Cretoxyrhina - WikiHQ

Cretoxyrhina (; 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 around , 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 may have occurred in waters as cold as . 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.

Taxonomy

Research history

thumb|left|upright=1.25|alt=Illustration of numerous English fossil shark teeth|[[Syntypes of C. mantelli (Fig. 1-9) and other species in the third volume of Recherches sur les poissons fossiles]]

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 the third volume of his book Recherches sur les poissons fossiles, published in 1843, Swiss naturalist Louis Agassiz 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. 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:

thumb|upright=1.25|alt=Fossil hunter George F. Sternberg preparing a fossil skull of the dinosaur Chasmosaurus|[[George F. Sternberg found many exceptionally well-preserved Cretoxyrhina skeletons including KUVP 247 and FHSM VP-2187.]]

Charles R. Eastman published his analysis of the skeleton in 1894. In the paper, he reconstructed the dentition based on the skeleton's disarticulated tooth set. Using the reconstruction, Eastman identified the many extinct shark species and found that their fossils are actually different tooth types of O. mantelli, which he all moved into the species. 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.

thumb|left|upright=1.5|alt=Interpretive drawings of two C. mantelli shark fossils|Exceptionally preserved C. mantelli skeletons KUVP 247 and FHSM VP-2187 from the [[Niobrara Formation]]

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 C. 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.