Product Code Database
Helicoprion (meaning "spiral saw") is an Extinction genus of shark-like Chondrichthyes in the order Eugeneodontiformes. Almost all Helicoprion consist of spirally-arranged clusters of fused teeth, called "tooth whorls", which in life were embedded in the Mandible. With the exception of the Palatoquadrate and lower jaws, the Cartilage skeleton of Helicoprion is unknown. The closest living relatives of Helicoprion (and other eugeneodonts) are the , though their relationship is very distant. The unusual tooth arrangement is thought to have been an adaption for feeding on soft-bodied prey, and may have functioned as a deshelling mechanism for hard-bodied such as and Ammonoidea. In 2013, study of the genus Helicoprion via morphometric analysis of the tooth whorls found that the genus contained only the species H. davisii, H. bessonowi and H. ergassaminon.
Fossils of Helicoprion have been found worldwide, with the genus being known from Russia, Western Australia, China, Kazakhstan, Japan, Laos, Norway, Canada, Mexico, and the United States (Idaho, Nevada, Wyoming, Texas, Utah, and California). These fossils are known from a 20 million-year timespan during the Permian, period from the Artinskian stage of the Cisuralian (Early Permian) to the Roadian stage of the Guadalupian (Middle Permian). More than 50% of the fossils referred to Helicoprion are H. davisii specimens from the Phosphoria Formation of Idaho. An additional 25% of fossils are found in the Ural Mountains of Russia, belonging to H. bessonowi.
The caseodontids have a fusiform (streamlined, torpedo-shaped) body plan, with triangular Fish fin. They have a single large and triangular dorsal fin without a fin spine, and a tall, forked Fish fin, which externally appears to be homocercal (with two equally sized lobes). This general body plan is shared by active, open-water predatory fish such as tuna, swordfish, and Lamnidae sharks. Eugeneodonts also lack Pelvic fin and Anal fin fins, and the genus Romerodus had broad keels along the side of the body up to the caudal fin. Fadenia and the smaller Ornithoprion had at least five well-developed gill slits, possibly with a vestigial sixth gill. No evidence has been found of the specialized gill basket and fleshy operculum present in living Chimaera. Based on the proportional size of caseodontoid tooth whorls, researcher Oleg Lebedev suggested that Helicoprion individuals with tooth whorls in diameter could reach in total length, rivaling the size of modern . The largest known Helicoprion tooth-whorl, specimen IMNH 49382 representing an unknown species, reached in diameter and in crown height, and would have belonged to an individual or more in length.
Helicoprion had an autodiastylic jaw suspension, meaning that the inner edge of the palatoquadrate was firmly attached (but not fused) to the chondrocranium at two separate points. These two attachment points are the dome-shaped ethmoid process at the front of the palatoquadrate, and the flange-like basal process at its upper rear corner. Autodiastylic jaws are common in early holocephalans, though in modern animals they can only be found in embryonic Chimaera. Another well-preserved specimen, USNM 22577+494391 (nicknamed the "Sweetwood specimen"), has demonstrated that the inner surface of the palatoquadrate was covered with numerous small (~2 mm wide) teeth. The palatoquadrate teeth were low and rounded, forming a "pavement" that scraped against the tooth whorl. When seen from behind, the palatoquadrate forms a paired jaw joint with the Meckel's cartilage. No evidence is seen for an Joint between the palatoquadrate and the hyomandibula.
Meckel's cartilage has an additional projection right before the joint with the palatoquadrate. This extra process, unique to Helicoprion, likely served to limit jaw closure to prevent the whorl from puncturing the chondrocranium. Another unique characteristic of Helicoprion is that the preserved labial cartilage forms a synchondrosis (fused joint) with the upper surface of Meckel's cartilage. This joint is facilitated by a long facet on the upper edge of Meckel's cartilage. The labial cartilage provides lateral support for the tooth whorl, widening near the root of each revolution. By wedging into the palatoquadrate while the mouth is closed, the upper edge of the labial cartilage helps to spread out the forces used to limit the extent of the jaw closure. The rear portion of the labial cartilage has a cup-like form, protecting the developing root of the last and youngest revolution.
Ramsay and colleagues further suggested that the whorl could have served as an effective mechanism for deshelling hard-shelled such as Ammonoidea and , which were abundant in Early Permian oceans. If a hard-shelled cephalopod was bitten head-on, the whorl could have served to pull the soft body out of the shell and into the mouth. During jaw closure, the palatoquadrates and tooth whorl combined to form a three-point system, equivalent to the set-up of an inverted three-point flexural test. This system was effective at trapping and holding soft parts to increase cutting efficiency and provide leverage against hard-shelled prey. At the three points of contact, the estimated bite force ranges between , with estimated bite stresses ranging from during initial prey contact. This large bite force may have allowed Helicoprion to expand its diet to , as its jaw apparatus was more than capable of cutting through skeletal elements of unarmoured Osteichthyes and other chondrichthyans.
More specifically, Helicoprion can be characterized as a member of Eugeneodontida, an order of shark-like euchondrocephalans that 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 the Eugeneodontida, Helicoprion is placed within the Eugeneodontida, 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 that incline backwards.
The other family, which contains Helicoprion, is sometimes called Agassizodontidae, based on the genus Agassizodus. Other authors, though, prefer the family name Helicoprionidae, which was first used 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.
In 1907 and 1909, Oliver Perry Hay described a new genus and species of eugeneodont, Lissoprion ferrieri, from numerous fossils found in phosphate-rich Phosphoria Formation on the border between Idaho and Wyoming. He also synonymized H. davisii with his new genus and species. However, Karpinsky separated the two species once more and transferred them to Helicoprion in 1911. H. ferrieri was initially differentiated using the metrics of tooth angle and height, but Tapanila and Pruitt (2013) considered these characteristics to be intraspecifically variable. As a result, they reassigned H. ferrieri as a junior synonym of H. davisii. Outside the Phosphoria Formation, H. davisii specimens have also been found in Mexico, Texas, and Canada (Nunavut and Alberta). H. davisii is characterized by its tall and widely spaced tooth whorl, with these becoming more pronounced with age. The teeth also noticeably curve forward. of Texas]]In a 1939 publication, Harry E. Wheeler described two new species of Helicoprion from California and Nevada . One of these, H, sierrensis, was described from a specimen (UNMMPC 1002) found in glacial moraine deposits in Eastern California, likely originating from the Goodhue Formation. Tapanila and Pruitt determined that the distinguishing shaft range of H. sierrensis was well within the variation found in H. davisii.
H. jingmenense was described in 2007 from a nearly complete tooth whorl (YIGM V 25147) with more than four volutions across a part and counterpart slab. It was discovered during the construction of a road passing through the Lower Permian Qixia Formation of Hubei Province, China. The specimen is very similar to H. ferrieri and H. bessonowi, though it differs from the former by having teeth with a wider cutting blade, and a shorter compound root, and differs from the latter by having fewer than 39 teeth per volution. Tapanila and Pruitt argued that the specimen was partially obscured by the surrounding matrix, resulting in an underestimation of tooth height. Taking into account intraspecific variation, they synonymized it with H. davisii.
Based on isolated teeth and partial whorls found on the island of Spitsbergen, Norway, H. svalis was described by Stanisław Siedlecki in 1970. The type specimen, a very large whorl with specimen number PMO A-33961, was noted for its narrow teeth that apparently are not in contact with each other, but this seems to be a consequence of only the central part of the teeth being preserved, according to Tapanila and Pruitt. Since the whorl shaft is partially obscured, H. svalis cannot be definitely assigned to H. bessonowi, but it closely approaches the latter species in many aspects of its proportions. With a maximum volution height of , H. svalis is similar in size to the largest H. bessonowi, which has a maximum volution height of .
In 1999, the holotype of H. bessonowi was stolen, but afterwards was shortly recovered with the aid of an anonymous fossil trade.LONG J. 2002. The Dinosaur Dealers. Mission: to uncover international fossil smuggling. Allen & Unwin, Melbourne, p. 53–57
In 1922, Karpinsky named a new species of Helicoprion, H. ivanovi, from Gzhelian (latest Carboniferous) strata near Moscow. However, this species has subsequently been removed from Helicoprion and placed as a second species of the related eugeneodont Campyloprion. In 1924, Karpinsky separated H. clerci from Helicoprion and reclassified it under the new genus name Parahelicoprion, but Parahelicoprion has recently been suggested to represent a junior synonym of this genus.
Joseph Leidy, who originally described Edestus vorax, argued that they represented the jaws of "plagiostomous" (chondrichthyan) fish. William Davies agreed, specifically comparing it to the jaws of Janassa, a Permian petalodont. On the other hand, J.S. Newberry suggested that the jaw-like fossils were defensive spines of a stingray-like fish. Woodward eventually settled on E.D. Cope's argument that they represented pectoral fin spines from fish similar to Pelecopterus (now known as Protosphyraena; then associated with Ptychodus).
Karpinsky's 1899 monograph on Helicoprion noted that the bizarre nature of the tooth whorl made reaching precise conclusions on its function difficult. He tentatively suggested that it curled up from the upper jaw for defensive or offensive purposes. This was justified by comparison to the upper tooth blades of Edestus, which by 1899 had been re-evaluated as structures belonging to the jaw.
Debates over the identity of Helicoprion
The other publication was Bendix-Almgreen's monograph on Helicoprion. His investigations reinterpreted the tooth whorl as a symphyseal structure wedged between the meckelian cartilages, which were separated by a gap at the front. A pair of cartilage loops, the symphyseal crista, seems to develop as a paired extension of the jaw symphysis where the meckelian cartilages meet at the back of the jaw. Each loop arches up before curling back inwards, tracing over the root of the tooth whorl. The largest and youngest teeth form at the symphysis near the back of the jaw. Over time, they are carried along the symphyseal crista, spiraling forwards, then downwards and inwards. The series of teeth accumulates into a spiraling structure, which is housed within the cavity defined by the symphyseal crista. The lateral and lower edges of the tooth whorl would have been obscured by skin during life. According to Bendix-Almgreen, the most likely use of the tooth whorl was as a tool for tearing and cutting prey against the upper jaw.
In the 1994 book Planet Ocean: A Story of Life, the Sea, and Dancing to the Fossil Record, author Brad Matsen and artist Ray Troll describe and depict a reconstruction based on the information gleaned by Bendix-Almgreen (1966). They proposed that no teeth were present in the animal's upper jaw, besides crushing teeth for the whorl to cut against. The two envisioned the living animal to have a long and very narrow skull, creating a long nose akin to the modern-day goblin shark. (1994). 9780898157789, Ten Speed Press. ISBN 9780898157789 A 1996 textbook by Philippe Janvier presented a similar reconstruction, albeit with sharp teeth at the front of the upper jaw and rows of low crushing teeth in the back of the jaw.
In 2008, Mary Parrish created a new reconstruction for the renovated Ocean Hall at the Smithsonian Museum of Natural History. Designed under the direction of Robert Purdy, Victor Springer, and Matt Carrano, Parrish's reconstruction places the whorl deeper within the throat. This hypothesis was justified by the argument that the teeth supposedly had no wear marks, and the assumption that the whorl would have created a drag-inducing bulge on the chin of the animal if located in a symphysial position. They envisioned the tooth whorl as a structure derived from throat denticles and designed to assist swallowing. This would hypothetically negate the disadvantages the tooth whorl would produce if positioned further forward in the jaw. This reconstruction was criticized for the overly intricate and potentially ineffective design of such a structure, if solely used to assist swallowing.
Lebedev (2009) found more support for a reconstruction similar to those of Bendix-Almgreen (1966) and Troll (1994). A tooth whorl found in Kazakhstan preserved radial scratch marks; the whorl was also found near several wide, tuberculated teeth similar to those of the putative caseodontoid Campodus. Lebedev's reconstruction presented a cartilage-protected tooth whorl in a symphysial position at the front of the long lower jaw. When the mouth was closed, the tooth whorl would fit into a deep longitudinal pocket on the upper jaw. Both the pocket in the upper jaw and the edges of the lower jaw would have been lined with dense rows of Campodus-like teeth. This was similar to the situation reported in related helicoprionids such as Sarcoprion and Agassizodus. As for Helicoprion
|
|
