The superfamily Plesiosauroidea is one of the two major traditional divisions of plesiosaurs, the group that typically have long necks.

Cryptoclidus in the Hunterian Museum, Glasgow, Scotland. Photo by Adam S. Smith. 2007.

The other major group (superfamily) of plesiosaurs is the Pliosauroidea (often just called pliosaurs). During the 1990s and 2000s, this simple division of plesiosaurs into two superfamilies was called into question and multiple studies have demonstrated that the picture is more complicated, with at least some ‘true’ plesiosaurs being located outside of these two clades. Within the Plesiosauroidea, there are a number of families including the long-necked Plesiosauridae, Cryptoclididae, Elasmosauridae, and the short-necked Polycotylidae also originated from this group.

In popular literature both the superfamily Plesiosauroidea, and also the larger group Plesiosauria, can be referred to as ‘plesiosaurs’, and this can sometimes result in confusion. See my article on different concepts of plesiosaur for the full story. On the Plesiosaur Directory I only ever use the informal term ‘plesiosaur’ to refer to the Plesiosauria. I use the word ‘plesiosauroid’ for the Plesiosauroidea.

Plesiosauroidea classification

Plesiosauroid genera


Abyssosaurus is a derived cryptoclidid plesiosaur from the Upper Hauterivian (Lower Cretaceous) of the Menya River, Chuvashia, Russia. It was named and described in 2011 by Alexander Yu Berezin (Berezin 2011). A partial skull associated with the holotype specimen (MChEIO, no. PM/1) was not described in the original 2011 paper but was discovered later in …


The holotype specimen (TMP 2007.0110001) consists of an almost complete skeleton lacking a skull. The neck contains 76 cervical vertebrae. This is a unique character of Albertonectes vanderveldei and the highest number of neck vertebrae known for any plesiosaur, surpassing the previous record-holder (Elasmosaurus) by four vertebrae. The distal-most caudal vertebrae (27th to 33rd) of Albertonectes …


The genus and species ‘Apractocleidus teretipes‘ was erected by Smellie (1916) for a specimen now regarded as old-adult individual of Cryptoclidus. The specimen was collected by Alfred Leeds and acquired by the Hunterian Museum, Glasgow. The taxon ‘Apractocleidus‘ is therefore considered a junior synonym of Cryptoclidus. The holotype and only specimen is mounted on display …


The mysterious plesiosaur Aristonectes is notable for its mouthful of pin-like teeth. A special feeding guild, the ‘trap guild’, has been proposed to accommodate Aristonectes and other plesiosaurs with similar dentition (Chatterjee and Small, 1989) such as Cryptoclidus, Kimmerosaurus, and Kaiwhekea. These plesiosaurs may have fed in a similar manner to the extant crabeater seal, …


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Cryptoclidus, often wrongly spelled ‘Cryptocleidus’ after Andrews (1909), is a moderately sized plesiosaur up to 3 metres long. It is known from a large number of individual specimens from the Oxford Clay Formation. Fossils of Cryptoclidus are relatively common, and provide a complete ontogenetic sequence from very young to old adult individuals. This makes Cryptoclidus …


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Edgarosaurus is a basal polycotylid (Druckenmiller, 2002) from the Thermopolis Shale


Elasmosaurus is one of the most widely recognised plesiosaur names and has become a stereotype for all elasmosaurids. However, it is relatively poorly known. The type and only known specimen of Elasmosaurus platyurus (ANSP 10081) includes the tip of the snout, occipital condyle, and the majority of the vertebral column. It is from the Sharon …


‘Gronausaurus‘ was named in 2013 (Hampe, 2013). It is now regarded as a junior synonym of Brancasaurus (Sachs et al. 2016). Under construction


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Welles (1943) derived the genus from the word “fisherman”, hence “fisherman lizard”. However, the genus translates more literally as “water beast reptile”. The type skeleton of Hydrotherosaurus was discovered in the Panoche Hills by Mr. Frank C. Paiva in 1937, and was excavated by the University of California Museum of Palaeontology, Berkeley, and Fresno State …


Lower Cretaceous plesiosaurs are rare, so Leptocleidus is important because it fills a gap in the fossil record of plesiosaurians. Leptocleidus was once considered to be a late surviving member of the family Rhomaleosauridae but it has recently been reidentified as a close relative of polycotylids. The fossils of all known species of Leptocleidus were …

Libonectes skull


Libonectes was erected for ‘Elasmosaurus morgani’. The pectoral girdle and a forelimb of the holotype are figured (most recently by Welles 1962, fig. 12) but now lost (Carpenter 1999). Libonectes morgani (Welles, 1949) is the type and only species. Type material of L. morgani (SMUSMP 69120) consists of a skull, cervical vertebrae and gastralia, from the …


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Microcleidus is a medium (4.27 m) to large (5.03 m) plesiosauroid with a small head and a long neck containing 38–39 cervical vertebrae. Watson (1909) erected Microcleidus to accommodate fossil material belonging to ‘P.’ homalospondylus and ‘P.’ macropterus (Watson 1911). Both species were considered valid by Seeley (1865), Blake (1876) and Watson (1911) on the basis of …


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‘Occitanosaurus’ was described by Sciau et al. (1990) and Bardet et al. (1999). The genus name was erected for ‘Plesiosaurus’ tournemirensis by Bardet et al. (1999). The taxon was originally regarded as an early elasmosaurid but is now considered a member of the Microcleididae. ‘Occitanosaurus’ was approximately 3.9 meters long. The type material (MMM J. …


The type skeleton of ‘Ogmodirus’ was collected in 1909 by C. Boyce from the upper Greenhorn Limestone Formation (Lower Turonian, Late Cretaceous) of Cloud County (near Aurora), Kansas (Storrs, 1999; Schumacher & Everhart, 2005). The specimen, KUVP 441, is a partial skeleton consisting of partial vertebral column (51 cervical vertebrae, 18 caudal vertebrae), limb, and girdle …


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Plesiosaurus was the first plesiosaur discovered and named. For a long time Plesiosaurus was treated as waste-basket taxon. This means that many different specimens were allocated to the genus Plesiosaurus under different species names, even when they were very different. By modern standards, many of these specimens deserve a generic name of their own, or …


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The holotype of Thalassomedon was discovered by R. L. Landberg in 1939 in Baca County, Colorado. Thalassomedon has a short and deep atlas-axis and 62 cervical vertebrae, and there is no pectoral or pelvic bar in the adult condition (Carpenter, 1999). The species name haningtoni is sometimes wrongly spelled ‘hanningtoni’ (as in Carpenter 1999) or …


Thililua has 30 cervical vertebrae. Each cervical has a longitudinal ridge on its lateral surfaces, this is a convergent feature with elasmosaurids. The skull of Thililua is relatively large with a short postorbital region and elongate rostrum. The type specimen (MHNGr.PA.11710) consists of a complete skull, entire cervical and pectoral series, and the first four …


T. seeleyi is the type and only species of Tricleidus. The holotype specimen (NHMUK R.3539) consists of disarticulated elements including most of skull and half the postcranium, from the Kosmoceras jason – Peltoceras athleta zones of the lowest deposits of the Oxford Clay.


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Sachs (2004) regarded ‘Woolungasaurus‘ as a junior synonym of Styxosaurus and assigned the type species (‘W. glendowerensis‘) to that genus under the new combination ‘Styxosaurus glendoweresnis‘. However, this placement was unsupported by unambiguous apomorphic characters (Kear 2007). Since ‘Woolungasaurus‘ has no unique diagnostic characters, Kear (2005) reassigned all specimens of it to Elasmosauridae indet. (indeterminate …