Two URI biologists solve mystery of a strange ancient shark

By Rudi Hempe
CELSnews editor

The team which unraveled the mysteries of the Helicoprion shark was composed of (l-r) Jason Ramsay, Cheryl Wilga, both URI; Alan Pradel, Dept. of Vertebrate Paleontology, American Museum of Natural History; Robert Schlader, Idaho Virtualization Lab, Idaho Museum of Natural History; Jesse Pruitt, Idaho Museum of Natural History and Leif Tapanila, Idaho State University. Not in photo was Dominique A. Didier of Millersville University who provided funding for the CT scanning of the fossil (shown right.)

Thanks to some high-tech imagery, two URI biologists and other colleagues have unraveled part of the mystery that has long surrounded one of the oddest looking ancient sharks ever to cruise the Earth’s oceans.

Dr. Cheryl Wilga of the Department of Biological Sciences has devoted her professional career to the study of how sharks’ jaws work, but this time she, along with her former grad student, Dr. Jason Ramsay,  is involved in a study revolving around a unique fossil –a whorled tooth.

The tooth belonged to a creature called Helicoprion, a shark that lived 290-270 million years ago. Ever since fossils of the tooth were discovered, there have been all sorts of theories about how the tooth was mounted on the shark and how it was employed.

The tooth is arranged in a strange spiral and it appears to be a series of teeth arranged in a whorl.

Actually, says Wilga, the spiral arrangement is a single tooth—it’s just that this tooth has many cusps arranged in a straight line. As the shark matured, the tooth grew into a spiral arrangement with the oldest cusp being in the center of the whorl and the most recent cusp being the largest one.

While there are many fossils of Helicoprion teeth found all over the world there is no fossil of the entire creature—sharks have cartilage instead of bones and therefore don’t leave a good fossil record. However shark teeth are quite distinctive and often the teeth serve as a clue as to the size of the fish. It is believed that Helicoprion species could reach 15 feet in length.

The study Wilga and Ramsay were part of involved a fossil that was found in 1950 in a phosphate mine in Idaho. The fossil was stored in a museum vault until recently when Jesse Pruitt of the Idaho Museum of Natural History thought it would be a good idea to study it using computer tomographic (CT) scans. Conducting CT scans is an expensive proposition but Dominique A. Didier of the Department of Biology at Millersville (PA) University had some funding and the fossil was scanned using a machine at the University of Texas. The project also involved Leif Tapanila of the Department of Geosciences at Idaho State University and the Idaho museum, Alan Pradel of the Department of Vertebrate Paleontology at the American Museum of Natural History and Robert Schlader of Idaho Visualization Lab at the Idaho museum.

Because the tooth whorl is preserved mostly as external impressions, a computer model of the whorl was generated and using specialized software the thickness of the tooth and root was determined. This was then scaled to match a surface scan of the fossil.

Reconstructions of Helicoprion since 1899. Earliest models (a-d) posited the whorl as an external defensive structure, but feeding reconstructions dominate more recent hypotheses (e-l). Credits (a) Woodward; (b) Simoens; (c) Karpinsky; (d) Obruchev; (e) Van den Berg in Obruchev; (f) John; (g) Carr; (h) Eaton; (i) Parrish in Purdy; (j) Troll in Matsen & Troll based on Bendix-almgreen; (k) Lebedev; (l) Troll & Ramsay. Configuration of gill slits and fins based on related fish, e.g. Caseodus and Ornithoprion. Bottom rendition is by artist Ray Troll based on the findings of Jason Ramsay and Cheryl Wilga. Photo courtesy of Royal Society Publishing.

The end result showed the tooth was a singular symmetrical structure that was attached to the symphysis between the lower jaws of the shark and occupied the full length of the mandibular arch (or jaws), said Wilga. This finding is in contrast to previous theories that the tooth was attached elsewhere on the creatures. One old theory even held that the tooth was merely ornamentation and another theory suggested the tooth could be shot out to snare some prey.

“This was a really bizarre shark,” commented Ramsay who received his doctorate just before the shark study began.

Their latest study indicates that the tooth grew out in a curved fashion as the shark aged. “Continual growth of the whorl pushed the tooth-root complex in a curved direction towards the front of the jaw where it eventually spiraled to form the base of the newest root material and this process continues to form successive revolutions,” the scientists said in their paper which was published Feb. 27 by Royal Society Publishing.

Retention of the whorled tooth necessitated that the jaw was quite narrow so that the whorl had structural support on either side, said Wilga. When the shark’s jaw closed, indications are it rotated inward bringing food into the back of the mouth “sort of like a radial arm saw in reverse,” explained Wilga. There is no indication that the upper jaw had any recess to prevent the tooth from cutting into it—rather, there probably was a structure that prevented the whorl from going too far up—“like a door stop,” says Wilga adding that the exact mechanics of the jaw action will be the subject of another paper that she and Ramsay are writing.

The scans showed how the jaw’s cartilage structures were arranged and also suggest that the main diet of these sharks consisted of soft-bodied fish and cephalopods (such as octopus, squid and cuttlefish). Long-tentacled ammonites, now extinct, could have been on the menu too. “The weird thing about these sharks is that their teeth did not show any signs of wear,” said Ramsay.

The upper jaw was close to being fused with the creature’s skull similar to today’s chimaera, also known as ratfish because of their long tail, noted Wilga.

Ramsay said as soon as he saw the scans he could determine from the markings where the jaw muscles were attached. “I saw those scans and immediately said I’m in on the study. That’s really my specialty—tooth and muscle function of sharks,” he said, adding that he and Wilga were able to assist the project because of their knowledge of how modern-day sharks’ jaws work.

Based on the recent findings, the scientists could then interpolate how the shark’s head looked like and those clues were presented to artist Ray Troll who created a new rendition of what the head of Helicoprions looked like (bottommost illustration in accompanying photo).

Ramsay, who is just starting out on his career, said he would like to study more fossils using scanning devices. There are thousands of fossils stored in museums all over the world, which could be re-studied by using some of the modern scanning techniques, he suggested.

Helicoprions had the perfect tool for going after fast prey with elongated appendages, said Ramsay. “They had a thin jaw with a perfect cutting device that could cut off the head and pull in the remainder.” Ramsay said the shark’s jaw had the capability of delivering tremendous cutting force. The biggest of the creatures could probably exert a force of 7500 newtons (about a million pounds of pressure per square inch). In contrast, noted Ramsay, “A car crusher has a force of only about 2,000 newtons.”