Helicoprion

Helicoprion, meaning ("Spiral Saw"), is an extinct genus of whorl-toothed shark that first arose in the oceans of the Late Carboniferous, approximately 280 million years ago, and survived the Permian-Triassic extinction event, and eventually went extinct during the Early Triassic, some 225 million years ago. Its fossils can be found in Russia and in the Western U.S. but no other part of the jaw or shark has ever been found.

The type species, H. bessonovi, was discovered and described in the Ural Mountains of Russia in 1899 by Russian paleontologist Andrzej P. Karpinski. The type specimen is a holotype based upon a single tooth-whorl. The tooth-whorl has puzzled paleontologists for decades, as it was unknown as to where it fit into the jaw, until a modern reconstruction determined that the most feasible place was within the shark's mouth.

Helicoprion is a member of the Order Eugeneodontida, members of which possessed a unique "tooth-worl" on the symphysis of the lower jaw as well as pectoral fins supported by long radials, and is closely related to Edestus. The most famous fossil specimens of Helicoprion are found in eastern Idaho, northern Utah, and the far central western part of Wyoming.

History
At first, specimens of H. bessonowi were thought to be the coiled shell of a somewhat odd ammonite. On closer inspection, it was discovered that they were a continuous whorl of teeth or perhaps dermal denticles from some kind of shark. In short order, the creature was named Helicoprion.

Russian paleontologist Andrzej P. Karpinski who was the first to find and describe a specimen, which he collected in 1899 in the Ural Mountains, invested years of his life in futile attempts to restore the position of the whorl, illustrating various possible positions of the tooth-whorl, from the top of the first dorsal fin, to hanging it from the tip of the tail, as well as placing the whorl on the tip of its nose.

Only a year after Karpinsky's description appeared in print, American paleontologist C. R. Eastman wrote: "Of the two leading theories as to the position of ... `spines' the first ascribes them to the jaws of a shark or skate, and the other to the median line of the back, some distance in advance of the dorsal fin." In a 1952 analysis of the tooth whorl, Russian paleoichthyologist Dimitri Obruchev decided that a location in the lower jaw "would only prevent the fish from feeding." He therefore placed the whorl in the upper jaw, "where it could serve as effective protection" acting as a shock absorber for the animal's head.

It was not until the discovery of the skull of a relative, Ornithoprion, that it was realized that the tooth-whorl was in the lower jaw.

Paleobiology
The tooth-whorl represented all of the teeth produced by that individual in the lower jaw, in that as the individual grew, with the older, smaller teeth being moved into the center of the whorl by the appearance of larger, newer teeth. Comparisons with other eugenodontids suggest that Helicoprion may have grown up to 10-15 feet long.