The Orthodonty of Helicoprion
by Robert W. Purdy
Helicoprion is a shark that swam in the Permian seas about 290 million years before the present time; it is an unusual shark in that it has a spiral dentition. These dentitions, other than crushed cartilage, are the only remains of this shark yet found. Spiral dentitions are not known from any living shark or any other vertebrate animal.
For over one hundred years scientists have tried to understand the biology of Helicoprion. How did the spiral tooth dentition fit into the shark’s jaws? 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.
Mary Parrish, scientific illustrator, was tasked with creating a restoration of this shark for the National Museum of Natural History’s new Ocean Hall. She sought the answers for the fitting of the tooth spiral into the shark from the museum’s scientific staff. Matt Carrano, curator of dinosaurs, Victor Springer, (senior scientist emeritus, Division of Fishes), and Bob Purdy (fossil shark specialist) tried to answer these questions for her.
First, the scientists asked Mary to use Ray Troll’s well researched reconstruction of Helicoprion as her primary reference. 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. Bob Purdy later decided that the Ray Troll illustration just didn’t seem right.
Bob then provided Mary with a restoration published in 1962 by an excellent fossil shark worker, Thomas Eaton. The scientists later decided that Eaton’s placement of the tooth spiral didn’t seem right either.
It was eventually decided that no previously published reconstruction of Helicoprion could be used as a reference for our reconstruction. 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?
We decided to look again at the scientific evidence. This evidence led us to the following conclusions:
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.
Vic Springer suggested that these dentitions may be from the throat (branchial) region rather than the jaws. His suggestion would resolve most the problems with current reconstructions. These problems and their history are recounted below.
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 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.
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.
Based on the above information provided by the scientists, Mary 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.
The illustration seen below is the final reconstruction of Helicoprion that was prepared by Mary Parrish under the direction of Robert Purdy, Victor Springer and Matt Carrano. The illustration was rendered in acrylic paint on gessoed masonite, then scanned and modified in Photoshop. Dozens of emails were exchanged between Mary and the scientists and many preliminary sketches were discussed before this illustration was approved.
Vic noted that Mary’s reconstruction had five gill slits instead of six or seven, as is found in living primitive sharks, such as the frilled shark and the six-gilled shark. Fossil shark skeletons, however, that were older than Helicoprion did have five gill slits. After resolving Vic’s final concern, Mary’s reconstruction was ready for our Ocean Hall and web site.
Other aspects of Mary’s Helicoprion reconstruction, such as the shape of the snout and body, are conjectural but are based on a figure in Zangerl (1981). The color we used for Helicoprion, as in most reconstructions, is hypothetical. The shape of the eye pupil is also hypothetical.
Someday better specimens of this shark may provide evidence to support or prove our reconstruction wrong. For now Mary’s reconstruction seems like the best science.
Bendix-Almgreem, S. E., 1966. New investigations on Helicoprion from the Phosphoria
Formation of south-east Idaho, U.S.A. Biologiske Skrifter undgivet af det Kongelige Danske Videnskabernes Selkab, v. 14, n. 5, pp.1-54, pl. 1-15.
Eastman, C. R., 1900. Karpinsky’s genus Helicoprion. A review. American Naturalist, v. 34, p. 579-582.
____________, 1902. Some carboniferous cestraciont and acanthodian sharks. Bulletin of the Museum of Comparative Zoology, v. 39, p. 55-99, pl. 1-7.
Eaton, Jr., T. H., 1962. Teeth of edestid sharks. University of Kansas Publications, Museum of
Natural History, v. 12, n. 8, pp. 347-362, 10 figs.
Hay, O. P., 1912. On an important specimens of Edestus; with description of a new species,
Edestus mirus. Proceedings of the U. S. National Museum, v. 42, n. 1884, pp. 31-38, pl. 1, 2.
Peyer, Bernhard, 1968. Comparative Odontology. The University of Chicago Press, pp.1-347, pl.1-96
Williams, M. E., 2001. Tooth retention in cladodont sharks: with comparison between primitive grasping and swallowing, and modern cutting and gouging feeding mechanisms. Journal of Vertebrate Paleontology, v. 21, n. 2, p. 214-226.
Zangerl, R., 1981.Chondrichthyes I: Paleozoic Elasmobranchii. In H. P. Schultze (ed.), Handbook of Paleoichthyology. Gustav Fischer Verlag, New York, 115 pp.
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