Unraveling the Nature of the Whorl-Toothed Shark

Reconstructing the anatomy of prehistoric sharks isn’t easy. With few exceptions – an exquisitely-preserved body fossil here, some calcified bits of skeleton there – teeth make up the majority of the shark fossil record. When those teeth come from a relatively recent species with close living relatives, it is not difficult to imagine what the extinct species might have looked like. The further back in time you go, though, the more bizarre sharks become. Sometimes teeth are not enough, and one especially unusual set of teeth has vexed paleontologists for over a century.

At first sight, the teeth didn’t look like they belonged to a shark at all. Coiled upon itself in a circular whorl, the bizarre tooth row superficially resembled the shells of extinct cousins of nautilus and squid called ammonites. After studying tooth whorls found in the Ural Mountains, though, the Russian geologist Alexander Petrovich Karpinsky recognized them for what they were. In 1899 he described them under the name Helicoprion as the remains of an ancient shark. Just how they fit in the shark’s mouth was another matter altogether.

No known shark had a similar buzz-saw arrangement of teeth, and, although an exact date for it could not be pinned down, Helicoprion had clearly lived long before the appearance of modern sharks. (Today we know that Helicoprion was a widespread genus of shark that persisted from about 290 to 270 million years ago.) Karpinsky was on his own, and in his original restorations he placed the tooth whorl on the shark’s snout with the teeth jutting from the outer edge. Not everyone agreed. While praising Karpinsky’s diligent description, in 1900 the paleontologist Charles Rochester Eastman balked at the restoration, writing “Few will be prepared to admit, however, that this highly fanciful sketch can be taken seriously, and, therefore, the least said about it the better.”  Drawing a comparison with similar fossils of a shark called Edestus, Eastman pointed out that some authorities believed such tooth-like structures were actually spines embedded on the back of the sharks. Which restoration was correct seemed impossible to say.

Karpinsky did not stick to his original idea. Taking the criticism of his peers seriously, he restored the whorls elsewhere on the shark’s body – on the dorsal fin, sticking out of the back, or extending from the upper tip of the tail. After all, some living sharks had spines or spikes on their bodies, and shark skin itself is made up of tiny, tooth-like structures called dermal denticles, so it was not out of the question that the objects might have been used for defense or ornamentation.

None of the arrangements were satisfying, but the discovery of more complete remains from related sharks confirmed that the whorls truly were teeth, after all.  This did little to resolve the mystery. Scientists were uncertain whether the tooth whorls were part of the upper or lower jaws – or both! – and no one could agree on how they were actually used. There was no shortage of inventive hypotheses – including the idea that the coils acted as a kind of shock-absorber or that they evolved as a lure to attract unwary ammonites – but two particular notions were prominently favored. Based upon well-preserved specimens from Idaho described by Svend Bendix-Almgreen in 1966, some authorities restored Helicoprion with a static, saw-blade arrangement of teeth at the front of its jaws for cutting or crushing, but others proposed that the teeth were actually embedded in an extendable tentacle. When feeding, Helicoprion would dart into a school and fish and start snapping out its lower jaw like a deadly, serrated party favor.

A new vision of Helicoprion created for the Smithsonian’s National Museum of Natural History took things in another direction entirely. By the 1990’s at the latest, it had generally been agreed that the whorls were parts of the shark’s jaws. Even as the snap-jawed idea fell out of favor, influential restorations by paleo-artist and Helicoprion-fan Ray Troll cradled the circular tooth blades within a bed of cartilage of flesh at the tip of the lower jaw with only part of the circular conveyor belt of teeth visible. This became the standard image of Helicoprion, but the experts at the Smithsonian disagreed.

Working with illustrator Mary Parrish, paleontologists Matt Carrano, Victor Springer, and Bob Purdy found themselves dissatisfied with all the previous interpretations of Helicoprion. Instead, they placed the tooth whorl inside the throat. They offered two lines of evidence for their decision. The teeth of Helicoprion didn’t appear to show signs of wear or breakage, the scientists argued, yet Paleozoic sharks are thought to have replaced their teeth much more slowly than modern sharks. If this were the case and the teeth were used for biting, then they should have been worn down. Furthermore, if the teeth grew slowly and were preserved by forming spiral shapes, then there should have been a large bulge beneath the shark’s lower jaw which may have created minute waves in the water and prematurely warned prey of the shark’s presence. Following the suggestion of Springer, the tooth whorl of Helicoprion was thus tucked away into the throat as a highly-specialized variety of throat denticle seen on shark gill arches. Posted in 2008, the explanation behind the illustration concluded, “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.”

But the Smithsonian reconstruction wasn’t entirely satisfying. If the tooth whorls really were a modified kind of throat denticle, then why did they grow in a spiral, with the teeth becoming increasingly longer and reaching higher into the animal’s throat as it aged? And a blade of teeth seems rather thin for a specialized cutting structure inside the mouth. How would the shark toss food backward in a way certain to drag across the strange structure? The throat-saw avoided the problems thought to have been caused by the whorls being placed in the jaw, but it created entirely new ones.

The appearance of Helicoprion had once again been thrown into question, but a 2009 paper by Oleg Lebedev threw support to the saw-jaw model. The study was based upon a Helicoprion specimen found in the 284-275 million year old rock of Kazakhstan – two sections of a single whorl that extended the southern range of the shark in the area – and Lebedev took the discovery of this opportunity to reevaluate how this strange shark might have fed.

A significant part of the mystery was the anatomy of the upper jaw. No one had ever found one. That the tooth whorl fit into the lower jaw had been confirmed by old Helicoprion finds and the discovery of related types, but even well-preserved specimens from Idaho that revealed some aspects of the head did not contain any parts of the upper jaw. Left to fill in the blank on the basis of other fossil sharks, paleontologists hypothesized that Helicoprion either had a narrow upper jaw with a few teeth – as in the related Sarcoprion – or a larger upper jaw which housed a second, upper tooth whorl. Lebedev suggested something different. Perhaps the tooth whorl fit into a pocket in the upper jaw lined with rows of much smaller teeth – a kind of special sheath for the lower tooth blade that gave Helicoprion a deeper upper jaw than had previously been imagined.

Evidence that Helicoprion was using this specialized equipment to bite was visible on some of the teeth themselves. Not only had Bendix-Almgreen noted some wear on Helicoprion teeth in the 1960’s, but Lebedev detected scratches one some of the best-preserved teeth of another previously-collected specimen. In general, though, such signs of feeding were relatively rare – a pattern consistent with the idea that Helicoprion was feeding on soft-bodied prey like squid and fish. The shark may not have been the scourge of the hard-shelled ammonites as had previously been thought. Since living pygmy sperm whales and Cuvier’s beaked whales have generally similar jaw anatomy – teeth in the lower jaw, but few or no teeth in the upper jaw – Lebedev suggested that these cetaceans might be the best living proxies for the way Helicoprion fed, especially since these whales frequently eat squid. In turn, this would hint that Helicoprion was a capable pursuit predator with a streamlined body, a point Lebedev supported by referring to more completely-known, allied fossil sharks such as Caseodus and Fadenia.

Of course, Lebedev’s hypotheses will require further discoveries and investigations to test, but it is probably close to what Helicoprion looked like. Contrary to the reasons outlined by the Smithsonian team, there is good evidence that the toothy buzzsaw of Helicoprion was housed within cartilage at the tip of the lower jaw and was used to bite down on soft-bodied prey (a mode of life that has further implications for the as-yet-unknown body shape of this prehistoric shark). Given how many times images of Helicoprion have changed over the past century, though, there’s little doubt that scientists and artists will continue to tweak its appearance. If they look carefully, the tip of the shark’s tooth whorl peeks out of the darkness, but the rest of its anatomy remains obscured by the shroud of incomplete preservation.

[Postscript: Investigations that will have a bearing on the upper jaw anatomy of Helicoprion are underway right now. Although Lebedev almost certainly got the overall shape of the shark correct, there may be evidence that Helicoprion had a narrow upper jaw, after all. I anxiously await the publication of the research currently underway.

There is obviously a long and rich history of debate surrounding Helicoprion, but, as far as I know, there is no comprehensive review covering how images of this shark have changed since 1899. For a fossil-loving historian of science – especially one who can read German and Russian! – the story of Helicoprion presents an open opportunity.]

Top Image: The tooth whorl of Helicoprion. (Wikimedia Commons)

References:

Eastman, C. (1900). Karpinsky’s Genus Helicoprion The American Naturalist, 34 (403) DOI: 10.1086/277706

Ellis, Richard. 2001. The Helicoprion Mystery. Natural History.

Lebedev, O. (2009). A new specimen of Helicoprion Karpinsky, 1899 from Kazakhstanian Cisurals and a new reconstruction of its tooth whorl position and function Acta Zoologica, 90, 171-182 DOI: 10.1111/j.1463-6395.2008.00353.x

Mutter, R.J. and Neuman, A. (2008). Jaws and dentition in an Early Triassic, 3-dimensionally preserved eugeneodontid skull (Chondrichthyes) Acta Geologica Polonica, 58 (2), 223-227

Purdy, R.W. 2008. The Orthodonty of Helicoprion. http://paleobiology.si.edu/helicoprion/