Helicoprion is an extinct genus of large shark-like cartilaginous fish that lived from the Early to the Middle Permian, about 290-270 million years ago. Helicoprion is a member of the Eugeneodontiformes, an extinct order of cartilaginous fish within the clade Holocephali, a group today represented only by chimaeras. It is also the type genus of the Helicoprionidae, a family of eugeneodonts characterised by distinctive tooth structures called tooth whorls. Helicoprion was first named in 1899 by Alexander Karpinsky on the basis of fossils discovered in Russia and Australia, the generic name meaning "spiral saw". Although numerous species were subsequently assigned to the genus, only H. bessonowi, H. davisii, and H. ergassaminon are recognized following a 2013 revision. The three species are distinguished by the shape and spacing of their tooth crowns.
Helicoprion is mainly known from its fused, spiral-shaped tooth whorls, which account for almost all documented fossils of the taxon. The position and function of these structures was long debated, but studies based on specimens preserving jaw cartilage indicate that they were positioned in the lower jaw and were specialised for grasping and slicing soft-bodied prey such as cephalopods. The whorl may also have aided in shelling or extracting the bodies of nautiloids and ammonoids. Based on the skeletal anatomy of smaller eugeneodonts, Helicoprion is estimated to have reached lengths between , with a general external appearance possibly comparable to that of tunas, swordfish, or mackerel sharks. Fossils of Helicoprion are known from marine deposits worldwide, indicating that it was pelagic and had a cosmopolitan distribution. The highest concentrations of these come from Idaho and Russia, which used to be covered by depp inland seas where Helicoprion may have congregated.
Research history
thumb|left|upright=0.8|alt=|[[Lithography|Lithographic figures of various eugeneodont fossils, the holotype of H. davisii (WAMAG 9080) being shown at the top (fig. 1)]]
The first known specimen of Helicoprion consists of a partial tooth whorl preserving 15 teeth, 14 of which are visible. It was discovered in Western Australia in a tributary of the Gascoyne River by a gold prospector named Mr. Davis, his first name being unknown. Now housed in the Western Australian Museum under the catalogue WAMAG 9080, the fossil was not found in situ, thus its precise stratigraphic origin remains uncertain.
thumb|right|upright=0.95|[[Lectotype tooth whorl of H. bessonowi (TsNIGR 1/1865)]]
More complete tooth whorls were discovered in the late 19th century by Alexander G. Bessonov of the Russian Academy of Sciences, in a quarry near the town of Krasnoufimsk in Sverdlovsk Oblast. He sent them to geologist Alexander Karpinsky, who first described them in a monograph published in Russian in 1899, followed later that year by a German translation. As the fossils he examined were sufficiently distinct from those of Edestus, Karpinsky assigned them to a new genus, which he named Helicoprion, with H. bessonowi as the type species. The generic name Helicoprion derives from the Ancient Greek (hélikos, "spiral") and (príōn, "saw"), in reference to the characteristic morphology of the tooth whorls, while the specific epithet bessonowi honors their discoverer. In his 1899 observations, Karpinsky also noted that the specimen described by Woodward in 1886 showed sufficient similarities with the Krasnoufimsk material to justify its provisional reassignment to Helicoprion, renaming it H. davisii. A few years later, in 1909, his colleague Oliver Perry Hay transferred the species once again, this time to Toxoprion, another newly established, related genus. In later publications, Karpinsky reaffirmed the position he had argued in 1899, an interpretation followed by several authors in the subsequent decades. His interpretation was finally confirmed in 1940, when German–American paleontologist Curt Teichert described much more complete fossils of H. davisii, which like the holotype, were also discovered in Western Australia.
Since the genus Helicoprion was established by Karpinsky in 1899, numerous additional species have been described from fossils found across the world, although most originate from North America. In a morphometric revision published in 2013, American paleontologists Leif Tapanila and Jesse Pruitt reassessed all of these historical species assignments. Their analysis showed that, among the roughly 10 named species, only three possess features distinctive enough to be considered valid: H. bessonowi, H. davisii, and H. ergassaminon, with the remaining taxa regarded as either synonymous or doubtful. H. ergassaminon was first described in 1966 by Danish ichthyologist Svend Erik Bendix-Almgreen in a monograph devoted to several Helicoprion fossils housed in the paleontological collections of various universities in Idaho, USA. As with the generic name, the specific epithet derives from the Ancient Greek (ergasamenon