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Extinct genus of shark

Cretoxyrhina (/k//r//ɪ/,/t//ɒ//k//s//i/'/r//h//aɪ//n//ə/; meaning 'Cretaceous sharp nose') is an extinct genus of large mackerel shark that lived about 107 to 73 million years ago during the late Albian to late Campanian of the Late Cretaceous. The type species, C. mantelli, is more commonly referred to as the Ginsu shark, first popularized in reference to the Ginsu knife, as its theoretical feeding mechanism is often compared with the "slicing and dicing" when one uses the knife. Cretoxyrhina is traditionally classified as the likely sole member of the family Cretoxyrhinidae but other taxonomic placements have been proposed, such as within the Alopiidae and Lamnidae. Measuring up to in length and weighing over , Cretoxyrhina was one of the largest sharks of its time. Having a similar appearance and build to the modern great white shark, it was an apex predator in its ecosystem and preyed on a large variety of marine animals including mosasaurs, plesiosaurs, sharks and other large fish, pterosaurs, and occasionally dinosaurs. Its teeth, up to long, were razor like and had thick enamel built for stabbing and slicing prey. Cretoxyrhina was also among the fastest swimming sharks, with hydrodynamic calculations suggesting burst speeds of up to It has been speculated that Cretoxyrhina hunted by lunging at its prey at high speeds to inflict powerful blows, similar to the great white shark today, and relied on strong eyesight to do so. Since the late 19th century, several fossils of exceptionally well preserved skeletons of Cretoxyrhina have been discovered in Kansas. Studies have successfully calculated its life history using vertebrae from some of the skeletons. Cretoxyrhina grew rapidly during early ages and reached sexual maturity at around four to five years of age. Its lifespan has been calculated to extend to nearly forty years. Anatomical analysis of the Cretoxyrhina skeletons revealed that the shark possessed facial and optical features most similar to that in thresher sharks and crocodile sharks and had a hydrodynamic build that suggested the use of regional endothermy. As an apex predator, Cretoxyrhina played a critical role in the marine ecosystems it inhabited. It was a cosmopolitan genus and its fossils have been found worldwide, although most frequently in the Western Interior Seaway area of North America. It preferred mainly subtropical to temperate pelagic environments but was known in waters as cold as 5 °C (41 °F). Cretoxyrhina saw its peak in size by the Coniacian, but subsequently experienced a continuous decline until its extinction during the Campanian. One factor in this demise may have been increasing pressure from competition with predators that arose around the same time, most notably the giant mosasaur Tylosaurus. Other possible factors include the gradual disappearance of the Western Interior Seaway.

Cretoxyrhina was first described by the English paleontologist Gideon Mantell from eight C. mantelli teeth he collected from the Southerham Grey Pit near Lewes, East Sussex. In 1843, Swiss naturalist Louis Agassiz published the third volume of his book Recherches sur les poissons fossiles, where he reexamined Mantell's eight teeth. Using them and another tooth from the collection of the Strasbourg Museum (whose exact location was unspecified but also came from England), he concluded that the fossils actually pertained to a single species of extinct shark that held strong dental similarities with the three species then classified in the now invalid genus Oxyrhina, O. hastalis, O. xiphodon, and O. desorii. Agassiz placed the species in the genus Oxyrhina but noted that the much thicker root of its teeth made enough of a difference to be a distinct species and scientifically classified the shark under the taxon Oxyrhina mantellii  and named in honor of Mantell. Most of these species were derived from teeth that represented variations of C. mantelli but deviated from the exact characteristics of the syntypes. In 1873, American paleontologist Joseph Leidy identified teeth from Kansas and Mississippi and described them under the species name Oxyrhina extenta. These teeth were broader and more robust than the O. mantelli syntypes from England. This all changed with the discoveries of some exceptionally well preserved skeletons of the shark in the Niobrara Formation in West Kansas. Charles H. Sternberg discovered the first skeleton in 1890, which he described in a 1907 paper: and 1965 respectively. The former is a partial skeleton consisting of a well preserved set of jaws, a pair of five gills, and some vertebra while the latter is a near complete skeleton with an almost complete vertebral column and an exceptionally preserved skull holding much of the cranial elements, jaws, teeth, a set of scales, and fragments of pectoral girdles and fins in their natural positions. Both skeletons are currently housed in the Sternberg Museum of Natural History. In 1968, a collector named Tim Basgall discovered another notable skeleton that, similar to FHSM VP 2187, also consisted of a near complete vertebral column and a partially preserved skull. This fossil is housed in the University of Kansas Museum of Natural History as KUVP 69102. In 1958, Soviet paleontologist Leonid Glickman found that the dental design of O. mantelli reconstructed by Eastman made it distinct enough to warrant a new genus—Cretoxyrhina. He also identified a second species of Cretoxyrhina based on some of the earlier Cretoxyrhina teeth, which he named Cretoxyrhina denticulata. Originally, Glickman designated C. mantelli as the type species, but he abruptly replaced the position with another taxon identified as 'Isurus denticulatus' without explanation in a 1964 paper, a move now rejected as an invalid taxonomic amendment. A 2013 study led by Western Australian Museum curator and paleontologist Mikael Siverson corrected the taxonomic error, reinstating the genus Cretoxyrhina and moving 'P'. vraconensis into it. This species was reaffirmed into the genus Cretoxyrhina by a 2013 study led by American paleontologist Michael Newbrey using additional fossil material of the same species found in Western Australia. efn|For some unclear reason, this study was first published online on BioOne in 2013, before being officially republished two years later in the Polish scientific journal Acta Palaeontologica Polonica. morphological, physiological, and paleoecological reconstructions, ontogenetic studies, and morphological variable based phylogenetic studies on Cretoxyrhina. Shimada's research on Cretoxyrhina helped shed new light on the understandings of the shark and, through his new methods, other extinct animals. The common name Ginsu shark, originally coined in 1999 by paleontologists Mike Everhart and Kenshu Shimada, is a reference to the Ginsu knife, as the theoretical feeding mechanisms of C. mantelli was often compared with the "slicing and dicing" when one uses the knife.

Cretoxyrhina bore a resemblance to the modern great white shark in size, shape and ecology, but the two sharks are not closely related, and their similarities are a result of convergent evolution. Siverson (1999) remarked that Cretoxyrhinidae was used as a 'wastebasket taxon' for Cretaceous and Paleogene sharks and declared that Cretoxyrhina was the only valid member of the family. Cretoxyrhina contains four valid species: C. vraconensis, C. denticulata, C. agassizensis, and C. mantelli. These species represent a chronospecies. The Late Albian–Mid Turonian interval sees mainly the reduction of lateral cusplets; C. vraconensis possessed lateral cusplets in all teeth except for a few in the anterior position, which would gradually become restricted only to the back lateroposteriors in adults by the end of the interval in C. mantelli. Tooth size of C. mantelli individuals inside the Western Interior Seaway peaked around 86 Ma during the latest Coniacian and then begins to slowly decline. a mechanism often informally described as "slicing and dicing" by paleontologists.

Analysis of skull and scale patterns suggests that C. mantelli had a conical head with a dorsally flat and wide skull. The rostrum does not extend much forward from the frontal margin of the braincase, suggesting that the snout was blunt. The jaws of C. mantelli were kinetically powerful. They have a slightly looser anterior curvature and a more robust build than that of the modern mako sharks, but otherwise were similar in general shape. The hyomandibula is elongated and was believed to swing laterally, which would allow jaw protrusion and deep biting. this was due to the formation's chalk having high contents of calcium, allowing calcification to become more prevalent. When calcified, soft tissue hardens, making it more prone to fossilization. These properties of the tail, along with other features such as smooth scales parallel to the body axis, a plesodic pectoral fin (a pectoral fin in which cartilage extends throughout, giving it a more secure structure that helps decrease drag), and a spindle shaped stocky build, show that C. mantelli was capable of fast swimming. Possessing regional endothermy (also known as mesothermy), it may have possessed a build similar to modern regionally endothermic sharks like members of the thresher shark and lamnid families, and thus efficiently function in the colder environments where Cretoxyrhina has been found. Fossils have been found in areas where paleoclimatic estimates show a surface temperature as low as 5 °C (41 °F). and the shortfin mako, the fastest extant shark, has been modeled at a speed of .

Although no fossil evidence for it has been found, it is inferred that Cretoxyrhina was ovoviviparous as all modern mackerel sharks are. In ovoviviparous sharks, young are hatched and grow inside the mother while competing against litter mates through cannibalism such as oophagy (egg eating). As Cretoxyrhina inhabited oligotrophic and pelagic waters where predators such as large mosasaurs like Tylosaurus and macropredatory fish like Xiphactinus were common, it is likely that it also was an r selected shark, where many infants are produced to offset high infant mortality rates. Similarly, pelagic sharks such as the great white shark, thresher sharks, mako sharks, porbeagle shark, and crocodile shark produce two to nine infants per litter. This was not mentioned in Newbrey et al. (2013). The graph below represents the length growth per year of FHSM VP 2187 according to Shimada (2008):efn|The chart is based only on vertebrae from FHSM VP 2187. It does not represent the genus Cretoxyrhina as a whole. The highest trophic level it occupied was a position shared only with large mosasaurs such as Tylosaurus during the latter stages of the Late Cretaceous. It played a critical role in many marine ecosystems. turtles, mosasaurs,

Although Cretoxyrhina was mainly an active hunter, it was also an opportunistic feeder and may have scavenged from time to time. Many fossils with Cretoxyrhina feeding marks show no sign of healing, an indicator of a deliberate predatory attack on a live animal, leading to the possibility that at least some of the feeding marks were made from scavenging.

The hunting strategies of Cretoxyrhina are not well documented because many fossils with Cretoxyrhina feeding marks cannot be distinguished between predation or scavenging. If they were indeed a result of the former, that would mean that Cretoxyrhina most likely employed hunting strategies involving a main powerful and fatal blow similar to ram feeding seen in modern requiem sharks and lamnids. A 2004 study by shark experts Vittorio Gabriotti and Alessandro De Maddalena observed that the modern great white shark reaching lengths of greater than commonly ram its prey with massive velocity and strength to inflict single fatal blows, sometimes so powerful that prey would be propelled out of the water by the impact's force. As Cretoxyrhina possessed a robust stocky build capable of fast swimming, powerful kinetic jaws like the great white shark, and reaches lengths similar to or greater than it, a hunting style like this would be likely. Israel, and Kazakhstan. In 2013, Mikael Siverson and colleagues noted that during the Turonian or early Coniacian, Cretoxyrhina individuals living offshore were usually larger than those inhabiting the Western Interior Seaway, with some of the offshore C. mantelli fossils like one of the syntypes yielding total lengths of up to , possibly The climate of marine ecosystems during the temporal range of Cretoxyrhina was generally much warmer than modern day due to higher atmospheric levels of carbon dioxide and other greenhouse gases influenced by the shape of the continents at the time. The interval during the Cenomanian and Turonian of 97–91 Ma saw a peak in sea surface temperatures averaging over 35 °C (95 °F) and bottom water temperatures around 20 °C (68 °F), about warmer than modern day. This interval also included a rise in global δ13C levels, which marks significant depletion of oxygen in the ocean, and caused the Cenomanian Turonian anoxic event. vertebrates like Cretoxyrhina were generally unaffected. The rest of the Cretaceous saw a progressive global cooling of Earth's oceans, leading to the appearance of temperate ecosystems and possible glaciation by the Early Maastrichtian.

Cretoxyrhina lived alongside many predators that shared a similar trophic level in a diverse pelagic ecosystem during the Cretaceous. During the Albian to Turonian, about 107 to 91 Ma, Cretoxyrhina was contemporaneous and coexisted with Mid Cretaceous pliosaurs. Some of these pliosaurs included Megacephalosaurus, which attained lengths of By the Mid Turonian, about 91 Ma, pliosaurs became extinct. It is thought that the radiation of sharks like Cretoxyrhina may have been a major contributing factor to the acceleration of their extinction. The ecological void they left was quickly filled by emerging mosasaurs, which also came to occupy the highest trophic levels. Large mosasaurs like Tylosaurus, which reached in excess of in length,

The causes of the extinction of Cretoxyrhina are uncertain. What is known is that it declined slowly over millions of years. Since its peak in size during the Coniacian, the size and distribution of Cretoxyrhina fossils gradually declined until its eventual demise during the Campanian. Siverson and Lindgren (2005) noted that the age of the youngest fossils of Cretoxyrhina differed between continents. In Australia, the youngest Cretoxyrhina fossils were dated approximately 83 Ma during the Santonian, while the youngest North American fossils known at the time (which were dated in the Early Campanian) were at least two million years older than the youngest fossils in Europe. The differences between ages suggests that Cretoxyrhina may have become locally extinct in such areas over time until the genus as a whole went extinct. It has been noted that the decline of Cretoxyrhina coincides with the rise of newer predators such as Tylosaurus, suggesting that increasing pressure from competition with the mosasaur and other predators of similar trophic levels may have played a major contribution to Cretoxyrhinas decline and eventual extinction. Another possible factor was the gradual shallowing and shrinking of the Western Interior Seaway, which would have led to the disappearance of the pelagic environments preferred by the shark; this factor does not explain the decline and extinction of Cretoxyrhina elsewhere. It has been suggested that the extinction of Cretoxyrhina may have helped the further increase the diversity of mosasaurs.