Pliosaurus (meaning 'more lizard') is an extinct genus of thalassophonean pliosaurid known from the Late Jurassic (Kimmeridgian and Tithonian stages) of Europe and South America.

The specimens were collected by Prof. William Buckland[9] at Market Rasen, Lincolnshire, from the Rasenia cymodoce ammonite zone of the lower Kimmeridge Clay Formation, dating to the early Kimmeridgian stage.

The ontogenetic stage of the holotype of P. 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.

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.

Housed at the University of Oslo Natural History Museum, the skeletons were found south of Sassenfjorden, from the southeast side of Mount Knerten, in the Arctic Spitsbergen island.

Due to the Arctic climate of Svalbard, the specimens were subjected to repeated freeze-thaw cycles before collection, extensively fracturing and degrading the material.

[5] 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".

[13][14] Morphological and histological characters, such as the presence of a tuberosity on the humerus 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.

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 to P. funkei.

[16] P. kevani is known solely from the holotype DORCM G.13,675, a nearly complete 2.0 metres (6.6 ft) long skull and mandible housed at the Dorset Museum, Dorchester, England.

The specimen was collected at Wyke Siltstone bed of Osmington Bay, from the Rasenia cymodoce ammonite zone, Kimmeridge Clay Formation of the Ancholme Group, dating to the early Kimmeridgian.

They were first identified as a pliosaurid skull by Richard Edmonds, Earth Sciences Manager for Dorset and East Devon Coast World Heritage Site Team.

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 in 2013.

The specific 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.

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.

The holotype of P. rossicus, PIN 304/1 housed at Paleontological Institute, Russian Academy of Sciences, consists of cranial and some postcranial remains of a relatively small pliosaur.

Halstead (1971) reassigned this species to Liopleurodon based on this symphyseal tooth count,[19] but Knutsen (2012) and Benson et al. (2013) referred it back to Pliosaurus, as it exhibits the diagnostic traits of the genus such as trihedral teeth.

[1] PIN 304/1 has also been interpreted as a juvenile by both Halstead (1971) and Storrs et al. (2000) based on its relatively small size and poorly developed dorsal blade and anteroventral ramus of the scapula.

grandis by Rozhdestvenskii (1947), but later referred to P. rossicus based on the presence of a similar number of mandibular symphyseal teeth with the holotye and their relative stratigraphic co-occurrence.

[1] 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 to P.

[1] Nicknamed "Westbury pliosaur I", it was first described by Taylor & Cruickshank (1993) and referred to P. brachyspondylus based on the absence of a ventral keel on the cervical vertebral centra.

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.

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 other pliosaurids as well.

[2][1] According to Knutsen (2012), Bardet et al. (1993) referred two additional mandibles to P. brachyspondylus based solely on a similar number of dentary and symphyseal teeth; BHN 2R.370, collected at the Moulin-Wibert quarry, from the Rasenia cymodoce ammonite zone of the Calcaires de Moulin-Wibert Formation of Nord-Pas-de-Calais, and MNHN cat.24.1 collected at Le Havre, Normandy, both from the early Kimmeridgian of France.

[2] Benson et al. (2013) discussed only BHN 2R.370 originally referred to Pliosaurus grandis, stating that P. carpenteri has a similar count of dentary and symphysial teeth, and thus the specimen cannot be identified to species level.

[1] Pliosaurus macromerus was first described and named by John Phillips in 1871, as a species of Pleiosaurus, on the basis of a large femur, OUMNH J.12498, and a series of unassociated vertebrae.

Lydekker (1889a) amended the Pleiosaurus macromerus into Pliosaurus, and reassigned NHMUK PV OR 39362 to this species with other material, based solely on large size.

The holotype, PIN 426 housed at Paleontological Institute, Russian Academy of Sciences, consists of partial cranium and postcranium of a medium-sized individual.

These assignments were criticized by other workers, including Tarlo (1960), Halstead (1971) and Storrs et al. (2000), because of the general and non-specific diagnosis for Strongylokrotaphus, and differences from the type species of Peloneustes.

[6][1] 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.