Parahelicoprion is an extinct genus of cartilaginous fish that lived during the Early Permian. The genus contains two species: P. clerci from the Arta Beds of the Ural Mountains of Russia, and P. mariosuarezi from the Copacabana Formation of Bolivia. Members of the genus possessed a row of large tooth crowns on the midline of the lower jaw, known as a tooth whorl. The characteristics of this whorl are unique to fishes of the order Eugeneodontiformes, and more specifically the family Helicoprionidae to which Parahelicoprion belongs. The genus name refers to Helicoprion, another eugeneodont from the Ural Mountains that bore a similar tooth arrangement.
The holotype of Parahelicoprion clerci, which consists only of tooth fragments, was badly damaged by mining and is broken into several pieces. That of the Bolivian P. mariosuarezi consists of nine partial teeth, the outer edges of which are broken off. Estimates of the extent of the complete whorl, body size, and ecology of Parahelicoprion are speculative as a result of its incomplete fossils, although it is assumed to have been very large, predatory, and potentially pelagic. When first described, P. clerci was considered a species of Helicoprion, although its initial describer, Alexander Karpinsky, later separated it into its own genus. It has since been suggested that this genus may indeed represent a junior synonym of Helicoprion or a paraphyletic, non-diagnostic taxon.
Discovery and naming
The type specimen of Parahelicoprion clerci was found in the Ural Mountains region of Russia, in strata dated to the Artinskian stage of the Cisuralian (early Permian) epoch. The fossil was embedded in a matrix of very fine-grain marlstone, and although it is well-preserved it is broken into multiple fragments. The first five portions of the specimen to be found, which consist of several teeth, were discovered incidentally by a miner and were badly damaged as a result poor handling during collection. Additional fragments of the holotype were identified in the years following the taxon's description, all in similar condition and found within one square meter of the first find. These remains include a single partial tooth described in 1925 and a collection of three partial teeth purchased by Georgii Fredriks and realized to correspond with the same fossil in 1926. Several fragments of the tooth whorl were sold to private collectors and were never scientifically examined. Pieces of a large dorsal fin spine found at the same site have also been suggested to have come from the same individual as the teeth. The rocks which produced the holotype specimen have, since its discovery, been defined as part of the Divya Formation (alternatively spelled the Divjinskian Formation). The exposure of the Divya Formation where the type specimen was found is near the town of Krasnoufimsk, and as of 1976 a cast of the P. clerci holotype was housed at the Krasnoufimsk Museum. The species is known only from the damaged holotype.
The type specimen of Parahelicoprion was initially named Helicoprion clerci by the Russian geologist Alexander Karpinsky in 1916. Karpinsky reassigned Helicoprion clerci to its own genus in a work published in 1924, although he had informally referred to it as Parahelicoprion in a series of publications two years prior and had suggested it may warrant its own genus when he first named the species. The given genus name derives from that of the related Helicoprion, meaning or , although due to the assumed shape of P. clerci's tooth whorl Karpinsky considered this name technically unfitting. The species name, P. clerci, honors Onésime Clerc, who at the time of its description was the president of the Ural Society of Natural Science Lovers. Clerc was responsible for bringing the holotype specimen of P. clerci to Karpinsky's attention.
A second species, Parahelicoprion mariosuarezi, was described and tentatively assigned to the genus in 1986 by Dagmar Merino-Roda and Philippe Janvier. This species, which is also based only on its type specimen (designated no. 6097, YPFB), was discovered in the Copacabana Formation of Yaurichambi, Bolivia, and was dated to the Asselian stage of the early Permian. The holotype, a partial set of teeth, was found preserved in a layer of calcarious red sandstone. The species is named in honor of Dr. Mario Suarez-Riglos, and the type specimen is currently housed in the collection of the Noel Kempff Mercado Natural History Museum.
Description
Both Parahelicoprion species are very incompletely known, and the only material which has been assigned confidently to the genus consists of fragments of a row of teeth termed a tooth whorl. While some authors have suggested the genus lacks defining, autapomorphous features, the tooth crowns of both species of Parahelicoprion are noted to share extremely long, backwards-sweeping, recessed lower sections (referred to as "wings" or "ribs" by researchers) which extend nearly to the base of the tooth root, as well as curved denticles or serrations along the edges of these sections. In addition to tooth whorls, it has been suggested that large, forward-arching fin spines under the form-genus Physonemus (also classified as Xystracanthus) may have belonged to Parahelicoprion and related eugeneodonts, with a fossil of Xystracanthus grandis or its potential synonym Physonemus mirabilis suggested to correspond to Parahelicoprion clerci. In a 2018 publication, researcher Serge Naugholnykh asserted personal belief that these spines correspond to edestoids, although sickle-like spine fossils are believed by other researchers to represent the copulatory organs of symmoriiform fishes or, alternatively, of holocephalans. Members of the order Eugeneodontiformes, which includes Parahelicoprion, are typically characterized as having lacked fin spines.In life, the tooth whorl of Parahelicoprion was positioned symphyseally (along the midline) in the lower jaw. In his 1916 description of P. clerci (as Helicoprion clerci), Karpinsky assumed the whorl of the species formed a very large spiral, and Naugholnykh has subsequently agreed that the shape of the complete whorl would have been indistinguishable from that of Helicoprion. In a 1924 publication, however, Karpinsky calculated that the complete whorl of P. clerci likely consisted of no more than 20 tooth crowns and did not form a spiral, although the precise number of tooth crowns is unknown. The description of P. mariosuarezi suggests that it likely possessed a very short tooth whorl situated at the tip of a greatly elongated pair of jaws, based on both the partial skull of Sarcoprion edax and the well-preserved fossils of the related caseodonts. Various other authors have agreed with a Sarcoprion-like interpretation, but have acknowledged that the shape of the complete whorl is not known and that the material is too fragmentary for such comparisons to be confidently made. The postcranial anatomy of eugeneodonts has been suggested to vary little between genera, indicating that, like their smaller, better known relatives, both species of Parahelicoprion possessed long, fusiform bodies with crescent-shaped caudal fins, and that they lacked pelvic and anal fins.
Parahelicoprion clerci
The known remains of Parahelicoprion clerci consist of badly damaged tooth fragments, all belonging to a single tooth whorl. During preparation by Karpinsky in 1916, three of these fragments were glued together in order to reflect their in-life articulation, and by 1926 these had again been assembled with additional associated material. The holotype specimen as recognized today spans 25 cm (10 in) in profile view and consists of six articulated tooth crowns, as well as an associated smaller tooth fragment. The surface texture of the teeth has been badly damaged by mining, and Karpinsky considered it likely that parts of the fossil had been destroyed or never collected. No other teeth are confidently associated with this species, although crushing teeth similar to those of Campodus or Orodus may have been present.
The teeth increase significantly in size towards the rear of the whorl, with the largest tooth crown tip (not including the serrated "rib") of the holotype measuring in height, across, and wide at the widest point, while the smallest known tip measures in height, across and in width. The morphology of the teeth also change depending on their position, with the teeth farthest forward being rounder and more asymmetrical than those at the back, which are nearly completely symmetrical in profile. The rear edges of the frontmost crown's tips thin to form sharp cutting blades, which are not serrated. In the rear crown tips, both the front and rear edges of the tooth are blade-like. The upper tips of all the known tooth crowns are uniquely rounded and blunt, rather than tapering to a point like those of many other related edestoids. As in other members of its family, the teeth of the whorl angle forward in the mouth. The "ribs" or "wings" of the teeth curve towards the back of the jaws and are extremely elongated. They are lined with very deep, downward-angled serrations along their forward edges and crenulations or corrugations along the sloping rear edges, both of which are absent from the tips of the crowns. The form of these serrations and crenulations is unique to this species. The base (or root) of Parahelicoprion's whorl is described as entirely fused, with no gaps or separations between segments forming separate teeth.
The histology of the holotype teeth is suggested to have originally been well-preserved, although on the exterior of the teeth the damage is severe enough to prevent study. The outer coating of the teeth, which is described by Karpinsky as being enamel, has been abraded away across most of the tooth fragments. This substance has alternatively been identified as enameloid (also called vitrodentine) in other helicoprionids and other fishes, rather than true enamel. The interior of the teeth remains well-preserved, and was composed of a spongy form of dentin identified by Karpinsky as vasodentin (a form common among fishes). Unlike other eugeneodonts, trabecular dentin (a porous form found in cartilaginous fish) was likely absent from the teeth. The interior structure of the known specimen could not be studied in detail, however, as Karpinsky did not want to damage the specimen through cross-sectioning.
Parahelicoprion mariosuarezi
The holotype whorl of Parahelicoprion mariosuarezi is larger than that of P. clerci. Merino-Rodo and Janvier (1986) state that it is likely nearly complete, with the exception of the tips of the teeth along the upper part of the whorl being broken off. Unlike the type species, serrations and crenulations are absent along most of the crown, and instead several large, pointed denticles protrude along the "wings" of the third to ninth tooth crowns. This species also preserves a single denticle-bearing parasymphyseal tooth (a paired tooth near the midline of the mouth) along the edge of the whorl, a tooth type which is not known from P. clerci. The holotype of P. mariosuarezi shows the smallest crown was positioned at the front of the preserved portion of the whorl and was significantly smaller than the next crown in the sequence. The whorl possesses only nine tooth crowns, far fewer than those of related genera such as Helicoprion. Despite being unpreserved, the outer cutting edges of the teeth in P. mariosuarezi are thought to have been similar to P. clerci, due to the preserved portions of the crowns being similar in shape and thickness. The rear portion of the whorl is described as being fused.
Estimated length