Of all the vexing fossil mysteries that have confounded paleontologists, few have been as persistent as that of Helicoprion – the name given to petrified whorls of elongate teeth that look like 270 million year old renditions on the theme of buzzsaw. What sort of animal did this Paleozoic remnant belong to, and where did the circular blade actually fit on the animal? Today, Idaho State University paleontologist Leif Tapanila and coauthors announce the answer to a conundrum that has puzzled paleontologists for over a century.

Russian geologist Alexander Petrovich Karpinsky coined the name Helicoprion in 1899. Even though the coiled fossils superficially resembled the shelled ammonites and nautilus paleontologists often found in the marine fossil record, Karpinsky realized that the petrifications were actually part of a shark-like fish. But there was no obvious indication of where such an unusual feeding apparatus might fit. Karpinsky’s best guess was that Helicoprion bore the toothy spiral on its nose, like a permanently-tensed party favor studded with a fearsomely pointed dentition.

Other paleontologists disagreed. While the American paleontologist Charles Rochester Eastman unabashedly praised the depth of scope of Karpinsky’s monograph – “not one in one hundred essays on paleontological subjects receives anything like the elaborate care and finish” that Karpinsky gave “the remarkable ichthyodorulites” called Helicoprion – the researcher brushed aside his colleague’s spiral-snouted restoration. “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,” Eastman wrote. Instead, the American paleoichthyologist pointed out that the “teeth” might actually be spines that jutted from elsewhere on the prehistoric shark’s body. (Karpinksy soon considered another alternative placement, too, with the whorl hanging off the putative shark’s tail.) Lacking a well-preserved Helicoprion with a whorl in place, though, the prehistoric enigma remained open to anyone who wanted to approach the problem.

View Images A gallery of Helicoprion hypotheses. Artwork by Ray Troll 2013.

Paleontologists and ichthyologists weren’t shy about proffering new ideas on the nature of Helicoprion. Over a century of speculation produced visions of sharks with whorls hanging off their snouts, lower jaws, dorsal fins, caudal fins, and even embedded deep in their throats. (Click on the image above, by Ray Troll, for a look at the gallery of hypotheses.) Even after paleontologists generally agreed that the teeth belonged at the tip of a long lower jaw, artists and scientists still played with what leeway they had. Was the fearsome spiral fully enclosed in the jaw, or did it hang down awkwardly in an external coil? The true anatomy of Helicoprion was frustratingly difficult to pin down.

When I wrote about Helicoprion in 2011, I highlighted the end-of-the-jaw placement for the tooth whorl as the most likely arrangement. But artist and major Helicoprion fan Ray Troll quickly got in touch with me to say that the classic image probably wasn’t correct, after all. New research was set to give Helicoprion a major makeover. That study has just been published today in Biology Letters, and focuses on a specimen found decades ago.

In 1966, paleontologist Svend Erik Bendix-Almgreen described a Helicoprion fossil that had been found 16 years earlier in the Waterloo Phosphate mine near Montpelier, Idaho. This specimen was special. Not only did it display a lovely tooth whorl – which Bendix-Almgreen suggested fit at the end of an elongate lower jaw – but the fossil also contained bits of cartilage from the upper jaw and skull.

Despite the extra material, though, Bendix-Almgreen thought that the specimen had been disarticulated and crushed so extensively that properly reassembling Helicoprion was impossible. The jaws sat in the Idaho Museum of Natural History for decades, one of thirty jaws in the institution’s collections, until student Jesse Pruitt started asking curator Leif Tapanila about the strange Permian fish. “He started poking around and asking questions about Helicoprion jaws,” Tapanila says, about “why the jaws were this way and not that.” In particular, Tapanila recalls, Pruitt wanted to know whether the coiled tooth row was a real feature of a living animal or something that happened after death – an artifact of death rather than a representation of life.

Tapanila and Pruitt concluded that the Helicoprion whorls really did have their buzzsaw shape in life, but they didn’t stop there. Along with their colleagues and input from Ray Troll, the researchers launched a new, detailed investigation into the museum’s Helicoprion stores. The fossil Bendix-Almgreen described, in particular, seemed to have the potential to yield new clues through CT scans that could visualize the internal secrets of the specimen. The scans, taken at the University of Texas High-Resolution X-ray CT Facility in Austin, “came out brilliant” Tapanila says. Not only was the fossil in better shape than expected, but the specimen elucidated two critical facets of the animal – that Helicoprion didn’t have an elongated jaw, and that it wasn’t really a shark.

View Images How the jaw of Helicoprion was arranged. Artwork by Ray Troll 2013.

Contrary to the popular long-jaw restorations, the tooth whorl of Helicoprion totally filled the lower jaw. The jaw joint sat right behind the weapon, and the spiral dentition was buttressed by jaw cartilage on either side. And, even stranger, Helicoprion didn’t have any upper teeth to speak of. The spiral of continually-added teeth was the creature’s entire dental armament.

Scraps of Helicoprion skull indicate that the fish wasn’t really a shark, either. Of course, as Tapanila points out, the word “shark” doesn’t have the simple definition we might expect. “‘Shark’ doesn’t have biological meaning anymore,” Tapanila told me, confiding “If I talk to a fish expert, and I say ‘shark,’ they get very angry.” Ichthyologists are rapidly rearranging the fish family tree and the definitions for different groups. All the same, the skull cartilage of Helicoprion included a very specific double connection that is characteristic of a group of cartilaginous fish called Euchondrocephali – commonly known as ratfish and chimeras.

Helicoprion was not a buzzsaw predecessor to great white or tiger sharks. The fish belonged to the lineage one branch over, near the evolutionary split where the ancestors of living sharks and ratfish parted ways. (And this pulls other weird prehistoric fish with fearsome teeth – such as the scissor-jawed Edestus – away from the shark line and into the ratfish line.) In general form, Tapanila and Troll expect, Helicoprion was an archaic member of the wider ratfish group that looked quite shark-like. And these predators reached impressive sizes. Tapanila estimates that a large Helicoprion would have been about 20 to 25 feet long.

View Images A new look for Helicoprion. Artwork by Ray Troll 2013.

After over 100 years, the mystery of the Helicoprion jaw is solved. That doesn’t make the prehistoric fish any less enigmatic. With only a single blade of teeth, how did Helicoprion actually catch and consume prey? Tapanila and other researchers are only just starting to investigate this question. Based on the new restoration, Tapanila suggests that “The analogy to a circular saw is almost perfect.” Not only was the tooth whorl shaped like a saw, but, Tapanila points out, “as the jaw closed [the tooth whorl] rotated the teeth backwards in a rotational saw motion.” Such a strategy would have worked well on squid and other soft-bodied cephalopods of the 270 million year old seas. But even with this realization, we are still left with the question of how such a strange arrangement – singular in the history of life on Earth – evolved in the first place.

Tapanila and colleagues are keeping after the enigmas surrounding the fish through studying Helicoprion fossils found in Idaho and elsewhere, including a lower jaw that’s even bigger than the one used in the new Biology Letters study. “You know the line from JAWS, ‘You’re going to need a bigger boat’? Well, I need a bigger CT machine,” Tapanila says. “I have the world’s largest Helicoprion specimen in the world sitting in my museum, and I see evidence for jaws.” The two-foot-wide jaw is too big for a conventional CT scanner, though. “It’s got all the features we hope,” Tapanila says, “but it’s massive, so I need to bring it to [a facility in] Pasadena.” That giant jaw will yield additional clues, and raise new questions. Tapanila suspects that the larger jaw belonged to a different species of Helicoprion than the one he and his team previously scanned, and the features of the bigger jaw might provide new information about how these buzzsaw fish differed across species and body sizes. There are still many secrets to draw out from the jaws of Helicoprion.

Yet, even with the remaining mysteries, to see the new vision come out of the rock is a dream come true for Ray Troll. “It’s been a twenty year quest for me,” he says, which all started “when I first saw a whorl and became obsessed by it.” “I’ve drawn the animal so many hundreds of times. Literally hundreds.” And not only is Troll “thrilled” to see the new research, but the new identity of Helicoprion is a bit of a personal victory. Troll heads the wonderfully geeky band Ray Troll and the Ratfish Wranglers. For so long, it seemed that the object of his endless fascination was a shark, but now, Troll says, “It’s really cool to have [Helicoprion] circle back around” to the ratfish side of the family tree. “My two obsessions have all converged,” Troll enthuses, in a spectacular ratfish relative that has for so long challenged scientists to chase after its circuitous spiral trail.

References:

Eastman, C. 1900. Karpinsky’s Genus Helicoprion. The American Naturalist, 34, 403: 579-582. 10.1086/277706

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. 10.1111/j.1463-6395.2008.00353.x