Pliosaurus

In 1824, William Daniel Conybeare established a new species of the genus Plesiosaurus, Plesiosaurus giganteus, to include all plesiosaurian specimens with shortened cervical vertebrae. Among the specimens that were classified in this taxon was a partial skeleton discovered in Market Rasen, Lincolnshire, England, collected by William Buckland.^[6] This specimen has since been stored in the Oxford University Museum of Natural History, where it has since been catalogued as OUMNH J.9245. Other fossils associated with this same individual are also stored in the museum, but their records tend to vary depending on the studies describing them since.^[5][1][4] Together it consists of teeth, a mandible, upper jaw, a partial spinal column, a femur, a tibia, and a fibula.^[7][5] Its ontogenetic stage is unknown, but the incomplete ossification of the proximal convexity of the tibia suggests that it is a juvenile or subadult specimen.^[4] In his article, Conybeare only refers specimens to this species, designating no type specimens and giving no detailed anatomical descriptions.^[6] As the referred specimens were then insufficiently characterized, the name Plesiosaurus giganteus was generally perceived as invalid in subsequent works,^[5] having since become a nomen oblitum.^[4]

In 1841, Richard Owen described the anatomy of the jaws of the Market Rasen specimen. Based on several distinctive features, he decided to make it the holotype of a subgenus accompanying a new species of Plesiosaurus, which he named Plesiosaurus (Pleiosaurus) brachydeirus.^[8] The name Pleiosaurus comes from the Ancient Greek πλειων (pleion, "more"), and σαῦρος (saûros, "lizard"),^{[9]: 564, 630 [10]} Owen named it this way because the appearance of the specimen described is closer to those of crocodilians than to those of other species then attributed to Plesiosaurus.^[8][11] The specific name brachydeirus also comes from Ancient Greek and comes from the words βραχύς (brakhús, "short"), and δειρή (deirḗ, "neck" or "throat"),^{[9]: 135, 153} in reference to its cervical vertebrae.^[8] Later that same year, Owen described the postcranial parts of the skeleton and relegated this taxon to a separate genus, but spelling it as Pliosaurus.^[11] In 1869, the same author erroneously claimed that another species formerly attributed to Pliosaurus, P. grandis, would be the type species of this genus.^[12] In 1871, John Phillips corrected most of Owen's taxonomic errors, recognizing P. brachydeirus as the type species and using the original spelling Pleiosaurus.^[7] Furthermore, the holotypic material of P. grandis is considered by many authors to be non-diagnostic and cannot be proven to belong to the genus.^[13][5][1] However, despite Phillips's requirement, the genus name Pliosaurus has since entered into universal usage and must be maintained according to the rule of article 33.3.1 of the ICZN.^[4] Currently, only the holotype and the fossils attributed to it constitute the only known specimen of this species.^[1][4]

In 1948, Nestor Novozhilov named the species P. rossicus on the basis of two more or less partial specimens discovered in two mines in the Lower Volga Basin in Russia (hence the name), and which have since been housed in the paleontological collections of the country's scientific academy. The holotype consists of a skull and postcranial remains from a relatively small specimen,^[14] since catalogued as PIN 304/1. This same specimen, which was originally a complete skeleton, was largely destroyed ten years earlier due to the exploitation of the oil shale from which it was discovered.^[15] However, pectoral elements associated with the latter are described by the same author in 1964.^[16] Based on its small size and poorly developed anatomical elements on the scapula, the holotype is interpreted as a juvenile.^[17][15][1] The second specimen, more imposing and since catalogued as PIN 2440/1,^[15][1] was discovered in May 1945 as a complete skeleton, but like the holotype, it was largely destroyed by mining operations. The only surviving remains of the latter are a rostrum, a proximal part of a humerus, a phalanx, and ribs fragments, which were originally described in 1947 as coming from a P. grandis by Anatoly Rozhdestvensky.^[18] In 1971, Beverly Halstead reclassified this species in the genus Liopleurodon because of its short mandibular symphysis (where the two halves of the lower jaw connect), and assigned the second specimen a complete hindlimb.^[17] However, it later turned out that this hindlimb actually originates from the holotype of the contemporary species P. irgisensis,^[15] which has since been recognised as a dubious.^[1][4] Although Halstead's classification was long recognised as valid, it was questioned in a 2001 thesis by Leslie F. Noè, who noted that, due to the shape of the teeth and the length of the mandibular symphysis, the species might represent a new genus.^[19] In a revision published in 2012, Espen M. Knutsen nevertheless reassigned the species to its original genus on the basis of diagnostic features shared with other lineages within the genus.^[1]

The species P. funkei was described in 2012 by Knutsen and colleagues based on two large specimens discovered in the Norwegian archipelago of Svalbard. The holotype, catalogued as PMO 214.135, consists of a partial skeleton preserving the anterior part of the jaws with teeth, various more or less preserved vertebrae, a complete right coracoid, a nearly complete right flipper, as well as ribs and gastralia (abdominal ribs). The larger referred specimen, catalogued as PMO 214.136, includes a partial skull preserving mostly its left posterior part, a few vertebrae and several unidentifiable fragmentary bones. Based on various morphological and histological characteristics, particularly in terms of increased bone density, these two specimens appear to have been adults. The fossils were collected at 2 km (1.2 mi) intervals during eight seasons of fieldwork conducted from 2004 to 2012 by Jørn Hurum in the southern Sassenfjorden.^[20] More precisely, they were discovered in 2006, excavated over the following two years,^[21] and officially reported at a Society of Vertebrate Paleontology conference in 2009, where their affinity with the genus Pliosaurus was already noted.^[22][23] Due to the Arctic climate of Svalbard, the specimens were subjected to repeated freeze-thaw cycles before collection, extensively fracturing and degrading the material. The specific name honours Bjørn Funke, the discoverer of the holotype, and his wife May-Liss Knudsen Funke, for their years of voluntary service to the paleontological collections of the University of Oslo Natural History Museum, where the specimens have since been housed. In their article, Knutsen and colleagues conclude that it cannot be definitively excluded that P. funkei and P. rossicus would represent different ontogenetic stages of the same species based on the available material. Nevertheless, the two taxa are still maintained as distinct on the basis that their respective holotypes exhibit proportionally very different humeral lengths.^[a][20] The rather remarkable size of the fossils led the Hurum's paleontological teams to nickname them "The Monster" for the holotype and "Predator X" for the referred specimen, which gave the taxon significant media coverage even before they were firmly described.^[24][23][25] This popularity led the species to appear in a 2009 documentary broadcast on the American television channel History, then in a 2010 low-budget science fiction horror film.^[25]

In a 2013 paper published in the mega journal PLOS ONE, Roger B. J. Benson and colleagues described three additional species of Pliosaurus, all of whose fossils were discovered in England. The first of these three species described was P. kevani, whose holotype consists of a large, nearly complete skull of a probable adult, which was discovered in a cliff at Osmington Mills Bay, Dorset. This specimen was collected over a period of eight years via fragmented parts weighing up to 60 kg (130 lb). Most of the fossils comprising the skull were taken without prior permission from loose or fallen boulders, while others were collected locally and purchased from landowners. The specimen was quickly identified as a pliosaurid by geologist Richard Edmonds.^[4] The remarkable size and completeness of this specimen, like "The Monster" and "Predator X", gave it widespread media coverage, to the point of earning the nickname "Weymouth Bay pliosaur" before its official description.^[23] The Dorset Museum's acquisition of the specimen was publicly announced in 2009, where it has since been catalogued as DORCM G.13,675. However, it was not officially opened by David Attenborough until July 2011. This discrepancy is due in part to the time taken to prepare the fossils, which took 200 hours for the lower jaw and over 365 hours for the rest of the skull. The specific name for this taxon honours Kevan Sheehan, owner of a small cafe overlooking the sea at Osmington Mills, who collected most of the holotype specimen during daily walks along the shore. In addition to the holotype skull, Benson and colleagues tentatively assigned two other large pliosaurid specimens discovered in the Cambridgeshire as P. cf. kevani os the basis of their tooth morphology. The first consists of a large, mostly postcranial skeleton catalogued as CAMSM J.35990, discovered in Stretham, while the second is a single tooth catalogued as LEICT G418.1965.108, discovered in Ely, which are stored at the Sedgwick Museum of Earth Sciences and the Leicester Museum & Art Gallery respectively.^[4]

The other two species described in the 2013 paper are P. westburyensis and P. carpenteri, both of whose holotypes were discovered in a quarry near Westbury, Wiltshire, and were subsequently donated to the Bristol Museum & Art Gallery, where they have since been catalogued as BRSMG Cc332 and BRSMG Cc6172, respectively. The holotype of the first species consists of a skull with some postcranial remains, while the second consists of a partial skeleton,^[4] respectively discovered on July 2, 1980,^[26] and May 12, 1994.^[27][28] Informally dubbed as the "first Westbury pliosaur",^[29][28] BRSMG Cc332 was first described in a 1993 paper by Michael A. Taylor and Arthur Cruickshank, in which they identified it as a specimen of P. brachyspondylus.^[26] The first anatomical description of BRSMG Cc6172 was not carried out until much later, by Judyth Sassoon and colleagues in 2012,^[28] although some historical details relating to its excavation had already been published in earlier works.^[27][29] In their publication, these two specimens are both classified in Pliosaurus, although without a species assignment,^[28] a point of view also followed in the genus revision conducted by Knutsen in the same year.^[1] Based on their morphological differences and their close stratigraphic levels within the same quarry, Sassoon and colleagues described these specimens as exhibiting intraspecific variation, and possibly sexual dimorphism within the same species, interpreting BRSMG Cc332 as a young male and BRSMG Cc6172 as an old female.^[28] However, in the article published the following year, Benson and his colleagues consider that the differences between them are relatively significant when observed in the context of specimens from other localities, and therefore justify a specific distinction. The specific name of the first cited species refers to the town of Westbury, while that of the second honours Simon Carpenter, discoverer of the holotype of the latter species.^[4] From June 2017 to February 2018, the holotype specimen of P. carpenteri, then nicknamed "Doris", was temporarily mounted as part of a special exhibition at Bristol Museum & Art Gallery.^[30]

Plesiosaurs are usually categorized as belonging to the small-headed, long-necked "plesiosauromorph" morphotype or the large-headed, short-necked "pliosauromorph" morphotype, Pliosaurus belonging to the latter category.^[31][4] Like all other plesiosaurs, it had a short tail, a barrel-shaped body, and all of its limbs modified into large flippers.^[32]

P. funkei and P. kevani are the largest known species of the genus and are among the largest pliosaurs ever discovered in the world.^{[20][25][4][33]} Even before their fossils were formally described in the scientific literature, the media estimated their lengths as ranging from 13 to 15 m (43 to 49 ft) for the P. funkei specimens and from 12 to 16 m (39 to 52 ft) for the holotype of P. kevani.^[23] However, more serious estimates published from 2012 reduce these estimates by 10 to 13 m (33 to 43 ft) for a skull exceeding 2 m (6 ft 7 in) in length.^[20][4] In 2023, David Martill and colleagues described four posterior cervical vertebrae from a large specimen putatively assigned as Pliosaurus sp., discovered in the Kimmeridge Clay of Abingdon, Oxfordshire. Based on comparisons made with other pliosaurid genera, the authors gave a body length estimate of 9.8 to 14.4 m (32 to 47 ft).^[32] However, in 2024, Ruizhe Jackevan Zhao reduced the size of these estimates. By comparing the specimens attributed to P. funkei and P. kevani, he concluded that the latter would not be significantly different in terms of measurements, giving rise to an estimate of the body length of 9.8 to 10.3 m (32 to 34 ft) for a body mass of approximately 12 t (12 long tons; 13 short tons). Regarding the cervical vertebrae discovered in Abingdon, the author acknowledges that they come from a larger individual, but which would have measured between 10.7 and 11.8 m (35 and 39 ft).^{[34]: 36–38}

Few estimates have been given for other species. The immature status of the holotype specimen of P. brachydeirus suggests that it could have reached larger measurements as an adult.^[4] The temporary skeletal mount of the holotype of P. carpenteri suggests a size of about 8 m (26 ft) long for a skull reaching 1.8 m (5 ft 11 in).^[30] The holotype skull of P. westburyensis is 1.7 m (5 ft 7 in) long, making it slightly smaller than P. carpenteri.^[28][4] In his 2024 publication, Zhao suggests that P. rossicus could have reached measurements similar to those of P. funkei and P. kevani.^[34]: 39

One of the main identifying features of Pliosaurus is that the teeth are often trihedral in cross-section, with flat and smooth labial surfaces (the side facing lips), but convex lingual surfaces (the side facing the tongue) bearing longitudinal enamel ridges. However, P. kevani is the only recognized species in which the teeth are sub-trihedral in shape, possessing slightly flattened labial surfaces with finely distributed enamel ridges. In some species such as P. kevani and P. carpenteri, the premaxillary teeth are anisodont, meaning they vary in size, which is not the case in P. brachydeirus or P. westburyensis.^[1][20][4]

The different species of Pliosaurus can also be distinguished by the number of teeth present in the premaxillae, maxillae (the latter two sometimes bearing caniniform dentition), the mandibular symphysis, and the dentary as a whole. However, since not all fossils are sufficiently well preserved, the dental counts of some species remain uncertain: P. brachydeirus possessed four to six premaxillary teeth, twenty-two or more maxillary teeth, thirty-five to thirty-seven dentary teeth, and more than seven to thirteen pairs of symphyseal teeth; P. rossicus had six premaxillary teeth and six pairs of symphyseal teeth; P. funkei had at least five premaxillary teeth and six pairs of symphyseal teeth; P. kevani had six premaxillary teeth, about twenty-five maxillary teeth, more than twenty-eight to thirty-seven dentary teeth, and more than six to fifteen pairs of symphyseal teeth; P. westburyensis had more than five premaxillary teeth, about twenty-five maxillary teeth, more than twenty-one dentary teeth, and possibly nine pairs of symphyseal teeth; and P. carpenteri had six premaxillary teeth, more than twenty-seven maxillary teeth, twenty-seven dentary teeth, and eight to nine pairs of symphyseal teeth.^[1][20][4]

The vast majority of recognized Pliosaurus species do not preserve any sufficiently complete postcranial skeletons, making comparisons difficult. A possible exception is P. kevani, one of whose referred specimens consists of a large postcranial skeleton. However, this skeleton cannot be diagnosed with certainty as belonging to a distinct or already known species, despite its current attribution.^[4] Thus, the postcranial anatomy of Pliosaurus is primarily known from vertebrae and limb elements documented in the holotype specimens of P. brachydeirus, P. funkei, and P. carpenteri, although rare diagnostic postcranial elements have also been reported in P. rossicus and P. westburyensis.^[1][20][4]

The cervical vertebrae of Pliosaurus are massive, short, and possess flattened centra that are subcircular to slightly oval in shape. The ventral surface of these vertebrae is an important criterion for distinguishing species. In P. brachydeirus, the ventral surface of the cervical vertebrae is marked by a pronounced longitudinal ridge, a feature absent in P. westburyensis and P. carpenteri. The cervical vertebrae of P. rossicus and P. funkei have rugose ventral surfaces lacking median ridges.^[1][20][4] Like other pliosaurids that lived during the Jurassic, Pliosaurus likely possessed double-headed cervical ribs.^[5][1][20] Like other pliosaurids, the pectoral vertebrae of P. funkei bear articulations for their respective ribs, partially on both the centra and neural arches. The dorsal vertebrae are approximately as long as they are wide, in contrast to the cervical vertebrae.^[20] Although the caudal vertebrae are poorly known in the genus, with only one documented in P. brachydeirus,^[5] they likely supported a tail fin, as documented in other plesiosaurs.^[35][36]

The few known elements of the pectoral girdle in the genus are currently documented only in P. rossicus and P. funkei.^[16][20] These elements are also present in P. carpenteri, but they have not been described in detail in studies concerning this species.^[29][4][30] The scapula of P. rossicus features a dorsally directed process, and the longitudinal axes of the scapulocoracoid openings intersect at an angle greater than 140°.^[16] The coracoid of P. funkei is among the largest identified in pliosaurids. In dorsal view, it is longer than wide, with its width tapering anteriorly. This coracoid also has a more elongate anteromedial process than that of P. rossicus, although this may be explained by ontogenetic factors.^[20]

Limb proportions also vary between species. For example, P. funkei is distinguished by particularly elongated forelimbs, which could reach up to 3 metres (9.8 ft), with humeri whose length exceeds seven times the average width of the cervical vertebrae. In comparison, P. rossicus has proportionally shorter limbs, with humeri less than 4.5 times the width of the cervical vertebrae. The radius and ulna of P. funkei are nearly equal in proportions, being about twice as long as they are wide. The phalanges are hourglass-shaped, becoming shorter and narrower distally.^[1][20] The tibia and fibula of P. brachydeirus are shorter than they are wide, a feature generally observed only in polycotylids.^[5] In P. carpenteri, the radius, ulna, tibia, and fibula are characterized by highly convex proximal articular surfaces.^[4]

Although being the type genus of pliosaurids, Pliosaurus was for nearly two centuries a poorly understood taxon due to the lack of an adequate description of the holotype of P. brachydeirus.^[28] This led in particular to many pliosaur genera since recognised as distinct being sometimes considered species, or even junior synonyms, of Pliosaurus. In 1960, Halstead (then called Tarlo) revised the taxonomy of Late Jurassic pliosaurids, making the first revision of the type genus. In his paper, he considered the following three species as valid: P. brachydeirus, P. brachyspondylus, and the newly described P. andrewsi.^[5] Simultaneously with the publication of the official description of P. funkei in 2012,^[20][21] Another article written by Knutsen alone and concerning the taxonomic revision of the genus Pliosaurus as a whole is published. According to him, the species P. brachydeirus, P. brachyspondylus, P. macromerus, P. rossicus^[b] and P. funkei are valid. However, he maintains the validity of both species P. brachyspondylus and P. macromerus on the basis of proposed neotypes, their original type specimens being deemed non-diagnostic. P. andrewsi, which was long considered a valid species of the genus, turns out to have too many morphological differences to be placed in Pliosaurus.^[1] In 2013, Benson and colleagues recognised the validity of the following six species: P. brachydeirus, P. rossicus, P. funkei, P. kevani, P. westburyensis, and P. carpenteri. As no formal petition to ICZN was made to designate the neotypes of P. brachyspondylus and P. macromerus, these two species are then considered as dubious.^[4]

In 1874, Harry Govier Seeley named a new family of plesiosaurs, Pliosauridae, to contain forms similar to Pliosaurus.^[37] Exactly how pliosaurids are related to other plesiosaurs is uncertain. In 1940, palaeontologist Theodore E. White considered pliosaurids to be close relatives of Elasmosauridae based on shoulder anatomy.^[38] However, in 1943, Samuel P. Welles thought that pliosaurids were more similar to Polycotylidae, as they both had large skulls and short necks, among other characteristics. He grouped these two families into the superfamily Pliosauroidea, with other plesiosaurs forming the superfamily Plesiosauroidea.^[39][40] Another plesiosaur family, Rhomaleosauridae, has since been assigned to Pliosauroidea,^[41][42] while Polycotylidae has been reassigned to Plesiosauroidea.^[43][44] However, in 2012, Benson and colleagues recovered a different topology, with Pliosauridae being more closely related to Plesiosauroidea than Rhomaleosauridae. This pliosaurid-plesiosauroid clade was termed Neoplesiosauria.^[44]

In 1960, Halstead considered Pliosaurus to be a close relative of Peloneustes, since both taxa had elongated mandibular symphyses.^[5] In 2001, F. Robin O’Keefe recovered Pliosaurus as the sister taxon of Brachauchenius.^[42] However, in 2008, Adam S. Smith and Gareth J. Dyke considered Pliosaurus to be the sister taxon of Peloneustes.^[41] In 2012, Patrick S. Druckenmiller and Knutsen recovered the genus Pliosaurus as a monophyletic group comprising the species P. brachydeirus, P. rossicus, P. funkei, P. brachyspondylus, and P. macromerus, although their cladogram also included an unspecified specimen catalogued as NHMUK R2439.^[45] In 2013, Benson and Druckenmiller named a new clade within Pliosauridae, Thalassophonea. This clade included the "classic", short-necked pliosaurids while excluding the earlier, long-necked, more gracile forms. Since the publication of this study, Pliosaurus has since been seen as being related to Gallardosaurus.^[46] In the same year, in order to keep the genus Pliosaurus as monophyletic again, Benson and colleagues removed Gallardosaurus, P. rossicus and its potential junior synonym P. irgisensis from their cladogram.^[4] In 2014 and 2018, two new species of Pliosaurus whose fossils were discovered in the Vaca Muerta Formation, Argentina, were respectively described under the names of P. patagonicus^[47] and P. almanzaensis.^[48] However, subsequent work finds that these two taxa do not appear to form a monophyletic grouping,^[49] a 2023 paper even classifying P. patagonicus among the Brachaucheninae, a subgroup of thalassophoneans whose representatives mainly lived during the Cretaceous.^[50]

The following cladogram follows Fischer et al. (2023),^[50] although the species P. brachyspondylus, P. macromerus and P. irgisensis are considered doubtful.^[4]

Plesiosaurs were well-adapted to marine life.^[51][52] They grew at rates comparable to those of birds and had high metabolisms, indicating homeothermy^[53] or even endothermy.^[51] A 2019 study by palaeontologist Corinna Fleischle and colleagues found that plesiosaurs had enlarged red blood cells, based on the morphology of their vascular canals, which would have aided them while diving.^[52] Plesiosaurs such as Pliosaurus employed a method of swimming known as subaqueous flight, using their flippers as hydrofoils. Plesiosaurs are unusual among marine reptiles in that they used all four of their limbs, but not movements of the vertebral column, for propulsion. The short tail, while unlikely to have been used to propel the animal, could have helped stabilise or steer the plesiosaur.^[35][36] Computer modelling by Susana Gutarra and colleagues in 2022 found that due to their large flippers, a plesiosaur would have produced more drag than a comparably-sized cetacean or ichthyosaur. However, plesiosaurs counteracted this with their large trunks and body size.^[54] Due to the reduction in drag by their shorter, deeper bodies, Judy Massare proposed in 1988 that plesiosaurs could actively search for and pursue their food instead of having to lie in wait for it.^[35]

Pliosaurus is interpreted by palaeontologists as a marine predator at the top of the food chain, with powerful cranial musculature that gave it an exceptionally strong bite, although its skull was relatively weak against twisting or lateral bending. Unlike basal thalassophoneans such as Peloneustes, which were better adapted to small, mobile prey, Pliosaurus appeared to favor a predatory strategy based on short, targeted bites, delivered to the back of the jaw where the force of pressure was greatest. This anatomical configuration suggests that it avoided violently shaking or twisting its prey, which could compromise the integrity of its skull. It likely captured a wide variety of marine prey, ranging from medium-sized fish to smaller marine reptiles, which it subdued by firmly immobilizing with its robust jaws before crushing or swallowing them in dismemberment. This strategy of combining muscular power and attack precision allowed it to adapt to a wide range of prey, characteristic of a generalist predator of the Jurassic seas.^{[55][26][4][33]}

In 2014, two studies conducted by Davide Foffa and his colleagues were published on biomechanical and CT analyses carried out on the holotype skull of P. kevani.^[56][33] The first study published that year focused on the specimen's rostral neurovascular network, with the authors reconstructing a complex system of vascular and nerve canals, preserved by a filling of sediment and pyrites, revealing ramifications of varying sizes up to 23 mm (0.91 in) in diameter. This architecture suggests a high degree of sensitivity in the anterior region of the skull, probably related to sensory or trophic functions, such as detecting prey in an aquatic environment via electroperception.^[56] The second study focuses on bite force. By modeling the mandibular musculature and reconstructing the geometry of the skull using computed tomography and finite element analysis techniques, the researchers determined that the force exerted ranged from 9,600 to 48,000 newtons (2,160 to 10,790 lbf) depending on the area of the lower jaws, a force comparable to, or even greater than, that of the largest living crocodilians. Since this power was concentrated in the back of the jaw, this suggests that P. kevani used a biting strategy designed to puncture or crush robust prey. However, despite this impressive force, the cranial structure has certain weaknesses when faced with bending or lateral torsional stresses. This also indicates that P. kevani did not kill its prey by violent jerks, but by a direct and powerful bite, optimized to quickly incapacitate large prey without excessive head movements.^[33] Such behavior was also suggested for the holotype of P. westburyensis in 1993.^[26] A Pliosaurus-like pliosaur has also been suggested to have inflicted bite marks upon an indeterminate ophthalmosaurid ichthyosaur by Nikolay Zverkov and colleagues in 2015.^[57]

P. brachydeirus, P. kevani, P. westburyensis, P. carpenteri and a possible specimen of P. rossicus are known from the Kimmeridgian and Tithonian stages of the Upper Jurassic in the Kimmeridge Clay Formation, England.^[4] This formation was deposited in a deep-sea marine environment reaching about 150 to 200 m (490 to 660 ft) depth, known as the Jurassic Sub-Boreal Seaway.^[58][59][60] Known invertebrates are mainly represented by ammonites and crustaceans.^[61]

In Russia, the first two known specimens of P. rossicus were discovered in Tithonian rocks of the Lower Volga Basin.^[18][14][16] Due to the abundant presence of the ammonite Dorsoplanites panderi in the type locality of the taxon, the stratigraphic unit thus bears this name.^[15][1][62] Little is known or published about Volga fossils from this period, although a fairly large number of invertebrates have been recorded. These include ammonites, bivalves, radiolarians, and dinoflagellates.^[63] Except P. rossicus and the dubious P. irgisensis,^[1] fossils of several contemporary marine reptiles have been discovered, including several species of ichthyosaurs in the region, notably the ophthalmosaurids Arthropterygius, Grendelius, Nannopterygius and Undorosaurus.^[57][64][65] In addition, fossils belonging to a metriorhynchid thalattosuchian are also known from contemporary sediments in this region.^[63] In 2015, a single tooth discovered at the summit of Mount Sheludivaya [ru] in Crimea was attributed to Pliosaurus by Zverkov on the basis of the typical trihedral shape of the genus. Dating to the late Valanginian of the Early Cretaceous, it represents the most recent known occurrence of the genus in the fossil record. Prior to this find, brachauchenines were the only pliosaurids thought to have persisted across the Jurassic–Cretaceous boundary.^[2][66]

In Svalbard, P. funkei is known from Tithonian-aged rocks of the Slottsmøya Member of the Agardhfjellet Formation.^{[22][1][20][45]} This unit consists of a mix of shales and siltstones and was deposited in a shallow water methane seep environment.^[21] The seafloor, which was located about 150 m (490 ft) below the surface, seems to have been relatively dysoxic, or oxygen-poor, although it was periodically oxygenated by clastic sediments.^[67] Despite this, near the top of the member, various diverse assemblages of invertebrates associated with cold seeps have been discovered; these include ammonites, lingulate brachiopods, bivalves, rhynchonellate brachiopods, tubeworms, belemnoids, tusk shells, sponges, crinoids, sea urchins, brittle stars, starfish, crustaceans and gastropods.^[68] Though direct evidence from Slottsmøya is currently lacking, the high latitude of this site and relatively cool global climate of the Tithonian mean that sea ice was likely present at least in the winter.^[69][70] A considerable number of plesiosaurs and ichthyosaurs are known from the Slottsmøya Member. As a large apex predator, P. funkei may have included some of them in its diet.^[20][21][67] Aside from P. funkei itself, the other plesiosaurs reported are Colymbosaurus, Djupedalia, Ophthalmothule, and Spitrasaurus, all of which belong to the family Cryptoclididae.^[67][71] The contemporary ichthyosaurs known from the Slottsmøya Member are ophthalmosaurids of the genera Arthropterygius, Brachypterygius, Undorosaurus, and Nannopterygius.^{[72][64][73][65]} Many of the fossils of these marine reptiles are preserved in three dimensions and partially articulated, a condition correlated with the high abundance of organic material in the sediments in which they were buried, as well as the absence of local invertebrates.^[67]

• List of plesiosaur genera
• Timeline of plesiosaur research