Plesiosauroidea

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

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.

Albertonectes

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.

Apractocleidus

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.

Aristonectes

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.

Brancasaurus

Under construction

Skeleton of Brancasaurus in side view. Form Sachs et al. (2016)

Skull of Brancasaurus in dorsal and lateral view, and showing the tip of the snout (C) in ventral view (from Wegner, 1914)

Colymbosaurus

Under construction

Cryptoclidus

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.

Djupedalia

Under construction

Edgarosaurus

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

Elasmosaurus

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.

Gronausaurus

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

Under construction

Hastanectes

Under construction

Hydrotherosaurus

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.

Leptocleidus

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.

Libonectes skull

Libonectes

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.

Lusonectes

Under construction

Mauisaurus

Microcleidus

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).

Muraenosaurus

Under construction

Restoration of the skeleton of Muraenosaurus in lateral view. From Andrews 1910.

Posterior cervical (neck) vertebra of Muraenosaurus in lateral and posterior view. From Andrews (1910).

Dorsal vertebra of Muraenosaurus in anterior view.

Nakonanectes

Nakonanectes is a small elasmosaurid plesiosaur from the Bearpaw Shale of Montana, USA. It is known from a single moderately complete specimen including a particularly fine skull. It has a relatively short neck for an elasmosaur consisting of ‘only’ 39-42 neck vertebrae.

Occitanosaurus

‘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.

Ogmodirus

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 elements.

Pantosaurus

Under construction

Plesiopterys

Under construction

Plesiosaurus

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.

Spitrasaurus

Under construction

Styxosaurus

The holotype specimen (KUVP 1301) of Styxosaurus is an articulated skull and anterior portion of the neck. It was found in the Niobrara Chalk near Hell Creek, Logan County, Western Kansas in 1890. It was described later that year as a new species Cimoliosaurus snowii by Williston (1890a&b), who subsequently (Williston, 1906) referred the species to Elasmosaurus.

Tatenectes

Under construction

Thalassomedon

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).

Thililua

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.

Tricleidus

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.

Vectocleidus

Under construction

Vectocleidus material on display in the Dinosaur Isle Museum, Isle of Wight, UK.

Vinialesaurus

Under construction

westphaliasaurus_skeleton

Westphaliasaurus

Under construction

Skeleton of Westphaliasaurus. The central slab of the skeleton was awaiting preparation at the time of this photograph, but was subsequently prepared. From Schwermann & Sander (2011).

Woolungasaurus

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‘.