Helicoprion

Helicoprion, meaning ("Spiral Saw"), is an extinct genus of whorl-toothed shark. first arose in the oceans of the Late Carboniferous 280 million years ago, 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.

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 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.

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.

Fossils
Fossils of Helicoprion bessonowi have been found in mid-Permian deposits in Russia, North America, Japan, and Australia. Curious spiral structures, some 10 inches (26 centimetres) across - or about the size of a large dinner plate.

Teeth
The individual teeth are serrated, and it's implied that Helicoprion was carnivorous. As there has yet to be an actual skull found, exactly how it captured or fed on its prey are subjected to a great deal of speculation. One hypothesis that it preyed on ammonites and that the teeth were specialized for the job of breaking into the ammonites' shells. Another idea was that the shark would swim into a school of fish and fling out the jaw, snagging prey on its many teeth. The most famous specimens of Helicoprion are found in eastern Idaho, northern Utah, and the far central western part of Wyoming.

Relationship with Edestus
It is now generally agreed that the structure is indeed a complex whorl composed of up to 180 teeth and must therefore have fit somehow into the mouth. Further specimens revealed that the teeth of Helicoprion most closely resembled those of a group of Paleozoic sharks known as edestoids. One of the best-known species, Edestus giganteus, was a 20-foot (6-meter) super-predator (about the same size of the modern white shark) with teeth that kept growing beyond the tip of its snout. The most likely orientation - based on the teeth of Edestus and related edestoid sharks - is that the teeth overhang from the lower jaw like the vertical blade of a circular saw, having coiled about themselves as new teeth were generated from behind. It is possible that Helicoprion used this buzz-saw arrangement to snag squid-like creatures with a sideways swipe of the head while swimming through a school of the soft-bodied molluscs. In any case, Helicoprion exemplifies some of the difficulties involved in reconstructing ancient creatures from only a few clues.

Restorations
A common question regarding Helicoprion is which of many life restorations found on the internet and in the scientific literature are correct? One restoration has the tooth whorl hanging out of the center of the lower jaw; another has the tooth whorl in the center on the inside of the lower jaw; still another has it set back in the lower jaw looking like a buzz saw; still another shows a spiral dentition in both the upper and lower jaw.

Early restorations
Ray Troll is known by fossil shark experts for having studied Helicoprion for many years. His illustrations of Helicoprion (as well as a beautiful life-size model by Gary Staub based on Ray’s illustrations) can be seen on the internet. However, it is widely accepted now that the Ray Troll illustration is incorrect.

It was eventually decided that no previously published reconstruction of Helicoprion could be used as a reference for an accurate model. A puzzling question is why would a new spiral form of tooth replacement for jaw teeth develop in a Permian shark when an efficient method for tooth replacement (non-spiral) had already evolved in sharks prior to the Permian? Most current reconstructions are based upon the Bendix-Almgreen’s (1966) interpretations of crushed specimens of the edestoid sharks Sarcoprion and Helicoprion. (Edestoid sharks have an arched or spiral series of teeth, which are usually the only portions of the sharks’ remains preserved as fossils.) None of the specimens used by Bendix-Almgreen to interpret the anatomical position of these dentitions possessed landmarks that would allow the identification of specific cranial cartilage. The reconstructions are based on the assumption that the dentitions were symphyseal ones (point where left and right jaws meet) in the upper and lower jaws.

History
Helicoprion was first described in 1899 by A. Karpinsky on the basis of an incomplete specimen from the Permian of the Ural region of Russia. He believed that this dentition hung out of the shark’s upper jaw. In his review of this paper, Charles R. Eastman (1900) stated that the teeth were spines positioned in front of the dorsal fin on the shark’s back. Both of these interpretations would have created drag and generated vibrations (see gallery below).

In 1902 Eastman described a new Carboniferous edestoid shark dentition (Campodus variabilis) that he described (p. 150) as "three series of coalesced anterior or symphyseal teeth." He considered this specimen as evidence that the spiral dentition of Helicoprion was symphyseal. Because Eastman’s specimen was not associated with skeletal remains, its position in the shark’s body could not be ascertained.

In 1912, O. P. Hay published a new species of Edestus based upon the articulated upper and lower arched dentitions. This was the first known edestoid specimen with associated upper and lower dentitions. He interpreted them as belonging to the symphyseal region of the upper and lower jaws; the specimen, however, did not possess any cartilage that could be identified with certainty to its position in the cranium. He did not consider the possibility that the dentition was branchial.

From the time of Hay’s paper (1912) until the present, paleontologists have assumed that the tooth whorls of Helicoprion and similar dentitions in other edestoids were jaw dentitions (see Zangerl, 1981:86), but no skeletal evidence existed to support their position.

Bendix-Almgreen (1966) studied several spiral dentitions of Helicoprion that had cartilage surrounding them. Although the cartilage possessed no landmarks to identify it as cranial cartilage, Bendix-Almgreen stated that the cartilage represented the jaws and that the spiral dentition was from the symphyseal region of the jaws. This interpretation creates the following problems:

1. As Bendix-Almgreen noted (1966:30) the teeth in these specimens show no sign of wear or breakage. Based on whole shark specimens, it appears that tooth replacement was slower in Paleozoic sharks than in living sharks (Williams, 2001). Jaw dentitions of Paleozoic sharks often exhibit tooth wear and breakage; in living sharks tooth wear and breakage are rare. This absence of wear and breakage in Helicoprion (a Paleozoic shark) suggests that the teeth were not used for biting.

2. If spiral dentitions were in jaw positions two problems arise. To understand one of these problems we have to know something about how sharks replace their teeth. Many sharks have a total of 50 or more teeth in their upper and lower jaws; each tooth in a biting position is followed by 3 or more teeth in varying stages of development. The innermost teeth are the youngest and least formed and the outermost or biting teeth are complete. In Devonian sharks (which are older than Helicoprion), Williams (2001) found that these sharks, unlike living sharks, did not shed their teeth but retained them on the outside of the jaws under the skin. The pressure of the succeeding tooth pushes the no longer functional teeth into bunches forming bumps around the head. With this method of tooth replacement, retaining dentitions in spiral forms suggest that the jaws or the skin surrounding them would have been significantly modified to preserve the dentition in this manner. The result would be a large bulge on the underside of the jaw, about 62 cm., (2 feet). Bio-mechanically this would require extreme adaptations for which there is no evidence. The second problem is with the largest teeth pointing toward the throat the older teeth would hang out of the jaws. This would create drag and generate waves through the water that would warn prey of the shark's presence. Helicoprion would lose the advantage (that evolved in their ancestors) of the placoid scales that cover their bodies. These scales, which look like squat, bent teeth, allow water to pass over the shark’s body without generating any waves. A fish nearby would not know the shark was coming. This adaptation seems unlikely, and no evidence exists yet to show that the spiral dentition occupied a jaw position.

Modern restorations
Where then does the dentition reside? A possible position is the throat cavity; this cavity could accommodate the dentition’s spiral form, and the dentition would not be subjected to the wear and breakage from biting prey that would occur in a jaw position. In the throat cavity, this dentition was probably supported by the cartilage between the basal margins of the right and left gill arches in sharks. New teeth for the spiral dentition probably originated on this basal cartilage. The teeth may be modified pharyngeal denticles, which occur on the gill arches and basal cartilage in sharks and other fishes. As a throat dentition, when the shark opens its jaws, the teeth would be presented to grab prey entering the mouth cavity. Closing the jaws, the teeth would move the prey toward the esophagus. This type of dentition would work well for catching soft-bodied prey.

Mary Parrish, scientific illustrator, reconstructed a Helicopiron with the spiral dentition in the throat. In Mary’s reconstruction, the jaw teeth are rounded bars. These have not been found yet associated with Helicoprion, but other edestoid sharks have jaw teeth like these. We, therefore, think that Helicoprion probably had similar teeth in its jaws.

Historical reconstructions of Helicoprion from earliest to latest.