The Smithsonian Magazine has a article about the accidental discovery of a fossil shark in Mammoth Cave.  The shark, Savivodus striatus, lived during the late Mississippian Period, about 330 million years ago.  This shark was probably about 20 feet long, but are known to be more than 30 feet long.

n November 2019, paleontologist John-Paul Hodnett found himself crawling through the winding hollows of Mammoth Cave National Park, toward a remote site that he suspected to hold a prehistoric treasure. Hodnett had recently been sent photos, taken by Mammoth Cave scientists Rick Olson and Rick Toomey, that appeared to show fossilized shark cartilage—a rare discovery indeed, since cartilage is not often preserved in the fossil record.

“I wasn’t exactly sure what I was going to see in the cave during my trip in November,” Hodnett tells Emma Austin of the Courier-Journal. “When we got to our target specimen my mind was blown.”

Embedded in the walls of the cave were parts of the head of an ancient shark, including teeth, the lower jaw and skull cartilage. The creature was about the size of a great white shark and lived some 330 million years ago; its remains, scientists say, offer new insight into the prehistoric creatures that once stalked the waters of North America.