Mystery of Prehistoric Shark's Spiral Saw-like Teeth Finally Solved

First Posted: Feb 27, 2013 11:33 AM EST

About 270 million years ago, a prehistoric shark prowled the depths of our ancient oceans. Named Helicoprion, it possessed teeth that looked similar to saw-like spirals on its jaws. For years, the teeth have baffled scientists, who were unable to understand where exactly they were positioned. New research, though, shows exactly how and where these teeth were located on the ancient, cartilaginous fish.

When the fossil of this shark's teeth was first discovered in 1899, Russian geologist, Alexander Petrovich Karpinsky, thought that the saw-like jaw might have been separated from the rest of the body and curled upward along its snout--think of a sawfish's saw curling upward. Other researchers thought that the saw might have been located on the shark's dorsal fin, lower jaw, throat or even along its tail.

Finally, in 1950, another specimen was discovered by a Danish paleontologist named Svend Erik Bendix-Almgreen. The specimen, named IMNH 37899, was housed in the Idaho Museum of Natural History and possessed some very telling cranial cartilage that showed that the whorl of teeth was actually housed within the fish's mouth. Even so, scientists were still baffled over whether the teeth extended from the lower lip, sat inside the mouth or further down in the throat.

Now, Leif Tapnila and colleagues have found the answer to the mystery. They used a high-power CT scan, which used X-rays to create a detailed computer image, in order to fully analyze the fossilized specimen. Once they finished imaging the specimen, they could clearly see that it contained both the lower and upper jaw of the animal, as well as the spiral of teeth.

For the first time ever, the scientists could see exactly how the jaws related to the teeth. The spiral was actually connected to the fish's lower jaw in the back of the mouth. Only about a dozen teeth poked up out of the lower jaw so that it could bite, while the rest of the teeth were stored inside and weren't used. These unused teeth were essentially the fish's "baby teeth" that it possessed when it was younger.

The finding actually supports the theory that unlike sharks that replace their teeth, Helicoprion retained its teeth permanently.

Yet the researchers weren't done yet. The team built a 3D model of the jaw using computer images to reveal how the spiral worked. They found that as the mouth closed, the teeth spun backward, rather like a saw blade. This motion slashed through the meat that the fish was biting into and make a clean cut.

So what did this fish eat? More than likely, it ate soft, fleshy animals such as squid. Since its teeth are ill-suited to cut through calcium carbonate shells, it probably focused on species that didn't possess these features.

The findings are published in the Royal Society journal Biology Letters.

To see pictures of what this fish may have looked like, check out the images here.

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