Plesiosauroidea is an extinct clade of carnivorous marine reptiles. They have the snake-like longest neck to body ratio of any reptile. Plesiosauroids are known from the Jurassic and Cretaceous periods. After their discovery, some plesiosauroids were said to have resembled "a snake threaded through the shell of a turtle", although they had no shell.
The Early Jurassic Epoch is the earliest of three epochs of the Jurassic Period. The Early Jurassic starts immediately after the Triassic-Jurassic extinction event, 201.3 Ma, and ends at the start of the Middle Jurassic 174.1 Ma.
Liopleurodon is an extinct genus of large, carnivorous marine reptile belonging to the Thalassophonea, a clade of short-necked pliosaurid plesiosaurs. Liopleurodon lived from the Callovian Stage of the Middle Jurassic to the Kimmeridgian stage of the Late Jurassic Period. It was the apex predator of the Middle to Late Jurassic seas that covered Europe. The largest species, L. ferox, is estimated to have grown up to 6.6 metres (22 ft) in length based on the largest known skull.
The Oxford Clay is a Jurassic marine sedimentary rock formation underlying much of southeast England, from as far west as Dorset and as far north as Yorkshire. The Oxford Clay Formation dates to the Jurassic, specifically, the Callovian and Oxfordian ages, and comprises two main facies. The lower facies comprises the Peterborough Member, a fossiliferous organic-rich mudstone. This facies and its rocks are commonly known as lower Oxford Clay. The upper facies comprises the middle Oxford Clay, the Stewartby Member, and the upper Oxford Clay, the Weymouth Member. The upper facies is a fossil poor assemblage of calcareous mudstones.
Cryptoclidus is a genus of plesiosaur reptile from the Middle Jurassic period of England, France, and Cuba.
Elasmosauridae is an extinct family of plesiosaurs, often called elasmosaurs. They had the longest necks of the plesiosaurs and existed from the Hauterivian to the Maastrichtian stages of the Cretaceous, and represented one of the two groups of plesiosaurs present at the end of the Cretaceous alongside Polycotylidae. Their diet mainly consisted of crustaceans and molluscs.
Muraenosaurus is an extinct genus of cryptoclidid plesiosaur reptile from the Oxford Clay of Southern England. The genus was given its name due to the eel-like appearance of the long neck and small head. Muraenosaurus grew up to 5.2 metres (17 ft) in length and lived roughly between 160 Ma and 164 Ma in the Callovian of the middle Jurassic. Charles E. Leeds collected the first Muraenosaurus which was then described by H. G. Seeley. The specimen may have suffered some damage due to the casual style of Charles Leeds’ collection. The first muraenosaur was recovered with pieces missing from the skull and many of the caudal vertebrae absent. Because the animal was described from Charles Leeds’ collection it was given the name Muraenosaurus Leedsi. M. leedsi is the most complete specimen belonging to the genus Muraenosaurus and also the only species that is undoubtedly a member of the genus. Two other species have been tentatively referred to as members of the genus Muraenosaurus: M. reedii and Muraenosaurus beloclis Seeley 1892, which in 1909 became the separate genus Picrocleidus.
Peloneustes is a genus of pliosaurid plesiosaur from the Middle Jurassic of England. Its remains are known from the Peterborough Member of the Oxford Clay Formation, which is Callovian in age. It was originally described as a species of Plesiosaurus by palaeontologist Harry Govier Seeley in 1869, before being given its own genus by naturalist Richard Lydekker in 1889. While many species have been assigned to Peloneustes, P. philarchus is currently the only one still considered valid, with the others moved to different genera, considered nomina dubia, or synonymised with P. philarchus. Some of the material formerly assigned to P. evansi have since been reassigned to "Pliosaurus" andrewsi. Peloneustes is known from many specimens, including some very complete material.
Polycotylidae is a family of plesiosaurs from the Cretaceous, a sister group to Leptocleididae. Polycotylids first appeared during the Albian stage of the Early Cretaceous, before becoming abundant and widespread during the early Late Cretaceous. Several species survived into the final stage of the Cretaceous, the Maastrichtian.
Leptocleidus is an extinct genus of plesiosaur, belonging to the family Leptocleididae. It was a small plesiosaur, measuring only up to 3 m (9.8 ft).
Simolestes is an extinct pliosaurid genus that lived in the Middle to Late Jurassic. The type specimen, BMNH R. 3319 is an almost complete but crushed skeleton diagnostic to Simolestes vorax, dating back to the Callovian of the Oxford Clay formation, England. The genus is also known from the Callovian and Bajocian of France (S.keileni), and the Tithonian of India (S.indicus). The referral of these two species to Simolestes is dubious, however.
Colymbosaurus is a genus of cryptoclidid plesiosaur from the Late Jurassic (Callovian-Tithonian) of the UK and Svalbard, Norway. There are two currently recognized species, C. megadeirus and C. svalbardensis. Both species are relatively medium-sized plesiosaurs, reaching 5 metres (16 ft) in length and 700 kilograms (1,500 lb) in body mass.
Pantosaurus is an extinct genus of plesiosaur from the Late Jurassic (Oxfordian) of what is now Wyoming. It lived in what used to be the Sundance Sea. It was originally named Parasaurus by Othniel Charles Marsh in reference to Plesiosaurus, but that name was preoccupied, and Marsh changed it. The species Muraenosaurus reedii is in fact a junior synonym of Pantosaurus. The holotype YPM 543 is a partial articulated skeleton, partially prepared to yield a distal humerus, four articulated carpals, a fragment of the coracoid, and several isolated cervical vertebrae from the Upper Member of the Sundance Formation. Other material includes USNM 536963, USNM 536965, UW 3, UW 5544 and UW 15938.
Leptocleididae is a family of small-sized plesiosaurs that lived during the Early Cretaceous period. They had small bodies with small heads and short necks. Leptocleidus and Umoonasaurus had round bodies and triangle-shaped heads. Leptocleidids have been found in what were shallow nearshore, freshwater and brackish habitats. Hilary F. Ketchum and Roger B. J. Benson (2010), transferred Brancasaurus, Kaiwhekea, Nichollssaura and Thililua to this family. However, Ketchum and Benson (2011) reassigned Kaiwhekea and Thililua to their original positions, as an elasmosaurid and a polycotylid, respectively.
Hauffiosaurus is an extinct genus of Early Jurassic pliosaurid plesiosaur known from Holzmaden of Germany and from Yorkshire of the United Kingdom. It was first named by Frank Robin O’Keefe in 2001 and the type species is Hauffiosaurus zanoni. In 2011, two additional species were assigned to this genus: H. longirostris and H. tomistomimus.
Eurycleidus is an extinct genus of large-bodied rhomaleosaurid known from the Early Jurassic period of the United Kingdom. It contains a single species, E. arcuatus. Like other plesiosaurs, Eurycleidus probably lived on a diet of fish, using its sharp needle-like teeth to catch prey. Its shoulder bones were fairly large, indicating a powerful forward stroke for fast swimming.
Bobosaurus is an extinct genus of sauropterygian reptile related to plesiosaurs. It is based on the holotype MFSN 27285, a partial skeleton found in Early Carnian-age rocks of the Rio del Lago Formation, northeastern Italy. Bobosaurus was named in 2006 by Fabio M. Dalla Vecchia and the type species is B. forojuliensis. It may be a pistosaurid, or closer to Plesiosauria. A recent cladistic analysis found it to be a pistosaur. It was relatively large animal, with more than 3 m (9.8 ft) in length.
Tricleidus is an extinct genus of cryptoclidid plesiosaur known from only specimen from the middle Jurassic of United Kingdom. It was first named by Andrews in 1909 and the type species is Tricleidus seeleyi. It was a relatively medium-sized plesiosaur, measuring 5 metres (16 ft) long and weighing 700 kilograms (1,500 lb).
Pistosauroidea is a group of marine reptiles within the superorder Sauropterygia that first appeared in the latter part of the Early Triassic and were the ancestors of plesiosaurs. Pistosauroids are rare in Triassic marine assemblages, and are represented by only a few fossils from central Europe, the United States, and China. Recent phylogenetic analyses consider the Triassic pistosauroids to be a paraphyletic grouping, meaning that they do not form a true clade. Plesiosauria is now placed within Pistosauroidea, while the traditional pistosauroids are successively more basal, or primitive, sauropterygians.
Spitrasaurus is an extinct genus of cryptoclidid plesiosauroid plesiosaur known from the uppermost Jurassic of central Spitsbergen, Norway and likely also Kimmeridge, England. It is named after a syllabic abbreviation for Spitsbergen Travel.
