Pliosauroidea is an extinct clade of marine reptiles. Pliosauroids, also commonly known as pliosaurs, are known from the Jurassic and Cretaceous periods. The pliosauroids were crocodile-like short-necked plesiosaurs with large heads and massive toothed jaws. They were sauropterygian swimming reptiles, and were not dinosaurs. They originally included only members of the family Pliosauridae, of the order Plesiosauria, but several other genera and families are now also included, the number and details of which vary according to the classification used.
The Plesiosauridae are a monophyletic group of plesiosaurs.
Elasmosauridae was a family of plesiosaurs. They had the longest necks of the plesiosaurs and survived from the Hauterivian to the Maastrichtian stages of the Cretaceous. Their diet mainly consisted of crustaceans and molluscs.
Umoonasaurus is an extinct genus of plesiosaur belonging to the family Leptocleididae. This genus lived approximately 115 million years ago during the Late Cretaceous period, in shallow seas covering parts of what is now Australia. It was a relatively small animal around 2.5 m (8 ft) long. An identifying trait of Umoonasaurus is three crest-ridges on its skull.
Plesiopleurodon is an extinct genus of Mesozoic marine reptiles, belonging to Sauropterygia, known from the Late Cretaceous of North America. It was named by Kenneth Carpenter based on a complete skull with a mandible, cervical vertebra, and a coracoid. In naming the specimen, Carpenter noted "Of all known pliosaurs, Plesiopleurodon wellesi most closely resembles Liopleurodon ferox from the Oxfordian of Europe, hence the generic reference." It was initially described as a pliosaur due to it short neck, a common trait of the family, although it is in the order Plesiosauria. However, later exploration into the relationships of both orders indicate that not all pliosaurs have short necks and not all plesiosaurs have long necks.
Pliosauridae is a family of plesiosaurian marine reptiles from the Earliest Jurassic to the early Late Cretaceous of Australia, Europe, North America and South America. Past the Turonian, they may have been replaced by the mosasaurs. It was formally named by Harry G. Seeley in 1874.
Cryptoclididae is a family of medium-sized plesiosaurs that existed from the middle Jurassic to the early Cretaceous period. They had long necks, broad and short skulls and densely packed teeth. They fed on small soft-bodied preys such as small fish and crustaceans.
Polycotylidae is a family of plesiosaurs from the Cretaceous, a sister group to Leptocleididae.
Nichollssaura is an extinct genus of leptocleidid plesiosaur from the Early Cretaceous Boreal Sea of North America. The type species is N. borealis, found in the early Albian age Clearwater Formation near Fort McMurray, Alberta, Canada. Nichollssaura fills an approximate 40-million-year gap in the fossil record of North American plesiosaurs.
Leptocleididae is a family of small-sized plesiosaurs that lived during the Early Cretaceous period. Leptocleidus and Umoonasaurus had round bodies and triangle-shaped heads. 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.
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.
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.
Rhomaleosauridae is a family of pliosauroid plesiosaurs from the Earliest Jurassic to the latest Middle Jurassic of Europe, North America, South America and possibly Asia. Most rhomaleosaurids are known from England, many specifically from lower Blue Lias deposits that date back to the earliest Jurassic, just at the boundary with the Triassic. In fact, to date only two undisputed rhomaleosaurids were named from outside Europe - the closely related Borealonectes russelli and Maresaurus coccai from Canada and Argentina, respectively. These two species are also the only Middle Jurassic representatives of the family. Rhomaleosauridae was formally named by Kuhn in 1961, originally proposed to include Rhomaleosaurus cramptoni and its relatives, which have short necks and large heads relatively to plesiosauroids like Elasmosaurus and Plesiosaurus, but longer necks and smaller heads relatively to advanced pliosaurids like Pliosaurus and Kronosaurus.
Leptocleidia is a clade of plesiosauroids. The group was erected in 2007 as Leptocleidoidea. Although established as a clade, the name Leptocleidoidea implies that it is a superfamily. Leptocleidoidea is placed within the superfamily Plesiosauroidea, so it was renamed Leptocleidia by Hilary F. Ketchum and Roger B. J. Benson (2010) to avoid confusion with ranks. Leptocleidia is a node-based taxon which was defined by Ketchum and Benson as "Leptocleidus superstes, Polycotylus latipinnis, their most recent common ancestor and all of its descendants".
Microcleididae is an extinct family of basal plesiosauroid plesiosaurs from the Early Jurassic of France, Germany, Portugal and the United Kingdom. Currently, the oldest and the most known microcleidid is Eretmosaurus from the middle Sinemurian of the United Kingdom. Microcleididae was formally named and described by Roger B. J. Benson, Mark Evans and Patrick S. Druckenmiller in 2012.
Djupedalia is an extinct genus of cryptoclidid plesiosauroid plesiosaur known from the uppermost Jurassic of central Spitsbergen, Norway. It is named after Øystein Djupedal, the former Minister of Education and Research who helped fund the fossil excavation.
Spitrasaurus is an extinct genus of cryptoclidid plesiosauroid plesiosaur known from the uppermost Jurassic of central Spitsbergen, Norway. It is named after a syllabic abbreviation for Spitsbergen Travel.
Thalassophonea is an extinct clade of pliosaurids from the Middle Jurassic to the early Late Cretaceous of Australia, Europe, North America and South America. Thalassophonea was erected by Roger Benson and Patrick Druckenmiller in 2013. The name is derived from Greek thalassa (θάλασσα), "sea", and phoneus (φονεύς), "murderer". It is a stem-based taxon defined as "all taxa more closely related to Pliosaurus brachydeirus than to Marmornectes candrewi". It includes the short necked and taxa that typify the family.
Atychodracon is an extinct genus of rhomaleosaurid plesiosaurian known from the Late Triassic - Early Jurassic boundary of England. It contains a single species, Atychodracon megacephalus, named in 1846 originally as a species of Plesiosaurus. The holotype of "P." megacephalus was destroyed during a World War II air raid in 1940 and was later replaced with a neotype. The species had a very unstable taxonomic history, being referred to four different genera by various authors until a new genus name was created for it in 2015. Apart from the destroyed holotype and its three partial casts, a neotype and two additional individuals are currently referred to Atychodracon megacephalus, making it a relatively well represented rhomaleosaurid.
Arminisaurus is a genus of pliosaurid plesiosaur that lived during the Lower Jurassic in present-day Germany. With Westphaliasaurus and Cryonectes, Arminisaurus is only the third plesiosaurian taxon that was described from the Pliensbachian stage. The holotype and only known specimen is a fragmentary skeleton, comprising an incomplete lower jaw, teeth, vertebrae and elements from the pectoral girdle and the paddles. The animal had an estimated body length of 3–4 metres (9.8–13.1 ft).
