| Pliosaurus | |
|---|---|
 | |
| P. kevani holotype skull | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Superorder: | †Sauropterygia |
| Order: | †Plesiosauria |
| Family: | †Pliosauridae |
| Clade: | †Thalassophonea |
| Genus: | †Pliosaurus Owen, 1842 |
| Type species | |
| †Pliosaurus brachydeirus (Owen, 1841) | |
| Species[3][4] | |
| Synonyms | |
| |
Pliosaurus (meaning 'more lizard') is anextinctgenus ofthalassophoneanpliosaurid known from theLate Jurassic (Kimmeridgian andTithonian stages) ofEurope andSouth America. This genus has contained many species in the past but recent reviews found only six (P. brachydeirus (type species),P. carpenteri,P. funkei,P. kevani,P. rossicus andP. westburyensis) to be definitively valid. One Patagonian speciesP. patagonicus likely belongs to a different genus withinBrachaucheninae. Currently,P. brachyspondylus andP. macromerus are considereddubious, whileP. portentificus is considered undiagnostic. Most European species ofPliosaurus would have measured around 10 metres (33 ft) long and weighed over 12 metric tons (13 short tons), though some potential specimens indicate a much larger size. Species of this genus are differentiated from otherpliosaurids based on sevenautapomorphies, including teeth that aretriangular in cross section. Their diet would have includedfish,cephalopods, andmarine reptiles.
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Pliosaurus brachydeirus is the (combinatio nova of the)type species of the genus. It was first described and named by the EnglishpaleontologistRichard Owen in1841, as a species of thewastebasket taxonPlesiosaurus in its own subgenusPleiosaurus, creatingPlesiosaurus (Pleiosaurus) brachydeirus.[5] Later that year or in1842, Owen published another study in which the species was relocated to its own genus, which he misspelled asPliosaurus.[6] As have been noted by several authors,Plesiosaurus (Pleiosaurus) is the original spelling ofPliosaurus, and therefore according to Article 32 of theICZN,Pleiosaurus is the correct spelling of the generic name as well. However, because this spelling had been abandoned since Phillips (1871),Pliosaurus should be preserved according to Article 33.3.1 of the ICZN.[3] Thegeneric name is derived from πλειων,pleion, meaning "more" andσαυρος,sauros, meaning "lizard" inAncient Greek, in reference to Owen's belief thatPliosaurus was more close in form to "saurians" (includingcrocodilians) thanPlesiosaurus was.[5] The etymology of thespecific name was not specified, but it probably refers to the shorter neck ofP. brachydeirus compared to the necks of other species then referred toPlesiosaurus.[6] The specific name has occasionally erroneously been spelled asbrachydirus, for example byRichard Lydekker (1889a, 1889b).[4]
P. brachydeirus is known from theholotype which includes seven specimens found in association and housed atOxford University Museum of Natural History, OUMNH J.9245, OUMNH J.9247 through OUMNH J.9301 and OUMNH J.10453. The specimen consists of a partialskull andlower jaw, severalaxial elements and limb material. Other specimens that are referable to this species include OUMNH J.9285, and OUMNH J.9192 through OUMNH J.9301, all described by Owen (1841–1842) and were found associated with the holotype. The specimens were collected by Prof.William Buckland[5] atMarket Rasen,Lincolnshire, from theRasenia cymodoce ammonite zone of the lowerKimmeridge Clay Formation, dating to the earlyKimmeridgian stage.[4][3]
Knutsen (2012) revised the validity of this species and was able to diagnose it on a basis of combination of traits.P. brachydeirus had approximately 70 teeth in the lower jaw (72 according to Bensonet al. (2013)), 8-9 or more pairs of symphyseal teeth in dorsal view (12 pairs according to Bensonet al. (2013)) and 5 or morepremaxillary teeth. It shows "type III" retroarticular process andautapomorphic (unique) traits of thecervical vertebrae, which had a smooth ventral surface and a ventral keel, unlike rounded to flat ventral surface seen in other species. Bensonet al. (2013) also noted that it lacks anisodont premaxillary dentition. Theontogenetic stage of the holotype ofP. brachydeirus is not known, but the rounded edge on the distal end of the femur and lack of separation between the femoral capitulum and trochanter suggests that it is from a relatively young individual. According to Bensonet al. (2013), the flat morphology of the proximal surface of the radius or tibia also suggests that it is a juvenile.[4][3]
P. carpenteri is known solely from theholotype BRSMG Cd6172, a nearly complete 1.8 metres (5.9 ft) longskull,mandible and postcranial skeleton, including at least 17 completevertebrae housed atBristol City Museum and Art Gallery, Bristol, England. The specimen was collected at the Westbury Clay pit,Wiltshire, from subdivision E4 of theAulacostephanus eudoxus ammonite zone, seven metres below theCrussoliceras Limestone of the Kimmeridge Clay Formation, dating to the late Kimmeridgian. Nicknamed "Westbury pliosaur II", it was first described by Sassoonet al. (2012) who, together with Knutsen (2012), assigned it tentatively toPliosaurus sp..[7][4] It was reassigned to its own species by Roger B. J. Benson, Mark Evans, Adam S. Smith, Judyth Sassoon, Scott Moore-Faye, Hilary F. Ketchum and Richard Forrest in2013. Thespecific name honors Simon Carpenter, the discoverer and collector of BRSMG Cd6172.[3]
Sassoonet al. (2012) originally ascribed the differences between BRSMG Cc332 (the holotype ofP. westburyensis) and BRSMG Cd6172 to intraspecific variation, with these specimens possibly beingsexual dimorphs, due to fact that both were collected from close stratigraphic levels of the same quarry.[7] However, Bensonet al. (2013) showed that the differences between them are relatively great even in the context of specimens from other localities. They diagnosedP. carpenteri based on a singleautapomorphy – unlike all otherthalassophoneans other than the proposed neotype ofP. brachyspondylus, the dorsal surface of the surangular lacks any fossa, and in contrast to all other specimens ofPliosaurus faces dorsally, not inclined to face dorsolaterally.P. carpenteri also possesses a unique combination of characters, including: low dentary alveolar count including only 18 postsymphysial alveoli, and a total count of 27; intermediate low count of syphysial alveoli including only 9; teeth fully trihedral, possessing a flat, anteroposteriorly broad labial surface lacking enamel ridges; mediolateral expansion of caniniform regions of the premaxilla and maxilla relatively pronounced, although this might be due to crushing; six closely spaced premaxillary alveoli; anisodont premaxillary dentition; diastema present between maxillary and premaxillary alveolar rows; premaxilla–parietal suture located level with the anterior region of the orbit; cervical centra lacking ventral ridge; and epipodials with highly convex proximal surfaces.[3]
P. funkei is known from two partial skeletons, theholotype PMO 214.135 and the referred PMO 214.136, from theSvalbard archipelago ofNorway. Housed at theUniversity of Oslo Natural History Museum, the skeletons were found south ofSassenfjorden, from the southeast side of Mount Knerten, in theArcticSpitsbergen island. Both individuals were collected by aNorwegian team led by Dr.Jørn Hurum within 2 kilometres (1.2 mi) of one another, approximately 30 metres (98 ft) below the Myklegardfjellet Bed from the black shales of theSlottsmøya Member, which is the uppermost out of four named members in theAgardhfjellet Formation. This stratigraphic horizon belongs to theDorsoplanites ilovaiskyi toDorsoplanites maximus ammonite zones (probably to the latter), of the middle Volgian stage which correlates with the middleTithonian stage. The skeletons were discovered during eight seasons of fieldwork (2004–2012 field seasons) in the Slottsmøya Member, that have yielded other skeletal remains of marine reptiles, including theplesiosauroidsColymbosaurus svalbardensis,Djupedalia andSpitrasaurus, and theichthyosaursCryopterygius andPalvennia.P. funkei was first described and named by Espen M. Knutsen, Patrick S. Druckenmiller and Jørn H. Hurum in2012. Thespecific name honors Bjørn Funke, the discoverer of the holotype, and his wife May-Liss Knudsen Funke for volunteering in the paleontological collections at the Museum.[8]
The holotype ofP. funkei is represented by the anterior portions of the upper and lower jaws (includingpremaxillary anddentary teeth), one nearly completecervical centrum and two partial cervical centra, three pectoral centra with neural arches, fifteen dorsal centra and eight neural arches, a complete rightcoracoid, numerousrib fragments andgastralia, and a complete right forelimb. The referred specimen is represented by five partial cervical centra, one partial dorsal centrum, and a partialskull including theoccipital condyle, a complete leftquadrate, a partial leftsquamosal and incomplete left surangular and articular. Several fragmentary and unidentified bones also pertain to PMO 214.136. Due to the Arctic climate of Svalbard, the specimens were subjected to repeatedfreeze-thaw cycles before collection, extensively fracturing and degrading the material.[8] PMO 214.136 was discovered in 2007, following the collection of approximately 20,000 fragments that compose PMO 214.135[9] which were found moistin situ and degraded upon drying during the preparation process (individual fragments are catalogued at the University of Oslo Natural History Museum by specimen number followed by a slash and a number). Estimates of skull length are approximately 1.6–2.0 m (5.2–6.6 ft) for the holotype and 2.0–2.5 m (6.6–8.2 ft) for PMO 214.136, suggesting a total body length of 10–12 m (33–39 ft) for the species, makingP. funkei one of the largest pliosaurs described so far.[8] Due to its large size and relative completeness, the species, nicknamed "Predator X" before its formal description, gained extensive media coverage, which claimed that it was "most fearsome animal ever to swim in the oceans".[10][11] Morphological andhistological characters, such as the presence of a tuberosity on thehumerus and a well developed anterior process on the coracoid, and abnormal hardening and increase in density of bone, indicate that both specimens were adult individuals. Even though none of the neural arches are fused to their centra in thevertebral column of both individuals (a possible juvenile trait), this feature is present in all large pliosaurids, and thus possiblypaedomorphic withinPliosauridae.[8]
Knutsenet al. (2012) diagnosed the species based on a unique combination of characters of the holotype.P. funkei has a possibly unique "type I" retroarticular process, unlikeP. brachydeirus,P. brachyspondylus andP. macromerus. UnlikeP. brachydeirus, its cervical centra possess a rugose ventral surface, but lack ventral keel. Finally, it possesses comparatively longer forelimbs than other known pliosaurids, with a long humerus, more than seven times the average width of cervical vertebral centra, in comparison toP. brachyspondylus andP. rossicus, which have humeri less than 4.5 times the cervical width. Its complete teeth count is not known, however, it has at least six pairs of teeth in the mandibular symphysis, and at least five premaxillary tooth pairs. The two known individuals ofP. funkei preserve mostly different regions of the skeleton and overlap only by cervical vertebrae. Nevertheless, these cervical vertebrae are morphologically indistinguishable, and both individuals were found in proximity to one another, at exactly the same stratigraphic horizon of the Slottsmøya member, strongly supporting the referral of PMO 214.136 toP. funkei. Knutsenet al. (2012) suggested thatP. funkei is more similar toP. rossicus and the proposed neotype ofP. macromerus than toP. brachydeirus and the proposed neotype ofP. brachyspondylus in its cranial morphology.[8]
Analysis of bones from the four flippers suggest that the animal cruised using only the fore-flippers, using the back flippers for extra speed when pursuing and capturing prey.
Predator X's brain was of a similar type and size, proportionally, to that of today'sgreat white shark, the team says.[12]
A television programme entitledPredator X first aired onHistory in 2009. It was also featured in the fourth episode of the BBC documentary seriesPlanet Dinosaur in 2011, where it is shown huntingKimmerosaurus, a smallerplesiosaur. The scene is based on damages to the one known skull ofKimmerosaurus, showing damage consistent with an attack of a very large animal withLiopleurodon-like dentition.[13]
P. kevani is known solely from theholotype DORCM G.13,675, a nearly complete 2.0 metres (6.6 ft) longskull and mandible housed at theDorset Museum, Dorchester, England. The specimen was collected at Wyke Siltstone bed ofOsmington Bay, from theRasenia cymodoce ammonite zone, Kimmeridge Clay Formation of the Ancholme Group, dating to the early Kimmeridgian. DORCM G.13,675 was collected over a period of eight years as pieces up to 60 kilograms (130 lb) in mass weathered out of the sea-cliff of Black Head. Most pieces were collected from loose or fallen blocks without any permits required, while other parts were collectedin situ and purchased from land owners. They were first identified as a pliosaurid skull by Richard Edmonds, Earth Sciences Manager for Dorset and East Devon CoastWorld Heritage Site Team. Due to its large size and completeness, the specimen gained extensive media coverage, and its acquisition was announced publicly in October 2009. Additional elements were later donated by Patrick Clarke and purchased from Shirley Swaine. DORCM G.13,675 went on display in Dorchester County Museum in July 2011, after being in preparation between March 2010 and March 2011. Preparation of the lower jaws took 200 hours and a further 365 hours were needed to complete preparation of the skull.P. kevani was first described and named by Roger B. J. Benson, Mark Evans, Adam S. Smith, Judyth Sassoon, Scott Moore-Faye, Hilary F. Ketchum and Richard Forrest in2013. Thespecific name honors Kevan Sheehan, the owner of a small café overlooking the sea at Osmington Mills, who collected most of the holotype during daily walks along the foreshore.[3]
Bensonet al. (2013) diagnosed the species based on fourautapomorphies of the holotype. The subrectangular sheet of the maxilla extends anteriorly on alveolar surface of the premaxilla to contact the distalmost premaxillary alveolus, while in other species ofPliosaurus an interdigitating premaxilla-maxilla suture is located midway between the mesialmost maxillary and distalmost premaxillary alveoli. Its pineal foramen is surrounded by a raised rim, while otherthalassophoneans have a shallow fossa containing anteroposteriorly oriented grooves or ridges, that extends anteriorly from the pineal foramen. The mesial postsymphysial dentary alveoli are everted to face dorsolaterally, and not dorsally as seen in other species. Finally, the lateral surface of the mandible dorsoventrallyconcave posteriorly, while other thalassophoneans show flat or weakly convex lateral surface of the postedentary bones.P. kevani also possesses a unique combination of characters, including: high dentary alveolar count including 22 postsymphysial alveoli and an estimated total count of 36–37; high count of symphysial dentary alveoli including at least 7, estimated as 14–15; subtrihedral teeth, possessing a suboval cross-section with slightly flattened labial surface bearing only thinly distributed enamel ridges; pronounced mediolateral expansion of caniniform regions of the premaxilla and maxilla; six closely spaced premaxillary alveoli; anisodont premaxillary dentition; and premaxilla–parietal suture located level with the anterior region of the orbit.[3]
CAMSM J.35990, a completepostcranial skeleton originally referred toStretosaurus macromerus, and later toPliosaurus sp., might also pertain toP. kevani. Although non-diagnostic at the species level, Bensonet al. (2013) differentiated it from most specimens ofPliosaurus based on its subtrihedral teeth, which are otherwise present only inPliosaurus kevani, and possibly also inGallardosaurus iturraldei from theOxfordian ofCuba. Even though it shares with the holotype ofP. kevani a very large body size and stratigraphically closer to it, otherwise the specimens cannot be directly compared. Therefore, it was tentatively referred to asPliosaurus cf.kevani. A subtrihedral tooth from the Kimmeridge Clay of Ely,LEICT G418.1965.108, is also referred toPliosaurus cf.kevani based on similar arguments.[3]
P. rossicus was first described and named byNestor Ivanovich Novozhilov in1948. Thespecific name is derived from the name ofRussia, where the holotype was found. Theholotype ofP. rossicus, PIN 304/1 housed atPaleontological Institute, Russian Academy of Sciences, consists of cranial and some postcranial remains of a relatively small pliosaur. It was collected at the right bank of theVolga River, ofChuvashia, European part ofRussia, from the Buinsk Mine oil shales,Dorsoplanites panderi ammonite zone, dating to the middle Volgian stage (also known as middleTithonian).[14] Novozhilov (1964) later also described some pectoral remains associated with the holotype.[15] The holotype preserved trihedral teeth like other members ofPliosaurus, and bears 6 tooth pairs in the mandibular symphysis, similar to OUMNH J.10454. Halstead (1971) reassigned this species toLiopleurodon based on this symphyseal tooth count,[16] but Knutsen (2012) and Bensonet al. (2013) referred it back toPliosaurus, as it exhibits the diagnostic traits of the genus such as trihedral teeth.[3] PIN 304/1 has also been interpreted as a juvenile by both Halstead (1971) and Storrset al. (2000) based on its relatively small size and poorly developed dorsal blade and anteroventral ramus of the scapula.[4]
Halstead (1971) referred a second, larger specimen PIN 2440/1 consisting of a partialrostrum and hind limb, toP. rossicus.[16] PIN 2440/1 was originally described asPliosaurus cf.grandis by Rozhdestvenskii (1947), but later referred toP. rossicus based on the presence of a similar number of mandibular symphyseal teeth with the holotye and their relative stratigraphic co-occurrence. Another specimen of large pliosaur was tentatively assigned toP. rossicus by Malakhov (1999). The specimen was collected from the lower Volgian (early Tithonian) ofKazakhstan, and represents postcranial remains No. 13-1958, at Institute of Zoology MS-AS RK.[4]
Knutsen (2012) suggested possible synonymy betweenP. macromerus andP. rossicus based on the presence of only six symphysial and five premaxillary alveoli in both. Nevertheless, he provisionally retainedP. rossicus as a separate species, as the stratigraphic ranges of the two taxa do not overlap, and the specimens were not adequate described.[4] However, a reexamination of NHMUK PV OR 39362 (proposed neotype ofP. macromerus) by Bensonet al. (2013), revealed that it had at least seven symphyseal tooth, but more likely nine. Therefore, they consideredP. rossicus to be a valid species ofPliosaurus based on the presence of an autapomorphic short symphysis containing only six alveoli. Based on the presence of this trait, they tentatively referred OUMNH J.10454 (and thus possibly OUMNH J.50376 and OUMNH J.50377) toP. ? rossicus.[3] These specimens were all collected at Chawley brick pit, of the upper part of the Lower Kimmeridge Clay, dating to the late Kimmeridgian, and were originally assigned toP. macromerus.[4] OXFUM J.10454 is a highly reconstructed and fragmentary specimen, with a total length of 287.5 cm (9.43 ft). Tarlo estimated that the skull length of this individual had originally been more than 3 m (9.8 ft),[17] however Bensonet al. (2013) argued that this cannot be currently determined. Apart from theautapomorphy noted above[3] and trihedral teeth,P. rossicus possesses the following combination of characters (based on its holotype): 5 premaxillary tooth pairs; cervical vertebrae with ornamented ventral surface, but lacking ventral keel; proportionally shorter humeri thanP. funkei, less than 4.5 times the average width of cervical centra, versus more than 7 times.[4]
P. westburyensis is known solely from theholotype BRSMG Cc332, a nearly completeskull and postcranial fragments, including fourcervical vertebrae housed atBristol City Museum and Art Gallery, Bristol. The specimen was collected at the Westbury Clay pit,Wiltshire, England, from subdivision E5 of theAulacostephanus eudoxus ammonite zone, one metre below theCrussoliceras Limestone of the Kimmeridge Clay Formation, dating to the late Kimmeridgian.[3] Nicknamed "Westbury pliosaur I", it was first described by Taylor & Cruickshank (1993) and referred toP. brachyspondylus based on the absence of a ventral keel on the cervical vertebral centra.[18] BRSMG Cc332 was assigned tentatively toPliosaurus sp. by Sassoonet al. (2012) and Knutsen (2012)[7][4] and reassigned to its own species by Roger B. J. Benson, Mark Evans, Adam S. Smith, Judyth Sassoon, Scott Moore-Faye, Hilary F. Ketchum and Richard Forrest in2013. Thespecific name honours the town ofWestbury near which the holotype was found.[3]
The mandible of BRSMG Cc332 is approximately 50 cm longer than CAMSM J.35991 (proposed neotype forP. brachyspondylus), but is shorter than the two French specimens referred toP. brachyspondylus. Knutsen (2012) distinguished BRSMG Cc332 fromP. brachyspondylus as the former has a "type IV" retroarticular process and a much lower degree of fusion between the anterior mandibular bones.[4] Bensonet al. (2013) diagnosed the species based on threeautapomorphies.P. westburyensis has widely spaced premaxillary alveoli, with interalveolar walls approximately half the anteroposterior length of a single alveolus. It also has a long, sheet-like process of the maxilla that extends back to the anterolateral part of the maxilla–frontal contact medial to the external naris, and terminates just anterior to midlength of the orbital. Finally, the suture between thepremaxilla andparietal bone is located around orbital midlength.P. westburyensis also possesses a unique combination of characters, including: low dentary alveolar count including only 18 postsymphysial alveoli; teeth fully trihedral in cross-section, possessing a flat, anteroposteriorly broad labial surface lacking enamel ridges; relatively slight mediolateral expansion of premaxilla and maxillary caniniform region; six premaxillary alveoli; lack of anisodont premaxillary dentition; lack of diastema between maxillary and premaxillary alveolar rows; and cervical centra lacking ventral ridge.[3]
Pliosaurus brachyspondylus was first described and named by Owen in1839, as a species ofPlesiosaurus. Thespecific name is derived from βραχυς,brachus, meaning "short" and σπόνδυλος,spondylos, meaning "vertebra" inAncient Greek.[19] It was named on the basis of a series of unassociatedcervical vertebrae as theholotype. These specimens were collected atHeadington Pits near Oxford,Oxfordshire, England, from theKimmeridge Clay. It was later noted by Pocock (1908), that the material was found in the lower part of the Kimmeridge Clay, as the pits expose the contact between the Kimmeridge Clay Formation and the underlying Corallian beds. Several other cervical centra from the same strata atWeymouth, Dorsetshire, that had been namedPlesiosaurus giganteus by Conybeare (1824), were also referred toPlesiosaurus brachyspondylus by Owen (1839). Eichwald (1868) reassignedP. brachyspondylus toPliosaurus but did not provide diagnosis to distinguish these vertebrae from those of the type species of the genus.P. giganteus was later synonymised by Lydekker (1889a) withP. brachydeirus, the type species.[4]
According to Tarlo (1959a), the holotypes ofP. giganteus andP. brachyspondylus have been lost or destroyed since their initial descriptions.[20] Therefore, he selected a single posterior cervical centrum, CAMSM J.29564, as theneotype ofP. brachyspondylus, whileP. giganteus became anomen oblitum, forgotten name, asP. brachyspondylus had traditionally been given priority over it. This centrum was the only not dorsal centrum from a series of thirty associated centra listed byHarry Seeley (1869) as belonging toP. brachyspondylus. They were collected at the Roswell (Roslyn) pit nearEly, Cambridgeshire, also in England, from between theAulacostephanus mutabilis orAulacostephanus eudoxus ammonite zones, of the lower Kimmeridge Clay Formation. Although the measurements of the neotype agree with these of the lost holotype, they are also similar to these of referred specimens ofP. brachydeirus (OUMNH J.9291-9301) and"Pliosaurus" andrewsi (NHMUK R.1243, cast of mid-cervical centrum referred toP. evansi). Additionally, Seeley's referral was based on a single character, "articular surface is very slightly concave, with a small round depression at the centre", a feature that is now known to be common in otherpliosaurids as well. Thus, Tarlo's choice of the neotype ofP. brachyspondylus relies solely on the similarity in size to the original material described by Owen (1839), which is also similar toP. brachydeirus.[4] The neotype centrum, CAMSM J.29564, differs from the holotype ofP. brachydeirus in the following two traits; it has ventral surface ornamentation but lacks a ventral keel. Nevertheless, the neotype forP. brachyspondylus is non-diagnostic from other species ofPliosaurus, and thus the name must be considered anomen dubium.[4][3]
In the same paper, Tarlo (1959a) described an associated pliosaur skeleton, CAMSM J.35991, and referred it toP. brachyspondylus.[20] CAMSM J.35991 consists of a completemandible, most of the axial skeleton and parts of theappendicular skeleton. It was found in 1889 in theAulacostephanus eudoxus ammonite zone of the lower Kimmeridge Clay, at the same locality as CAMSM J.29564 (the neotype forP. brachyspondylus). Knutsen (2012) suggested to replace the neotype forP. brachyspondylus, with CAMSM J.35991 as the new neotype, because both specimens and possibly the lost holotype are from the same locality and horizon, and similar in size. CAMSM J.35991 is much more complete and can be distinguished from all other species ofPliosaurus.[4] Bensonet al. (2013) agreed with this suggestion, but consideredP. brachyspondylus to beThalassophonea indet., until a petition to the ICZN is made.[3] Based on CAMSM J.35991, Knutsen (2012) provisionally diagnosedP. brachyspondylus as aPliosaurus with approximately 60 (58 according to Bensonet al. (2013)) teeth in the lower jaw, 9 pairs of symphyseal teeth in dorsal view. It had "type II" retroarticular process and proportionally shorter humeri thanP. funkei. As also seen in CAMSM J.29564, it had cervical centra with ventral surface ornamentation, but lacking a ventral keel. According to Bensonet al. (2013), the flat morphology of the proximal surface of the radius or tibia suggests that CAMSM J.35991 is a juvenile.[4][3]
According to Knutsen (2012), Bardetet al. (1993) referred two additionalmandibles toP. brachyspondylus based solely on a similar number of dentary and symphyseal teeth;BHN 2R.370, collected at the Moulin-Wibert quarry, from theRasenia cymodoce ammonite zone of theCalcaires de Moulin-Wibert Formation of Nord-Pas-de-Calais, andMNHN cat.24.1 collected atLe Havre,Normandy, both from the early Kimmeridgian ofFrance.[4] Bensonet al. (2013) discussed only BHN 2R.370 originally referred toPliosaurus grandis, stating thatP. carpenteri has a similar count of dentary and symphysial teeth, and thus the specimen cannot be identified to species level. However it was referred toPliosaurus indet. as it has a broad, dorsolaterally facing surangular fossa, bounded laterally by a fossa and ridge.[3] Another specimen, BRSMG Cc332, was referred toP. brachyspondylus by Taylor and Cruickshank (1993) and toPliosaurus sp. by Sassoonet al. (2012) and Knutsen (2012),[18][7][4] but was reassigned to its own speciesP. westburyensis by Bensonet al. (2013).[3]
Pliosaurus macromerus was first described and named byJohn Phillips in1871, as a species ofPleiosaurus, on the basis of a largefemur, OUMNH J.12498, and a series of unassociated vertebrae. Thespecific name is derived from μακρός,makros, meaning "long" inAncient Greek andmēros, meaning "thigh" inLatin, from Greek, in reference to the large size of OUMNH J.12498 (a thigh bone).[21] These specimens were collected during the excavation of theGreat Western Railway nearSwindon,Wiltshire, probably from thePectinatites hudlestoni andPavlovia pallasioides ammonite zone, Upper Kimmeridge Clay, of the Swindon Clay and Cemetery Beds. Lydekker (1889a) amended thePleiosaurus macromerus intoPliosaurus, and reassigned NHMUK PV OR 39362 to this species with other material, based solely on large size.[4] NHMUK PV OR 39362, a complete skull and mandible, was first described by Owen (1869) who referred it toPliosaurus grandis. It was collected at Kimmeridge Bay,Dorsetshire, from theAulacostephanus autissidorensis orPectinatites elegans ammonite zones of the Upper Kimmeridge Clay Formation,[4] dating to the late Kimmeridgian or earlyTithonian.[3]
Tarlo (1959b) erected OUMNH J. 10441, one of the partial cervical vertebrae from the collection of unassociated remains listed by Philips (1871), as thelectotype ofP. macromerus.[17] It was assigned toP. macromerus based on its large size, similar to those ofP. westburyensis andP. funkei.[4] Tarlo (1959b) described the lectotype as "somewhat roughened", while Lydekker (1889a) described it as being "very coarse and rough" in reference to its ventral rugosity, resembling that ofP. brachyspondylus. Tarlo (1959b) also erected a new generic name forP. macromerus,Stretosaurus, due to the unusual scapular morphology of a specimen he described and referred to it, CAMSM J.35990.[17] It was found atStretham, southwest of Ely inCambridgeshire, probably from the early KimmeridgianAulacostephanus mutabilis ammonite zone. It represents an associated fragmentary skull and a relatively completepostcranial skeleton. Tarlo (1959b) referred CAMSM J.35990, and two other very large, anterior cervical vertebral centra, CAMSM J.29562 and CAMSM J.30057, toStretosaurus macromerus based on similar size and ornamentation of the cervical vertebrae. Knutsen (2012) noted that this ornamentation is shared withP. westburyensis and the proposed neotype ofP. brachyspondylus.[4]
Based on similar number of mandibular symphyseal teeth with NHMUK PV OR 39362, Tarlo (1959b) also assigned toS. macromerus a partialmandible, OUMNH J.10454, represented by an anterior rostral tip and associated trihedral teeth.[17] Nicknamed "Cumnor mandible", it was collected at Chawley brick pit,Oxfordshire, alongsideCumnoria prestwichii. This locality most likely belongs to theAulacostephanus eudoxus orA. autissidorensis ammonite zone, of the upper part of the Lower Kimmeridge Clay. Knutsen (2012) noted that there is no overlap between the cranial portions of CAMSM J.35990 and NHMUK PV OR 39362 or OUMNH J.10454. Halstead (1989) referred a more recently discovered, complete specimen (NHMUK R8928) toP. brachyspondylus based on its long mandibular symphysis. Using this specimen he re-identified the "scapula" of CAMSM J.35990 as anilium, but in spite of their similar ilial morphology, reassignedS. macromerus toLiopleurodon.[4] According to Noèet al. (2004), the specimens ofS. macromerus should be referred toP. macromerus as the nameStretosaurus is invalid, and NHMUK PV OR 39362 and CAMSM J.35990 exhibit theautapomorphic trihedral teeth ofPliosaurus.[22]
Knutsen (2012) suggested to replace the lectotype ofP. macromerus, with NHMUK PV OR 39362 as aneotype, because the lectotype (as well as the remainingsyntypes) is indistinguishable from all species ofPliosaurus, apart fromP. brachydeirus due to vertebral ornamentation. NHMUK PV OR 39362 is much more complete and can be distinguished from all other species ofPliosaurus. NHMUK PV OR 39362 has been traditionally assigned toP. macromerus, and even though it has previously been assigned toP. grandis, Lydekker (1889a) showed that there is no evidence that the syntypes ofP. grandis even belong toPliosaurus, and there is no overlapping material between them and NHMUK PV OR 39362.[4] Bensonet al. (2013) agreed with the suggestion to assign a neotype ofP. macromerus, but considered it to beThalassophonea indet., until a petition to the ICZN is made.[3] Based on NHMUK PV OR 39362, Knutsen (2012) provisionally diagnosedP. brachyspondylus as aPliosaurus with at least 50 teeth in the lower jaw (probably 54), at least 7 pairs (probably 9) of symphyseal teeth and 5 pairs of premaxillary teeth. It also has "type III" retroarticular process. Bensonet al. (2013) noted that it lacks anisodont premaxillary dentition.[4][3]
Other specimens previously referred toP. macromerus are currently assigned to other species ofPliosaurus, rendering this species anomen dubium. Knutsen (2012) referred CAMSM J.35990 toPliosaurus sp., stating that it is non-diagnostic at the species level. Bensonet al. (2013) agreed that it cannot be confidently diagnosed as a distinct species, or referred to an existing species with certainty. Nevertheless, they differentiated CAMSM J.35990 from most specimens ofPliosaurus based on its subtrihedral teeth, which are otherwise present only inPliosaurus kevani, and possibly also inGallardosaurus iturraldei from theOxfordian ofCuba. Furthermore, it shares withP. kevani a very large body size and stratigraphically closer toP. kevani, but cannot be otherwise compared to it. Therefore, it was tentatively referred to asPliosaurus cf.kevani.[3] According to Knutsen (2012), OUMNH J.10454 is referable toP. macromerus, together with two associated fragments, OUMNH J.50376 and OUMNH J.50377, each constituting one ramus of a lower jaw from a single individual. All three specimens were collected from the same pit, are of similar size, and have "type III" retroarticular process. The referral toP. macromerus was based on this trait, a suggested similar symphyseal tooth count (six) between OUMNH J.10454 and the proposed neotype ofP. macromerus, and their occurrence at approximately the same stratigraphic level.[4] However, a reexamination of NHMUK PV OR 39362 (proposed neotype ofP. macromerus) by Bensonet al. (2013), revealed that it had at least seven symphyseal tooth, but more likely nine. Therefore, they consideredP. rossicus to be a valid species ofPliosaurus based on the presence of an autapomorphic short symphysis containing only six alveoli, and tentatively referred OUMNH J.10454 (and thus possibly OUMNH J.50376 and OUMNH J.50377) toP. ? rossicus. Another possible difference between NHMUK PV OR 39362 and OUMNH J.10454, is that the latter had greater mandibular tooth count of approximately 60,[3] although this is possibly an artifact of its reconstruction according to Knutsen (2012).[4]
P. irgisensis was first described and named byNestor Ivanovich Novozhilov in1948, under the namePeloneustes irgisensis. Thespecific name is derived from the name of theMaly Irgiz River, where the holotype was found. Theholotype, PIN 426 housed atPaleontological Institute, Russian Academy of Sciences, consists of partial cranium and postcranium of a medium-sized individual. The cranial remains of PIN 426 are currently suffering from the effects ofpyrite decay, and its associated remains have been lost. It was collected at Savel-evsk Mine No. 1, of easternSaratov Oblast, European part ofRussia, from the middle Volgian stage (also known as middleTithonian). Novozhilov (1964) reassigned the material to a new genus,Strongylokrotaphus, meaning "round" (strongylos) "temple" or "head" (krotaphos) inAncient Greek. These assignments were criticized by other workers, including Tarlo (1960), Halstead (1971) and Storrset al. (2000), because of the general and non-specific diagnosis forStrongylokrotaphus, and differences from the type species ofPeloneustes.P. irgisensis might be synonymous withP. rossicus based on their similar sizes and stratigraphic co-occurrence, as the only difference between the species isontogenetic. Knutsen (2012) considered the material to be non-diagnostic and referredP. irgisensis toPliosauridae indet, as dentition, which is autapomorphic inPliosaurus, was not described from PIN 426.[4] Bensonet al. (2013) also considered this species to be anomen dubium, referring it toThalassophonea indet, and stating that PIN 426 requires re-description.[3]
Pliosaurus portentificus is known from the late Kimmeridgian of England. Considered by Knutsen (2012) and Bensonet al. (2013) to be undiagnostic, with its holotype specimen most likely being a juvenile individual of one of the other species ofPliosaurus.[4][3]
The size ofPliosaurus has been debated among paleontologists. Specimens ofP. funkei were initially estimated to reach a total body length of 10–13 metres (33–43 ft), with estimated skull lengths of 1.6–2 m (5.2–6.6 ft) for the holotype (PMO 214.135) and 2–2.5 m (6.6–8.2 ft) for the referred specimen (PMO 214.136).[8] After comparing the specimens ofP. funkei withP. kevani (DORCM G. 13675),P. cf.kevani (CAMSM J.35990) andP. macromerus (anterior cervical vertebrae), Zhao (2024) concluded that these specimens were not significantly different in body size, resulting in a body length estimate of 9.8–10.3 metres (32–34 ft) and body mass over 12 metric tons (13 short tons).[24] Some specimens may even be 11.7–14.4 metres (38–47 ft) long and are based on a giant mandible and giant neck vertebrae that may be referred toPliosaurus.[25][3]
The forelimb length of the holotype ofP. funkei was estimated up to 3 m (9.8 ft), suggesting that the animal had proportionally bigger flippers than other pliosaurs compared to the skull size and dimensions of the vertebrae.[8] Analysis of bones from the four flippers suggest that the animal cruised using just two fore-flippers, using the back pair for extra speed when pursuing and capturing prey.P. funkei brain was of a similar type and size, proportionally, to that of today'sgreat white shark.[12][8] In 2014, Foffa and colleagues estimated the bite force ofP. kevani which produced varying results from 9,617–16,927 N (981–1,726 kgf; 2,162–3,805 lbf) at the anterior dentary tooth to 27,685–48,728 N (2,823–4,969 kgf; 6,224–10,954 lbf) at the 36th dentary tooth pair.[26]
The monophyly of at least one of the two Patagonian species withinPliosaurus,P. patagonicus andP. almanzaensis, has been questioned by recent cladistics analyses. Both taxa did not form a monophyletic clade with other European species in the 2019 study,[27] while the 2023 study recoveredP. patagonicus as a sister taxon ofLuskhan withinBrachaucheninae andP. almanzaensis within the monophyleticPliosaurus, the latter analysis of which is reproduced below:[28]
Media related toPliosaurus at Wikimedia Commons
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