340 million-year-old 'nail tooth' shark found deep inside Mammoth Cave in Kentucky

Researchers collecting fossils deep inside Kentucky's Mammoth Cave have uncovered the remains of a never-before-seen species — an absolutely tiny ancient shark with nail-like teeth.

The newfound species is called Clavusodens mcginnisi, or McGinnis' nail tooth. The animal lived 340 million years ago and belonged to a group of sharks known as obruchevodid petalodonts, or "chipmunk sharks" because of their tiny size and unusual teeth.

These sharks grew to just 3 to 4 inches (7.5 to 10 centimeters) long and fed on crustaceans, worms and other prey on the seafloor, according to a statement released by the National Park Service (NPS). It's thought their tiny size helped them avoid large predators while foraging.

Researchers unveiled McGinnis' nail tooth in a study published Jan. 24 in the Journal of Paleontology. The species name is named after David McGinnis, a retired National Park Service superintendent, and their teeth, which resemble old iron nails.

Related: 325 million-year-old shark graveyard discovered deep within Mammoth Cave harbors new fossilized species

Mammoth cave

A team of researchers plucked the fossils out of rocks inside Mammoth Cave, the longest cave system in the world. Mammoth Cave is a treasure trove for ancient fish fossils, with more than 70 different species identified within its walls, according to a 2024 study published in the journal Parks Stewardship Forum.

Underground rivers and streams forged the cave's passages, which are made up of rocks that have preserved marine sediment from the Carboniferous period (358.9 million to 298.9 million years ago). The cave has given researchers a unique window inside these rocks, and therefore the marine environments they preserve, according to the 2024 study.

Researchers have been documenting vertebrate fossils in Mammoth Cave since 2019. The work is challenging because of the cave's often cramped conditions and limited space to move equipment in and out.

"Many of these sites have low ceilings requiring crawling for long distances on hands and knees, and at times belly crawling," the researchers wrote in the new study. "The fish fossils are frequently encountered in the cave ceilings or walls."

The newly discovered fossils came out of the Ste. Genevieve rock formation and date back to 330.9 million to 323.2 million years ago. The fossils are the oldest obruchevodid petalodonts discovered so far, according to the study.

Researchers suspect that these ancient sharks may have lived in reef-like systems filled with marine invertebrates called crinoids, or sea lilies, that attached themselves to the sea floor like plants. However, that assessment remains speculative until researchers carry out more studies on the ancient fish and their habitat, according to the study.

Discoveries like McGinnis' nail tooth enable researchers to learn more about long-extinct sharks and the ecosystems they inhabited.

"The fossil discoveries in Mammoth Cave continue to reveal a wealth of new information about ancient shark species," Barclay Trimble, the superintendent at Mammoth Cave National Park, said in the statement. "Researchers and volunteers collected samples from the main Mammoth Cave system and from smaller isolated caves throughout the park that have provided new data on previously known ancient sharks and revealed several species that are brand new to science."