Parahelicoprion

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 'coiled saw' or 'spiral saw', 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

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