves. These included high-resolution CT scans, as well as X-ray diffraction and fluorescence scanning procedures. They discovered a scarcely seen chemical composition across the specimens—at the time of their death, their marine habitat contained very low oxygen levels along with a high acidity. This combination would have dramatically slowed down bacterial decay, giving cartilage the time it needed to fossilize.

The low oxygen–high acid pairing also explains why there are so few bony fish in the same rock formations, too. Despite their prevalence at the time, their skeletons would have eroded too quickly to become preserved. The same goes for the sharks’ teeth.
Sharkansas contains some of the most complete Paleozoic shark skeletons in the world, including Ozarcus mapesae, Cosmoselachus mehlingi, and Carcharopsis wortheni. The latter is a newly discovered species that resembles a present-day barracuda more than a shark. With the Fayetteville Shale’s unique preservation, paleontologists will be better able to trace how the two species are related across millions of years. In the meantime, understanding the oceanic chemical composition that facilitates such delicate fossilizations will help other researchers identify similar areas around the world that may contain even more sharks to study.

 

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