A McGinnis's nailbill (Clavusodens mcginnisi) is pictured hunting for a crustacean in a reef-like forest of crinoids during the Carboniferous period. (Image credit: NPS Illustration/Benji Paysnoe)
Scientists examining fossils deep within Kentucky's Mammoth Cave have stumbled upon the remains of a previously unknown species – a tiny ancient shark with teeth that resemble fingernails.
The new species is called Clavusodens mcginnisi, or McGinnis' fingernail. The creature lived 340 million years ago and belonged to a group of sharks known as hoop-petalodonts, or “chipmunk sharks,” because of their diminutive size and unusual teeth.
These sharks were only 3 to 4 inches (7.5 to 10 centimeters) long and fed on crustaceans, worms, and other prey on the seafloor, according to information released by the National Park Service (NPS). Their small size is thought to have allowed them to hide from larger predators while feeding.
Scientists introduced the McGinnis tooth-nail in a study published Jan. 24 in the Journal of Paleontology. The species was named after David McGinnis, a retired National Park Service superintendent, and their teeth resemble antique iron nails.
Mammoth Cave
A team of researchers has been extracting fossils from rocks inside Mammoth Cave, the longest cave system in the world. Mammoth Cave is a treasure trove of ancient fish fossils, with more than 70 different species found within its walls, according to a 2024 study published in the journal Parks Stewardship Forum.
Underground rivers and streams carved out cave passages formed by rocks that preserved marine sediments from the Carboniferous period (358.9 million to 298.9 million years ago). The cave provides researchers with a unique opportunity to study these rocks and, therefore, the marine environments they contain, according to a 2024 study.
Since 2019, scientists have been documenting vertebrate fossils in Mammoth Cave. The work is complicated by the confined spaces and narrow passages for moving equipment.
“Many of these sites have low ceilings, requiring crawling for considerable distances on hands and knees, and sometimes on the stomach,” the researchers note in the new study. “Fish fossils are often found on the ceilings or walls of caves.”
The newly discovered fossils were excavated from the Sainte-Geneviève rock formation and date back to 330.9 million to 323.2 million years ago. According to the study, these finds are the oldest known hoop-shaped petalodonts.
Scientists speculate that these ancient sharks may have lived in reef-like systems filled with marine invertebrates known as crinoids, or sea lilies, that were attached to the ocean floor like plants. However, this remains hypothetical until researchers conduct more studies of the ancient fish and their habitats, according to the study.
Discoveries like McGinnis' claw tooth give researchers a deeper understanding of long-extinct shark species and the ecosystems they lived in.
“Fossil discoveries at Mammoth Cave continue to reveal new information about ancient shark species,” said Barkley Trimble, superintendent of Mammoth Cave National Park. “Researchers and volunteers have collected specimens from the main Mammoth Cave system
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