Mystery of the Whorl Tooth Shark of Idaho Revealed

Whorl teeth fossil: all that we have left of Helicoprion. CREDIT: Tapanila et al. (2013)

The spiral-shaped whorl of the ancient shark Helicoprion has left paleontologists questioning its functionality for over a century.  First described in 1899, Helicoprion, which is more closely related to the now extinct ratfish, inhabited the oceans from the Pennsylvanian period (310 million years ago) through the Early Triassic period (250 million years ago). Because only the whorl is preserved, scientists and artists have gotten very creative in trying to figure out what this mysterious critter actually looked like; some the ideas, shown below, include placement of the whorl on the tail, fins, or face.

Reconstructions of Helicoprion with different whorl morphology with (l) being the most recent reconstruction. CREDIT: RAY TROLL

Using CT scans, a team of scientists led by Dr. Leif Tapanila of Idaho State University’s Department of Geosciences have discovered the function of the buzz tooth whorl.  The CT scan of a Helicoprion whorl known as IMHN 37899 or ‘Idaho 4’ revealed the cartilage structure of its head, including the upper jaw, lower jaw, and a piece of cartilage that extends from the lower jaw and braces against the base of the outermost whorl. The whorl itself is a singular fused structure of the lower jaw that takes up the entire mandibular arch. 

Previous reconstructions suggested that the whorl extended beyond lower jaw and curled underneath the chin, whereas this reconstruction suggests that the whorl grew inside of the lower jaw. The whorl grows continually throughout the shark’s life with each new tooth pushing the rest of the teeth forward in a curved direction, creating the iconic spiral.  Helicoprion’s teeth were pushed backwards when the lower jaw closed, creating a slicing mechanism that cut food and forced it into the back of the mouth. 

Interested in learning more about Helicoprion? The Idaho Museum of Natural History in Pocatello, ID is running a special exhibition until the end of 2013, featuring the artwork of Ray Troll.  Tickets are $5 for adults and $1 for children K-12.

Original article: Tapanila, L., Pruitt, J., Pradel, A., Wilga, C., Ramsay, J., Schlander, R., and Didier, D., "Jaws for a spiral-tooth whorl: CT images reveal novel adaptation and phylogeny in fossil Helicoprion." Biology letters 9, no. 2 (2013).