Fossil sharks of Mammoth Cave: the science behind the 2023 National Fossil Day artwork

Mammoth Cave National Park in Kentucky holds the longest known cave system in the world, at over 400 miles. It is better known for Pleistocene fossils such as bats, sabertooth cat, short-faced bear, peccary, tapir and mastodon, but recent research on Mississippian sharks inspired the 2023 National Fossil Day artwork. In a short time paleontologists have identified more than 100 species of cartilaginous fishes (class Chondrichthyes) from nearly every rock layer that forms the cave, including one of the largest sharks of the Mississippian period and several species found only at the park.

The cave and its rocks

The park was designated July 1, 1941 and covers 52,830 acres, including parts of the Green and Nolin Rivers. The cave began forming about 14 million years ago, in the Miocene, when the early Green River cut through protective sandstone and shale to softer limestone below, and underground streams dissolved passages through limestones and sandstones laid down as far back as 350 million years ago. Then Kentucky lay south of the equator, under a tropical sea, while Pangea was nearly complete. The cave-forming Mississippian rocks (359-323 million years ago) are, oldest to youngest, the St. Louis, Ste. Genevieve, Girkin and Big Clifty Formations; the Haney Formation is also part of the sequence. Early explorers had noted well-preserved Mississippian invertebrates: crinoids, blastoids, horn corals, brachiopods and nautiloids.

Finding sharks in the dark

Cave paleontologists are both cavers and paleontologists. Passages are narrow and far from light, so packs must balance fossil tools, food, water and caving gear, and team members wear helmets with head lamps, gloves and knee pads. Tools are mostly small chisels and rock hammers to free delicate teeth, plus occasionally a lightweight battery-powered circular saw. Getting to known shark sites means going off-trail through very narrow or low passages. Some sites mean crawling on hands and knees for almost half a mile, some mean descending a narrow drill borehole on ropes, and one means paddling small kayaks along underground rivers where shark teeth and spines line the walls and ceiling. Samples travel in cotton-lined plastic tubes or cases to the park museum.

The cave preserves fossils well, thanks to constant temperature, slow erosion and protection from rain, wind and sunlight, so teeth, spines and even skeletal cartilage survive. The 100-plus species come from four of the five rock layers. The older St. Louis and Ste. Genevieve Formations have the greatest diversity, perhaps because they formed in deeper water just before Pangea assembled; the younger Girkin and Haney rocks, laid down in shallower water after Pangea formed, have given only a few sharks so far.

The Mississippian rock layers of Mammoth Cave and the distribution of chondrichthyans in them. Image from NPS's page.

New species

• "Trimble's shark", found by Superintendent Barclay Trimble, belongs to the ctenacanths, a group that appeared about 400 million years ago in the Devonian and died out at the end of the Permian; the name means "comb-spine." Its tooth crown resembles those of some modern sharks, but the root keeps older traits. From the teeth it was probably 6-7 feet long.
• A new species of Janassa, a fish found almost worldwide from the Mississippian to the Permian. Permian skeletons from Germany and England show a skate-like body, though it is more closely related to modern ratfish. The Mammoth Cave species has teeth with more fine ridges and rounder cusps, was about 1-2 feet long, and probably ate brachiopods, snails and bryozoans.

Saivodus striatus

Saivodus striatus was one of the largest sharks of the Mississippian seas, also a ctenacanth. Before, it was known only from isolated teeth in Europe and North America. At Mammoth Cave a side passage held a partial skull with a nearly complete lower jaw almost half a meter (2 feet) long, with teeth about 2.5 centimeters (1 inch) wide, suggesting an individual at least as large as an average living white shark, today's largest predatory fish. Larger teeth from the St. Louis through the Haney Formation suggest sharks exceeding the largest white shark recorded, and still larger teeth from Ireland suggest possibly 11 meters (36 feet).

Estimated average body size of Saivodus striatus compared with a modern great white shark and an average human. Image from NPS's page.

The Mammoth Cave fossils show how marine vertebrates adapted to changing environments while Pangea formed, and more discoveries are expected as exploration continues.

Sources

• National Park Service, Mammoth Cave National Park, "Fossils of the 2023 National Fossil Day Artwork", https://www.nps.gov/articles/000/fossils-of-the-2023-national-fossil-day-artwork.htm; the artwork and reconstructions by artists, and photographs of people, are left out; rewritten in hubnx's own words.