In 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.
Today, Kentucky is a landlocked state, making it seem like an odd place for shark fossils to surface. But during the Late Mississippian period—which spanned from about 330 million to 325 million years ago, according to Gizmodo’s George Dvorsky—much of North America was covered by shallow seas. The Mississippian is a subset of the Carboniferous epoch, which has been dubbed the “Golden Age of Sharks”; after an extinction event during the late Devonian period that killed off some 75 percent of all species on Earth, including many marine lineages, sharks emerged as the ocean’s apex predators.
Hodnett, who works at Maryland's Dinosaur Park and is an expert on Paleozoic sharks, tells CNN’s David Williams that he found the fossilized remains of around 150 individual sharks, belonging to between 15 and 20 different species, within Mammoth Cave. Those remains include shark teeth, which are the most commonly found vertebrate fossils because the creatures shed and replace their teeth throughout their lives. But there is “hardly ever any record at all of sharks teeth coming from these rocks,” Hodnett says of Mammoth Cave. “So that was exciting.”
Also exciting was discovering the remnants of a shark’s head, which included not only teeth, but also the lower jaw and skull cartilage. Hodnett has identified the specimens as belonging to Saivodus striatus, a Paleozoic species that “is a ctenacanth shark, a group of sharks that are distant cousins to the modern shark family lineage,” Hodnett tells Gizmodo. “All ctenacanths bare elongated dorsal spines, similar to those seen in living horn sharks and dogfish.”
Saivodus striatus could grow to up to 30 feet long, making it “one of the largest ctenacanths to evolve,” Hodnett adds. The individual found in Mammoth Cave was likely a bit smaller, spanning between 15 and 20 feet. Though these marine creatures rivalled great whites in size, they may have had different feeding habits; the teeth of Saivodus striatus were better suited to grabbing soft-bodied prey than cutting through flesh.
The species is primarily known to paleontologists through “isolated teeth” found in North America and Europe, Hodnett says. But inside Mammoth Cave, tucked safely away from the elements, the newly jaw and skull parts have been well preserved over the ages. Hodnett and his colleagues plan to present their preliminary findings in October at a meeting of the Society of Vertebrate Paleontology, though questions still remain. It’s not clear, for instance, how much of the shark is embedded within the walls of the cave.
"Caves are a very special environment, so it's not ideal to be removing big chunks of rock out of it and damage the internal environment by doing this," Hodnett tells CNN.
Experts are exploring the possibility of using a non-invasive method like photogrammetry to create a 3D model of the fossil. But Hodnett is already looking forward to the possibility of future discoveries that might shed light on prehistoric marine life in the Kentucky region.
"We literally just scratched the surface, and the sharks are just coming out from that scratch," he tells CNN. "So, hopefully, with more field work, we'll get another good batch of specimens to kind of help get at least some more rich diversity."