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Cretoxyrhina

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Cretoxyrhina
Temporal range: Albian-Campanian, 107.59–73.2 Ma
[1][2][3][4][5][6]
Zahn eines Cretoxyrhina mantelli.JPG
Cretoxyrhina mantelli tooth from New Jersey, USA; Naturhistorisches Museum (Vienna)
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Class: Chondrichthyes
Order: Lamniformes
Family: Cretoxyrhinidae
Genus: Cretoxyrhina
Glickman, 1958
Type species
Cretoxyrhina mantelli
Agassiz, 1843
Species
  • C. mantelli Agassiz, 1843 (type)
  • C. denticulata Glickman, 1957
  • C. vraconensis Zhelezko, 2000
  • C. agassizensis Underwood & Cumbaa, 2010
Synonyms[7][8][9]
  • Oxyrhina mantellii Agassiz, 1843
  • Isurus mantelli Agassiz, 1843
  • Pseudoisurus mantelli Agassiz, 1843
  • Isurus denticulatus Glickman, 1957
  • Pseudoisurus vraconensis Zhelezko, 2000
  • Telodontaspis agassizensis Underwood & Cumbaa, 2010

Cretoxyrhina (/ˌkrˈɪtɔːksrhnə/; 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 early Maastrichtian of the Late Cretaceous period. 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 mechanisms 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 8 meters (26 ft) in length and weighting up to 3,400 kilograms (3.3 long tons; 3.7 short tons), Cretoxyrhina was one of the largest sharks of its time. Thanks to the discoveries of many exceptionally well-preserved skeletons of Cretoxyrhina, it is the anatomically best understood extinct shark to date. These fossils suggest that it may have had a similar appearance and build to the modern great white shark. The possession of extreme hydrodynamic features suggests that Cretoxyrhina may have been capable of burst speeds up to 70 kilometers per hour (43 mph), making it one the fastest swimming sharks. Its teeth were razor-like and had thick enamel, built for stabbing and slicing prey. Cretoxyrhina had large eyes that took up as much as one-third of the skull, and thus acute vision, suggesting that it used mainly eyesight to hunt prey. In a 2008 study, the lifespan of Cretoxyrhina was estimated to be 38 years, though a 2013 analysis reduced that to 21 years. Both studies found that Cretoxyrhina grew rapidly during the first few years of its life, doubling in size by three years of age and reaching sexual maturity at around four–five years of age.

Cretoxyrhina has been found worldwide, being most frequent in the Western Interior Seaway area of North America. It was an apex predator in its ecosystem, preying on a variety of marine animals including mosasaurs and plesiosaurs, large fishes including other sharks, and occasionally dinosaurs. Nevertheless, it may have faced competition from other contemporaneous predators such as pliosaurs, giant mosasaurs like Tylosaurus, and large sharks like Cardabiodon. Cretoxyrhina may have possessed the earliest forms of regional endothermy in mackerel sharks, allowing it to live in waters as cold as 5 °C (41 °F).

Taxonomy[edit]

Research history[edit]

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 Swiss naturalist Louis Agassiz in the third volume of his 1843 book Recherches sur les poissons fossiles. Using eight teeth collected by paleontologist Gideon Mantell from the Southerham Grey Pit in England and another from the collection of the Strasbourg Museum (whose exact location was unspecified but also came from England), he noted the strong dental similarities with the three species then classified in the now-invalid genus Oxyrhina, O. hastalis, O. xiphodon, and O. desorii.[10][a] Agassiz noted that the much thicker root of C. mantelli teeth made enough of a difference to be a distinct species from either and scientifically classified the shark under the taxon Oxyrhina mantellii[b] and named in honor of Mantell.[10]

During the late 19th century, paleontologists described numerous species that are now synonymized as Cretoxyrhina mantelli. According to some, there may have been as much as almost 30 different synonyms of O. mantelli at the time.[14] In 1870, French paleontologist Henri Sauvage identified teeth from France that greatly resembled the O. mantelli syntypes from England. However, the teeth also included lateral cusplets (small enameled cusps that appear at the base of the tooth's main crown), which are not present in the syntypes. This led him to describe the teeth under the species name Otodus oxyrinoides based on the lateral cusplets.[15] 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.[15][16]

This all changed with the discoveries of a number of 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:[14]

The remarkable thing about this specimen is that the vertebral column, though of cartilaginous material, was almost complete, and that the large number of 250 teeth were in position. When Chas. R. Eastman, of Harvard, described this specimen, it proved so complete as to destroy nearly thirty synonyms used to name the animal, and derived from many teeth found at various former times.

— Charles H. Sternberg, Some animals discovered in the fossil beds of Kansas, 1907
George F. Sternberg found many rare 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.[15][17] This skeleton however, which Sternberg had sold to the Ludwig Maximilian University of Munich, was destroyed in 1944 by allied bombing during World War II.[15][14] In 1891, Sternberg's son George F. Sternberg discovered a second O. mantelli skeleton now housed in the University of Kansas Museum of Natural History as KUVP 247. This skeleton was reported to measure 6.1 meters (20 ft) in length and consists of a partial vertebral column with skeletal remains of a Xiphactinus as stomach contents and partial jaws with about 150 teeth visible. This skeleton was considered to be one of the greatest scientific discoveries of that year due to the unexpected preservation of cartilage.[14] George F. Sternberg would later discover more O. mantelli skeletons throughout his career. His most notable finds were FHSM VP-323 and FHSM VP-2187, found in 1950[18] and 1965[19] respectively. The former is a partial skeleton consisting of a well-preserved set of jaws and 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, and a set of scales in their natural positions. Both skeletons are currently housed in the Sternberg Museum of Natural History.[20] 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.[21]

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.[15][22] He also identified a second species of Cretoxyrhina based on some of the earlier Cretoxyrhina teeth, which he named Cretoxyrhina denticulata.[23][24] In 2000, Russian paleontologist Viktor Zhelezko described Pseudoisurus vraconensis based on tooth material from Kazakhstan,[25] which a 2013 study led by Australian paleontologist Mikael Siverson moved the into the genus Cretoxyrhina.[7] In 2010, paleontologists Charlie Underwood and Stephen Cumbaa described Telodontaspis agassizensis from teeth found in Lake Agassiz in Manitoba.[26] This species would also be moved into Cretoxyrhina by a 2013 study led by American paleontologist Michael Newbrey.[9]

Between 1997 and 2008, paleontologist Kenshu Shimada published a series of papers where he analyzed the skeletons of C. mantelli including those found by the Sternbergs using modernized techniques to extensively research the possible biology of Cretoxyrhina. Some of his works include the development of more accurate dental,[27] morphological, physiological,[28] and paleoecological reconstructions,[29] ontogenetic studies,[30] and morphological-variable based phylogenetic studies[31] 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.[9]

Etymology[edit]

Cretoxyrhina is a portmanteau of the word creto (short for Cretaceous) prefixed to the genus Oxyrhina, which is derived from the Ancient Greek ὀξύς (oxús, "sharp") and ῥίς (rhís, "nose"). When put together they mean "Cretaceous sharp-nose", although Cretoxyrhina is believed to have had a rather blunt snout.[28] The type species name mantelli translates to "from Mantell", which Louis Agassiz named in honor of English paleontologist Gideon Mantell for supplying him the syntypes of the species.[10] The species name denticulata is derived from the Latin denticulus (small tooth) and āta (to have), together meaning "having small teeth". This is a reference to the appearance of lateral cusplets in most of the teeth in C. denticulata. The species vraconensis derived from the word vracon and the Latin ensis (from), meaning "from Vracon", which is a reference to the Vraconian substage of the Albian stage in which the species was discovered.[25] The species name agassizensis is derived from the name Agassiz and the Latin ensis (from), meaning "from Agassiz", named after Lake Agassiz where the species was discovered.[26] Coincidentally, lake itself is named in honor of Louis Agassiz.[32] 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.[33]

Phylogeny and evolution[edit]

Lamniformes

Mitsukurinidae

Odontaspididae

Pseudocarchariidae

Megachasmidae

Alopiidae

Cretoxyrhina

Cetorhinidae

Lamnidae

Possible phylogenetic relationship between Cretoxyrhina and modern mackerel sharks based on Shimada (2007)[34]

Cretoxyrhina was most like the modern great white shark in size, shape and ecology. However, the two sharks are not closely related, and their similarities are a result of convergent evolution.[30] Since the coinage of the genus, Cretoxyrhina has been traditionally grouped within the Cretoxyrhinidae, a family of lamnoid sharks that traditionally included other genera resulting in a paraphyletic or polyphyletic family.[35] However, Siverson (1999) remarked that Cretoxyrhinidae was used as a 'wastebasket taxon' for various Cretaceous and Paleogene sharks and declared that Cretoxyrhina was the only valid member of the family.[36]

Cretoxyrhina contains four valid species C. vraconensis, C. denticulata, C. agassizensis, and C. mantelli. These species represents a chronospecies. The earliest fossils of Cretoxyrhina are dated about 107 million years old and belong to C. vraconensis.[3] The genus would progress by C. vraconensis evolving into C. denticulata during the Early Cenomanian which evolved into C. agassizensis during the Mid-Cenomanian which in turn evolved into C. mantelli during the Mid-Turonian. This progression was characterized by the reduction of lateral cusplets and the increasing size and robustness of teeth.[2][37] 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 anteriors, which would gradually become restricted only to the posteriors in adults by the end of the interval in C. mantelli.[2] The Late Cenomanian–Coniacian interval was characterized by a rapid increase in tooth (and body) size and significant decrease of crown/height-crown/width ratio.[1][2] Tooth size peaked around 86 Ma during the latest Coniacian and then begins to slowly decline.[1] The youngest fossil of C. mantelli was found in the Bearpaw Formation of Alberta, dating as 73.2 million years old.[5] However, a single tooth identified as Cretoxyrhina sp. was recovered from the nearby Horseshoe Canyon Formation and dated as 70.44 million years old, suggesting that Cretoxyrhina may have survived into the Maastrichtian. However, the Horseshoe Canyon Formation possesses only brackish water deposits despite Cretoxyrhina being a marine shark, making a likely possibility that the fossil was reworked from an older layer.[6]

Phylogenetic studies through morphological data conducted by Shimada (2005) suggested that Cretoxyrhina may alternatively be congeneric with the genus of the modern thresher sharks. However, the analysis was met with uncertainty due to a lack of data for Cretoxyrhina. Because of this, the study also stated that the results are premature and may be inaccurate, recommending that Cretoxyrhina is kept within the family Cretoxyrhinidae.[31]

Another possible model for Cretoxyrhina evolution, proposed in 2014 by private researcher Cajus Diedrich, suggests that C. mantelli was congeneric with the mako sharks of the genus Isurus and was part of an extended Isurus lineage beginning as far the Aptian stage in the Early Cretaceous. According to this model, the Isurus lineage was initiated by 'Isurus appendiculatus' (Cretolamna appendiculata), which evolved into Isurus denticulatus[c] in the Mid-Cenomanian, then 'Isurus mantelli' (Cretoxyrhina mantelli) at the beginning of the Coniacian, then Isurus schoutedenti during the Paleocene, then Isurus praecursor, where the rest of the Isurus lineage continues.[d] The study claimed that the absence of corresponding fossils during the Maastrichtian (72-66 Ma) was not a result of a premature extinction of C. mantelli, but merely a gap in the fossil record.[38] However, Shimada and another paleontologist Phillip Sternes published a poster in 2018 which voiced doubt over the credibility of this proposal, noting that the study's interpretation is largely based on data that has been arbitrarily selected and failing to cite neither Shimada (1997)[e] nor Shimada (2005), which are key papers regarding the systematic position of C. mantelli.[8]

Biology[edit]

Morphology[edit]

Dentition[edit]

Teeth of juvenile and sub-adult C. mantelli from the Carlile Shale.

Distinguishing characteristics of Cretoxyrhina teeth include a nearly symmetrical or slanted triangular shape, razor-like and non-serrated cutting edges, visible tooth necks (bourlette), and a thick enamel coating. The dentition of Cretoxyrhina possesses the basic dental characteristics of a mackerel shark, with tooth rows closely spaced without any overlap. Anterior teeth are straight and near-symmetrical, while lateral teeth are slanted. The side of the tooth facing the mouth is convex and possesses massive protuberance and nutrient grooves on the root, whereas the labial side, which faces outwards, is flat or slightly swollen.[27] Juveniles possessed lateral cusplets in all teeth,[26] and C. vraconensis consistently retained them in adulthood.[7] However, lateral cusplets were only retained up to the lateral teeth in adulthood in C. denticulata and C. agassizensis and only up to the posterior teeth in C. mantelli.[26] The anterior teeth of C. vraconensis measure 2.1–3.5 centimeters (0.8–1.4 in) in height,[7] while the largest known tooth of C. denticulata measures 3 centimeters (1.2 in).[26] C. mantelli teeth are larger, measuring 3–4 centimeters (1–2 in) in average slant height. The largest tooth discovered from this species may have measured up to 8 centimeters (3 in).[9]

The dentition of C. mantelli is among the best documented and well-understood of all extinct sharks, thanks to fossil skeletons like FHSM VP-2187, which consists of a near-complete articulated dentition. Other C. mantelli skeletons, such as KUVP-247 and KUVP-69102, also includes partial jaws with some teeth in their natural positions, some of which were not present in more complete skeletons like FHSM VP-2187. Using these specimens, the dental formula was reconstructed by Shimada (1997), it was S4.A2.I3.L5.P7(+?)S1?.A3.L5.P10(+?). This means that from front to back, C. mantelli had: four symphysials, two anteriors, three intermediates, five laterals, and seven or more posteriors for the upper jaw and possibly one symphysial, three anteriors, five laterals, and ten or more posteriors for the lower jaw. The structure of the tooth row shows a dental structure suited for a feeding behavior similar to modern mako sharks, having large spear-like anteriors to stab and anchor prey and curved laterals and posteriors to cut it to bite-size pieces,[27] a mechanism often informally described as "slicing and dicing" by paleontologists.[33] In 2011, paleontologists Jim Bourdon and Mike Everhart reconstructed the dentition of multiple C. mantelli individuals based on their associated tooth sets. They discovered that two of these reconstructions show some notable differences in the size of the I1 tooth and lateral profiles, concluding that these differences could possibly represent sexual dimorphism or individual variations.[39]

Reconstruction of C. mantelli.

Skull[edit]

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.[28] Similar to modern crocodile sharks and thresher sharks, C. mantelli had proportionally large eyes, with the orbit roughly taking up one-third of the entire skull length, giving it acute vision. As a predator, good eyesight was important when hunting the large prey Cretoxyrhina fed on. In contrast, the more smell-reliant contemporaneous anacoracid Squalicorax's less advanced orbital but stronger olfactory processes was more suitable for an opportunistic scavenger.[28][40]

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.[28]

Exceptionally preserved C. mantelli skeletons KUVP-247 and FHSM VP-2187 from the Niobrara Formation.

Skeletal anatomy[edit]

Most species of Cretoxyrhina are only represented by fossil teeth and vertebra. Like all sharks, the skeleton of Cretoxyrhina was made of cartilage not bone, which is less capable of fossilization. However, fossils of C. mantelli from the Niobrara Formation have been found exceptionally preserved;[41] this was due to the formation's chalk having high contents of calcium, allowing calcification to become more prevalent.[42] When calcified, soft tissue hardens, making it more prone to fossilization.[41]

Numerous skeletons consisting of near-complete vertebral columns have been found. The largest vertebra were measured up to 87 millimeters (3 in) in diameter. Two specimens with the best-preserved vertebral columns (FHSM VP-2187 and KUVP 69102) have 218 and 201 centra respectively and nearly all vertebra in the column preserved; only portions of the tail tip are missing for both. Estimations of tail length calculates a total vertebral count of approximately 230 centra, which is unique as all known extant mackerel sharks possess a vertebral count of either less than 197 or greater than 282 with none in between. As a pelagic shark, C. mantelli possessed five gill slits, and large circular vertebral centra in the trunk region, and thus had a spindle-shaped body with a stocky trunk.[28]

An analysis of a partially complete tail fin fossil shows that Cretoxyrhina had a lunate (cresent-shaped) tail most similar with modern lamnid sharks, whale sharks, and basking sharks. The transition to tail vertebrae is estimated to be between the 140th-160th vertebrae out of the total 230, resulting in a total tail vertebral count of 70-90 and making up approximately 30-39% of the vertebral column. The transition from precaudal (the set of vertebrae before the tail vertebrae) to tail vertebrae is also marked by a vertebral bend of about 45°, which is the highest possible angle known in extant sharks and is mostly found in fast-swimming sharks, such as lamnids.[43] Along with other features such as smooth scales parallel to the body axis, a plesodic pectoral fin (a fin where its cartilage extends throughout all the fin), and a spindle-shaped stocky build, this shows that C. mantelli was capable of fast swimming.[28][43]

Physiology[edit]

Thermoregulation[edit]

As a regional endotherm, Cretoxyrhina may have possessed red muscles closer to its body axis like this porbeagle shark.

Cretoxyrhina represents one of the earliest forms and possible origins of endothermy in mackerel sharks.[44] 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,[44] where red muscles are closer to the body axis compared to ectothermic sharks, and a system of specialized blood vessels called rete mirabile (Latin for "wonderful nets") is present, allowing metabolic heat to be conserved and exchanged to vital organs. This morphological build allows the shark to be partially warm-blooded,[45] and thus efficiently function in the colder environments Cretoxyrhina has been found in. Fossils have been found in areas where paleoclimatic estimates show a surface temperature as low as 5 °C (41 °F).[44] Regional endothermy in Cretoxyrhina may have been developed in response to increasing pressure from progressive global cooling and competition from mosasaurs and other marine reptiles that had also developed endothermy.[44]

Hydrodynamics and locomotion[edit]

Cretoxyrhina possessed highly dense overlapping placoid scales parallel to the body axis and patterned in parallel kneels separated by U-shaped grooves, each groove having a mean width of about 45 micrometers. This formation of scales allows efficient drag reduction and enhanced high-speed velocity capabilities, a morphotype only seen in the fastest known sharks.[28][44] Cretoxyrhina also had the most extreme case of a "Type 4" tail fin, having the highest known Cobb's angle (curvature of tail vertebrae) and tail cartilage angle (49° and 133° respectively) ever recorded in mackerel sharks.[46][44] A "Type 4" tail fin structure represents a build with maximum symmetry of the tail fin lobes, which is most efficient in fast swimming and among sharks, are only found in lamnids.[46]

A. FHSM VP-323 B1-B9. Portions of the vertebral column from FHSM VP-2187 C. Vertebral centrum from C. mantelli D1-D2. Vertebral centrum from C. agassizensis E. Associated centra of C. cf. agassizensis.

A 2017 study by PhD student Humberto Ferron analyzed the relationships between the morphological variables including the skeleton and tail fin of C. mantelli and calculated an average cruising speed of 12 km/h (7.5 mph) and a burst swimming speed of around 70 km/h (43 mph), making Cretoxyrhina possibly one of the fastest sharks known.[44] For comparison, the modern great white shark reaches speeds of up to 56 km/h (35 mph)[47] and the shortfin mako, the fastest extant shark, has been clocked at a speed of 70 km/h (43 mph).[48]

Life history[edit]

Reproduction[edit]

Although no fossil evidence for it has been found, it is heavily implied that Cretoxyrhina was ovoviviparous as all modern mackerel sharks are. In ovovivipary, young are hatched and grow inside a parent's womb while competing against litter-mates through cannibalism such as oophagy or 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 large numbers of 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.[30]

Growth and longevity[edit]

Like all mackerel sharks, Cretoxyrhina grew a growth ring in its vertebrae every year and is aged through measuring each band. However, due to the rarity of well-preserved vertebrae, only few Cretoxyrhina individuals have been aged. In Shimada (1997), a formula for calculating the total body length of Cretoxyrhina using the centrum (the body of a vertebra) diameter of a vertebra was developed and is shown below, with TL representing total body length and CD representing centrum diameter (The diameter of each band).[30]

TL = 0.281 + 5.746CD

Using this formula, measurements were first conducted on the best-preserved C. mantelli specimen FHSM VP-2187 by Shimada (2008). The measurements showed a length of 1.28 meters (4 ft) and weight of about 16.3 kilograms (36 lb) at birth, and rapid growth in the first two years of life, eventually doubling the former length within 3.3 years. From then on, size growth became steady and gradual, growing a mean estimate of 21.1 centimeters (8 in) per year until its supposed death at 15 years of age, at which it had grown to 5 meters (16 ft). Using the von Bertalanffy growth model on FHSM VP-2187, the maximum lifespan of C. mantelli was estimated to be 38.2 years. By that age, it would have grown over 7 meters (23 ft) long and weigh as much as 3,400 kilograms (3.3 long tons; 3.7 short tons).[30]

Scale diagram of notable C. mantelli individuals

A remeasurement conducted by Newbrey et al. (2013) found that C. mantelli and C. agassizensis reached sexual maturity at around four–five years of age and revised the measurements of the growth rings in FHSM VP-2187. The lifespan of FHSM VP-2187 and maximum lifespan of C. mantelli was also revised to 18 and 21 years respectively using the new measurements. Measurements were also conducted on other C. mantelli skeletons and a vertebra of C. agassizensis yielding similar results of rapid growth in early stages of life.[9] Such rapid growth within mere years could have helped Cretoxyrhina better survive by quickly phasing out of infancy and its vulnerabilities, as a fully grown adult would have few natural predators.[30] The study also identified a syntype tooth of C. mantelli from England and calculated the individual's maximum length of 8 meters (26 ft), making the tooth the largest known specimen yet.[9][f]

The graph below represents the length growth per year of FHSM VP-2187 according to Shimada (2008)[g][30]

Paleobiology[edit]

A Cretoxyrhina and two Squalicorax circling around a dead Claosaurus

Prey relationships[edit]

The powerful kinetic jaws, high-speed capabilities, and large size of Cretoxyrhina suggest a very aggressive predator.[28][29] Indeed, Cretoxyrhina's association with a diverse number of fossils showing signs of devourment confirms that it was an active apex predator that fed on much of the variety of marine megafauna in the Late Cretaceous.[29][50] The highest trophic level it occupied was a position only shared with large mosasaurs such as Tylosaurus during the latter stages of the Late Cretaceous. It played a critical role in many marine ecosystems.[50]

Pteranodon specimen with a tooth embedded in a neck vertebra

Cretoxyrhina mainly preyed on other active predators including ichthyodectids (a type of large fish that includes Xiphactinus),[29] plesiosaurs,[29][51] turtles,[52] mosasaurs,[29][53][33] and other sharks.[29] Most fossils of Cretoxyrhina feeding upon other animals consist of large and deep bite marks and punctures on bones, occasionally with teeth embedded in them.[33] Isolated bones of mosasaurs and other marine reptiles that have been partially dissolved by digestion, or found in coprolites, are also examples of Cretoxyrhina feeding.[33][53] There are a few skeletons of Cretoxyrhina containing stomach contents known, including a large C. mantelli skeleton (KUVP 247) which possesses skeletal remains of the large ichthyodectid Xiphactinus and a mosasaur in its stomach region.[29] There was also an occasion where Cretoxyrhina may have occasionally fed on pterosaurs, evidenced by a set of cervical vertebrae of a Pteranodon from the Niobrara Formation with a C. mantelli tooth lodged deep between them. Although it cannot be certain if the fossil itself was a result of predation or scavenging, it was likely that Pteranodon and similar pterosaurs were natural targets by predators like Cretoxyrhina, as they would routinely enter water to feed and thus be well within reach.[54][55]

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, leading to the possibility that at least some of the feeding marks were made from scavenging.[29] Remains of partial skeletons of dinosaurs like Claosaurus and Niobrarasaurus show signs of feeding and digestion by C. mantelli. They were likely scavenged carcasses swept into the ocean due to the paleogeographical location of the fossils being that of an open ocean.[56][57]

Hunting strategies[edit]

Restoration of C. mantelli breaching and attacking a Pteranodon

The hunting strategies of Cretoxyrhina are not well documented because many fossils with Cretoxyrhina feeding marks cannot be distinguished between predation or scavenging. As most of these fossils show no signs of healing, if they were indeed a result of predation, 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 (which is most similar with Cretoxyrhina in morphology and ecological role) reaching lengths of greater than 4 meters (13 ft) 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.[51]

Paleoecology[edit]

Range and distribution[edit]

Anterior tooth of Cretoxyrhina mantelli from the Menuha Formation of southern Israel
Cretoxyrhina sp. teeth from Arnager, Denmark, at the Geological Museum, Copenhagen.

Cretoxyrhina had a cosmopolitan distribution with fossils having been found worldwide. Notable locations include North America, Europe,[58] Israel,[59], the Cotinguiba Formation of Brazil,[60] and Kazakhstan.[7] Cretoxyrhina mostly occurred in temperate and subtropical zones.[2] It has been found in latitudes as far north as 55° N, where paleoclimatic estimates calculate an average sea surface temperature of 5–10 °C (41–50 °F). Fossils of Cretoxyrhina are most well known in the Western Interior Seaway area,[59] which is now the central United States and Canada.[61] During a speech in 2012, Mikael Siverson noted that Cretoxyrhina individuals living offshore were usually larger than those inhabiting the Western Interior Seaway, with some of such offshore C. mantelli fossils yielding total lengths of up to 8 meters (26 ft), possibly 9 meters (30 ft).[62]

Habitat[edit]

The subtropical regions of the Late Cretaceous that Cretoxyrhina inhabited were dominated by sharks and turtles like the Cretodus and Protosphargis in this fossil.

Cretoxyrhina inhabited mainly temperate to subtropical pelagic oceans. A tooth of Cretoxyrhina found in the Horseshoe Canyon Formation in Alberta (a formation where the only water deposits found consist of brackish water and no oceans) suggests that it may have, on occasion, swam into partially fresh-water estuaries and similar bodies of water. However, a rework from an underlying layer may be a more likely explanation of such occurrence.[63] 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.[64]

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 7–8 °C (45–46 °F) warmer than modern day.[64] Around this time, Cretoxyrhina coexisted with a radiating increase in diversity of fauna like mosasaurs.[65] 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.[64] Although this event led to the extinction of as much as 27% of marine invertebrates,[66] vertebrates like Cretoxyrhina were generally unaffected.[67] 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.[64] Subtropical areas retained high biodiversity of all taxa, while temperate ecosystems generally had much lower diversity. In North America, subtropical provinces were dominated by various sharks, turtles, and mosasaurs such as Tylosaurus and Clidastes, while temperate provinces were mainly dominated by plesiosaurs, hesperornithid seabirds, and the mosasaur Platecarpus.[68]

Competition[edit]

Cretoxyrhina may have faced competition with mosasaurs like Tylosaurus

Cretoxyrhina lived in a very diverse pelagic ecosystem. It was contemporaneous with many predators that shared a similar trophic level during the Cretaceous.[50] Most of these predators included large marine reptiles, some of which already occupied the highest trophic level when Cretoxyrhina first appeared.[69] During the Albian to Turonian about 107 to 91 Ma, Cretoxyrhina was contemporaneous and coexisted with Mid-Cretaceous pliosaurs. Some of these pliosaurs included Kronosaurus and Megacephalosaurus, which both attained lengths of 9 meters (30 ft).[70][71] By the Mid-Turonian about 91 Ma, however, pliosaurs became extinct.[67][72] It is thought that the radiation of sharks like Cretoxyrhina may have been a major contributing factor to the acceleration of their extinction.[73] However, the ecological void they left was quickly filled by emerging mosasaurs, which also came to occupy the highest trophic levels.[74] Large mosasaurs like Tylosaurus, which reached in excess of 12 meters (39 ft) in length,[75] may have provided strong competition with Cretoxyrhina, and evidence of interspecific interactions such as bite marks from either have been found.[29] There were also many sharks that occupied a similar ecological role with Cretoxyrhina such as Cardabiodon[9] and Dwardius, with the latter showing evidence of direct competition with C. vraconensis based on intricate distribution patterns between the two.[76]

A 2010 study by paleontologists Corinne Myers and Bruce Lieberman on competition in the Western Interior Seaway calculated the competitive relationships Cretoxyrhina faced by various species. It determined that Cretoxyrhina was likely to have faced heavy competition with Squalicorax falcatus, Squalicorax kaupi, and Tylosaurus spp., but was unlikely to face competition from other predators such as Platecarpus spp. or Xiphactinus spp.. The study also concluded that such competitions were not severe enough to force major changes on Cretoxyrhina such as species distribution.[77]

Notes[edit]

  1. ^ These are now identified as Cosmopolitodus hastalis and Isurus desori;[11] Oxyrhina xiphodon is now considered conspecific with Cosmopolitodus hastalis.[12]
  2. ^ The original spelling made by Agassiz ended with -ii. However, later authors dropped the extra letter, spelling it as mantelli. Although this is a clear misspelling, it is maintained as the valid spelling of the species according to ICZN Article 33.4 due to its predominant usage over the original.[13]
  3. ^ Not to be confused with Cretoxyrhina denticulata.
  4. ^ 'Isurus appendiculatus' and 'Isurus mantelli' are scientific names proposed and used by Diedrich (2014). The original and more widely accepted scientific names of these taxa are shown in parenthesis.[38]
  5. ^ Shimada (1997) is cited in Diedrich (2014). However, the latter failed to use the citation in any of his claims regarding the systematic placement of C. mantelli, using it only in stratigraphic and historical notes.[38]
  6. ^ This syntype is in the collection of the British Museum of Natural History cataloged as NHMUK PV OR 4498.[49] However, this was not mentioned in Newbrey et al. (2013).[9]
  7. ^ The chart is based only on vertebrae from FHSM VP-2187. It does not represent the genus Cretoxyrhina as a whole.[30]

See also[edit]

References[edit]

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External links[edit]