Helicoprion - the Sprial Saw shark
When: Late Permian (~290-270 million years ago)
Where: Fairly globally distributed
What: Helicoprion is a fossil shark. Though it has been known to science for over 100 years, it is still poorly understood. Teeth are the part of it that has ever been found. This not that strange for a fossil shark, as sharks do not ossify their skeletons - they are cartilaginous all of their lives. Thus it is extremely rare to find a fossilized shark skull or skeleton. What makes Helicoprion such a scientific puzzle is the arrangement of these teeth. They are in a spiral, with the smallest teeth in the center. Modern sharks go though dozens upon dozens of teeth in their lifetimes, and fossil sharks were no different. What was very different however, is these extinct sharks retained their older teeth, instead of shedding them as we see in all modern forms. There are well preserved fossil sharks from the Devonian which incorporated their older teeth into bumps and ridges on their heads, but they did not develop an elaborate spiral as seen in Helicoprion.
The biggest problem with reconstructing Helicoprion is determining the location of the spiral. Some of the first studies of this shark located the spiral on the upper jaw (the first small image above), but closely related fossil finds have shown unarguably this structure was located on the lower jaw. However, even with the position on the lower jaw certain, there are other debates and various reconstructions, two more of which are shown above. A recent study conducted by researchers at the Smithsonian concluded that these placements outside of the body were not realistic, as this would create too much drag when the shark swam, not only slowing it down but alerting it’s prey as it approached. They have interpreted the spiral to be located in the throat region of Helicoprion - their reconstruction is the large one shown above.