through skeletal elements of unarmoured bony fish and other chondrichthyans. Classification Skull data from IMNH 37899 reveals several characteristics, such as an autodiastylic jaw suspension without an integrated hyomandibula, which confirm the placement of Helicoprion within the chondrichthyan subgroup Euchondrocephali. In contrast to their sister group Elasmobranchii (containing true sharks, rays, and kin), euchondrocephalans are primarily an extinct group. Living members of Euchondrocephali are solely represented by the order Chimaeriformes in the subclass Holocephali. Chimaeriforms, commonly known as chimaeras or ratfish, are a small and specialized group of rare deep-sea cartilaginous fish. The relationship between Helicoprion and living chimaeras is very distant, but had been previously suspected based on details of its tooth anatomy. More specifically, Helicoprion can be characterized as a member of Eugeneodontida, an order of shark-like euchondrocephalans which lived from the Devonian to Triassic periods. Eugeneodonts have simple, autodiastylic skulls with reduced marginal dentition and enlarged whorls of blade-like symphysial teeth on the midline of the jaw. Within Eugeneodontida, Helicoprion is placed within Edestoidea, a group of eugeneodonts with particularly tall and angled symphysial teeth. Members of the Edestoidea are divided into two families based on the style of the dentition. One family, the Edestidae, has relatively short tooth blades with roots which incline backwards. The other family, which contains Helicoprion, is sometimes called Agassizodontidae, based on the genus Agassizodus. However, other authors prefer the family name Helicoprionidae, which was first utilized 70 years prior to Agassizodontidae. Helicoprionids (or agassizodontids) have large, cartilage-supported whorls with strongly arched shapes. Helicoprionids do not shed their teeth; instead, their tooth whorls continually add new teeth with bases inclined forwards at the top of the whorl. As most eugeneodonts are based on fragmentary tooth remains, concrete phylogenetic relationships within the group remain unclear. History and species Three species of Helicoprion are currently considered valid via morphometric analyses, differing in the proportions of the upper, middle and lower sections of the tooth crown. These differences are only apparent in adult individuals past the 85th tooth of the spiral. H. davisii and synonyms The first specimen of Helicoprion to be described was WAMAG 9080, a 15-tooth fragment of a tooth whorl found along a tributary of the Gascoyne River in Western Australia. Henry Woodward described the fossil in 1886 and named it as the species Edestus davisii, commemorating the man who discovered it. Upon naming H. bessonowi in 1899, Alexander Karpinsky reassigned E.