The Beacon

Mystery of a Prehistoric Fish's Bizarre Body Part Solved

The many faces of Helicoprion. Artwork © Ray Troll / Idaho State University 2013

One of the tricks of paleontology is figuring out how all those fossils fit together to form a whole animal.

The placement of the puzzle pieces wasn’t so clear with the bewildering spiral jaw of one 270 million year old “ghost shark.” Scientists have known since 1899 that the six-meter long shark-like Helicoprion (which means “spiral saw”) had a whorl of fused teeth somewhere in its makeup. But the original discovery of this mysterious body part found it detached from the rest of the animal’s body, with no good indication of its location.

The Russian geologist who unearthed this first fossil theorized that it was a weapon curling up out of the Helicoprion’s mouth, like a sawfish’s saw that had spiraled upwards. Others thought it was the lower jaw, curled below the giant fish’s chin.  Still others couldn’t imagine having such a monstrosity for a face, and guessed that it was located somewhere along the shark’s back—perhaps growing from its dorsal fin—or on its tail.

Less than a decade after the initial fossil was discovered, an American found an intact fossil of the toothy spiral in the creature’s mouth. But this did not settle the controversy over its attachment to the upper or lower jaw or its position inside or outside of the mouth.

Now a team at Idaho State University using a CT scanner on one well-preserved fossil specimen has been able to look back through the millennia and see the ghost shark as it would have been. Their digital reconstruction of the Helicoprion indicates that the spiral began at the back of the mouth, near the hinge that joins the upper and lower jaws, and curled forward, filling the place where a tongue would be. As the whorl continued to grow, it curled underneath and fused with the lower jaw, remaining entirely within the creature’s mouth. Without any teeth in its upper jaw, the Helicoprion used its one curling row of teeth to slice up its soft-bodied prey.

The same team also learned that the Helicoprion is not a true shark, but is a chimaera—not to be confused with the mythological lion-like creature. Chimeras or “ghost sharks” make up a group of cartilaginous fish that branched off from sharks more than 400 million years ago. Living species include the plownose chimaera, or “elephant fish”, and may be the oldest group of fish still alive today.

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