After a century of colourful guesses, CT scans have revealed what's really going on inside the nightmarish jaw of Helicoprion, a large, 270 million-year-old cartilaginous fish with an elaborate whorl of teeth set in the middle of its mouth.

In 1899, Russian geologist, Alexander Petrovich Karpinsky, gave this six-metre-long fish the name Helicoprion, meaning "spiral saw", based on a fragmentary fossil found in Kazakhstan. Because the saw he was describing had been separated from the rest of the body, Karpinsky couldn't be sure where it would have fit, so initially he suggested that it started in the fish's mouth, and curled upwards along the snout as an external coiled mass of fused-together teeth. Think a sawfish's saw, only curled upwards. Further guesses were made during the early 1900s by a number of researchers from around the world, including American palaeontologist Charles Rochester Eastman. Eastman had issues with the idea that such an unwieldy apparatus could have possibly sat inside this poor creature's face. Publishing in a 1900 edition of The American palaeontologist, Eastman favoured the idea that the whorl protruded from somewhere along the length of the fish's back, acting as some sort of defensive display, perhaps.

A few years later, Karpkinsky followed Eastman's train of though, and suggested that the Helicoprion's whorl could have formed part of the animal's tail, or perhaps extended from its dorsal fin, or sat lower down on its back. In 1907, American ichthyologist, Oliver Perry Hay, found a fossilised specimen that was still sitting in its natural position, and judging from this, favoured the jaw theory. But did it sit in the upper or lower jaw? And did it sit in both? Such questions were impossible to answer with the few and fragmentary specimens these researchers had to work with.

Regardless, the general consensus in the earliest hypothetical reconstructions of Helicoprion was that this terrible, toothy whorl surely served a defensive purpose. Later this century, this perception has changed, and researchers moved towards the idea that the whorl was used mainly for feeding, and therefore was associated with the creature's jaw.

In 1950, a crucial Helicoprion whorl specimen was discovered by Danish palaeontologist Svend Erik Bendix-Almgreen in the Waterloo Mine near Montpelier, Idaho. Named IMNH 37899 and housed in the Idaho Museum of Natural History, it was first described by Bendix-Almgreen in 1966. It might have been seriously crushed and disarticulated, but along with the 117 discernible serrated tooth crowns sitting on a spiral with a diameter of 23 cm was some very telling cranial cartilage. This proved for the first time that at least some of the whorl was contained inside Helicoprion’s mouth.

But that didn't limit the possibilities. Over the past fifty years, researchers have suggested that the whorl extended awkwardly from the lower lip, curling underneath the chin; sat inside the mouth where the tongue should be; or perhaps sat further down towards the throat.

Now a team led by Leif Tapanila from the Department of Geosciences at Idaho State University, and curator of the Idaho Museum of Natural History, have gained unprecedented insight into the structure of Helicoprion’s skull. IMNH 37899 was scanned using an ACTIS scanner at the University of Texas High-Resolution X-ray CT Facility, and from this, a scaled, 3-D computer-generated model of the animal's skull was generated.

“Our reconstruction posits that the tooth whorl is a singular, symphyseal [fused] structure of the lower jaw that occupied the full length of the mandibular arch,” the team reported in Biology Letters yesterday. This means that instead of extending past the lower jaw and coiling underneath the chin, as had been previously suggested, the whorl grew inside the lower jaw. This way, just as sharks have multiple rows of teeth that are continuously replaced, Helicoprion had a partly concealed tooth factory that began near the area where the upper and lower jaws meet, ran over the mouth wear the tongue would be if it had one, and then into the cartilage supported by the lower jaw (see first image).

“Continual growth of the whorl pushes the tooth–root complex in a curved direction towards the front of the jaw, where it eventually spirals to form the base of the newest root material, and this process continues to form successive revolutions,” the researchers say. “At some time, prior to a complete 360 degree evolution of spiral growth, tooth crowns are concealed within tessellated cartilage on the upper jaw.”

As Helicoprion didn’t have any teeth on his upper jaw, the team suggests that the predatory fish would have broken down its soft-bodied prey, such as cephalopods and small fish, by repeatedly slicing them with a single row of serrated teeth. When it closed its closed its lower jaw, the whorl of teeth were pushed backwards, “providing an effective slicing mechanism for the blade-like serrated teeth and forcing food to the back of the oral cavity”.

Tapanila and colleagues suggest that the Helicoprion’s jaw could have extended past 50 cm long, and some tooth whorls would have boasted some 150 teeth. The team also says that the creature is not a shark, as others have assumed, but a chimaera (Holocephalan), which is a group of cartilaginous fish also known as ratfish or ghost sharks that branched off from the sharks 400 million years ago. "It was always assumed that the Helicoprion was a shark, but it is more closely related to ratfish, a Holocephalan,” says Tapanila. "The main thing it has in common with sharks is the structure of its teeth, everything else is Holocephalan."

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