Prehistoric ghost shark Helicoprion’s spiral-toothed jaw explained

By Becky Crew | February 27, 2013

... 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 ...

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 ...

“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 where the tongue would be if it had one, and then into the cartilage supported by the lower jaw ...

http://blogs.scientificamerican.com/running-ponies/2013/02/27/prehistoric-ghost-shark-helicoprions-spiral-toothed-jaw-explained/