Helicoprion: The Permian Fish With a Spiral Buzzsaw Tooth Whorl

Helicoprion was a large cartilaginous fish that lived in the world’s oceans during the Permian Period, roughly 290 to 270 million years ago. It is famous for its “tooth whorl”, a spiral of teeth that looks like a coiled circular saw. For more than a century scientists could not agree where this strange structure sat in the animal’s body, until CT scans published in 2013 showed it filled the lower jaw and that Helicoprion was a relative of today’s ratfishes rather than a true shark.

Background: the Permian seas

The Permian Period lasted from about 299 to 252 million years ago. During this time almost all land was united in the supercontinent Pangaea, surrounded by the global ocean Panthalassa. The seas were home to ammonoids, brachiopods, early ray-finned fishes and many kinds of cartilaginous fishes.

Cartilaginous fishes, or chondrichthyans, have skeletons made mainly of cartilage instead of bone. Today the group includes sharks, rays and the less familiar ratfishes, also called chimaeras. Because cartilage rarely fossilises, many extinct members are known mainly from their teeth and spines, which are much tougher. Helicoprion is a classic example.

Discovery: a puzzle from the Urals

Helicoprion was named in 1899 by the Russian geologist Alexander Karpinsky, based on a spiral tooth whorl found in the Ural Mountains of Russia. The name combines the Greek words for “spiral” and “saw”. Karpinsky’s species was Helicoprion bessonowi.

Since then, tooth whorls have been found in Permian rocks in many parts of the world, including Russia, the western United States, Canada, Mexico, Japan and Australia. The richest source is the Phosphoria Formation of Idaho and neighbouring states, where phosphate mining in the 20th century uncovered dozens of specimens.

Where did the whorl go?

The problem was that the whorls were almost always found on their own, without the rest of the skeleton. Karpinsky suggested that the coil grew from the tip of the upper jaw and curled upward over the snout. Other researchers placed the whorl inside the throat, on the dorsal fin, on the tail, or projecting down from the lower jaw like a curved blade.

A key clue came in 1966, when the Danish palaeontologist Svend Erik Bendix-Almgreen described an Idaho specimen in which the whorl was preserved together with traces of jaw cartilage. This suggested the whorl belonged in the lower jaw. However, the exact arrangement remained unclear, and reconstructions continued to vary widely.

The 2013 CT-scan study

In 2013 a team led by Leif Tapanila of Idaho State University and the Idaho Museum of Natural History published a study in the journal Biology Letters. They used computed tomography (CT) scanning to examine the same Idaho specimen, catalogued as IMNH 37899, which preserved cartilage of the jaws around the whorl.

The scans revealed the three-dimensional shape of both jaws. They showed that the whorl sat at the symphysis, the point where the two halves of the lower jaw meet, and that it filled almost the entire length of the lower jaw. The upper jaw was firmly attached to the braincase, a feature also seen in modern chimaeras.

The study also re-examined Helicoprion’s relationships. Its jaw structure suggested that it was a eugeneodont, an extinct group of cartilaginous fishes more closely related to chimaeras than to sharks. This means the popular name “buzzsaw shark” is not strictly accurate.

The artist Ray Troll, who had long been fascinated by Helicoprion, worked with the team to produce new reconstructions. The findings were shown to the public in an exhibition at the Idaho Museum of Natural History.

How the tooth whorl worked

The whorl was built up throughout the animal’s life. New, larger teeth formed at the outer end, while older, smaller teeth were pushed into the centre of the spiral. Unlike most sharks, Helicoprion did not shed its old teeth; they were kept and coiled inward.

When Helicoprion closed its mouth, the whorl would have moved backward and acted like a rotating saw, drawing prey into the mouth while slicing it against the upper jaw. A 2015 study by Jason Ramsay and colleagues modelled the movement of the jaws and supported this slicing action.

The arrangement seems best suited to soft-bodied prey. Many researchers suggest that Helicoprion fed on cephalopods, squid-like relatives of octopuses, and possibly on fish. Hard-shelled prey would have been more difficult to handle.

Relatives: the eugeneodonts

Helicoprion was not the only fish with unusual teeth. The eugeneodonts, the group to which it belonged, appeared in the Carboniferous and included several genera with teeth arranged in rows or partial spirals along the middle of the jaw. One of the best known is Edestus, from the Carboniferous of North America and Europe, which had curved rows of blade-like teeth in both jaws.

In many of these fishes, old teeth were kept rather than shed, forming a growing series along the jaw. Helicoprion took this pattern to an extreme, coiling its lifetime’s teeth into a tight spiral of more than one full turn. Studying these relatives helps scientists understand how such a strange structure could evolve step by step.

Several species of Helicoprion have been named over the years, often on the basis of small differences in whorl shape. A 2013 review by Tapanila and Jesse Pruitt suggested that many of these differences reflect growth and variation, and recognised a smaller number of valid species, including H. davisii and H. bessonowi.

Size and appearance

Because no complete body has been found, Helicoprion’s overall size is uncertain. Estimates are based on the size of the whorls and jaws compared with related fishes. Proposed lengths for the largest individuals commonly fall in the range of about 5 to 8 metres, although smaller figures have also been suggested.

Its body shape is also partly guesswork. Modern reconstructions usually show a streamlined fish resembling a shark, with the whorl hidden inside the lower jaw and only the tips of the teeth visible.

Myths and facts

Older illustrations often show the whorl sticking out of the mouth or curling over the nose. The CT evidence shows that it was enclosed in the lower jaw. Helicoprion is also widely called a shark; it was a cartilaginous fish, but its closest living relatives are the ratfishes. Finally, it did not survive into the age of dinosaurs. Eugeneodonts as a group lasted into the Triassic, but Helicoprion itself is known only from the Permian.

By the numbers

Timeline

• c. 290 million years ago: Helicoprion appears in the early Permian seas.
• c. 270 million years ago: Helicoprion becomes rare and disappears from the record in the middle Permian.
• c. 252 million years ago: The Permian–Triassic extinction devastates marine life.
• 1899: Alexander Karpinsky names Helicoprion from a whorl found in the Urals.
• 1966: Svend Erik Bendix-Almgreen describes an Idaho whorl with jaw cartilage.
• 2013: Tapanila and colleagues publish CT scans placing the whorl in the lower jaw.
• 2015: Ramsay and colleagues model how the jaw and whorl worked.

Why it matters

Helicoprion shows how strange and inventive evolution could be, producing a feeding apparatus unlike any living animal. Its century-long puzzle also illustrates how new technology, such as CT scanning, can solve old problems using specimens that have sat in museum drawers for decades. Like the giant Megalodon and the armoured Dunkleosteus, it has become one of the most popular prehistoric fishes.