Elasmosauridae

The Elasmosauridae is a group of very long-necked plesiosaurs. They form a major component of Late Cretaceous plesiosaur assemblages. The definition of Elasmosauridae has changed in recent years as our understanding of plesiosaur evolution has improved. Older definitions included many Jurassic long-necked forms (Bardet et al. 1999; Cruickshank et al. 2002). However, today Elasmosauridae has a more restricted definition and contains exclusively Cretaceous taxa. Synapomorphies (shared derived characters) of the clade Elasmosauridae include (Bardet et al. 1999):

– Ventral cheek margin weakly excavated
– Pterygoids on palate closed anteriorly (lacks anterior pterygoid vacuity)
– Platycoelus (flat articulation surfaces) to the vertebrae
– Greater than 40 cervical vertebrae
– Cervical centrum length greater than height
– Anterior cervical vertebrae with lateral keel

Elasmosaurid classification

Elasmosaurid sub-clades

Elasmosaurid genera

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. The distal-most caudal vertebrae (27th to 33rd) ofAlbertonectes …

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. These plesiosaurs may have fed in a similar manner to the extant crabeater seal, …

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. It is from the Sharon …

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. Paiva in 1937, and was excavated by the University of California Museum of Palaeontology, Berkeley, and Fresno State …

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. morgani (SMUSMP 69120) consists of a skull, cervical vertebrae and gastralia, from the …

Morturneria

The taxon was originally named ‘Turneria seymourensis‘ by Chatterjee and Small (1989) but this genus was preoccupied and so Chatterjee and Creisler (1994) later revised the name to Morturneria seymourensis. The holotype specimen is TTU P 9219, an incomplete skull and mandible plus associated cervical vertebrae, from the Lopez de Bertodano Formation of Seymour Island, …

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 …

Styxosaurus

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). The species name haningtoni is sometimes wrongly spelled ‘hanningtoni’ (as in Carpenter 1999) or …

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‘. 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 …

Several elasmosaurid genera are considered nomina dubia or are regarded as junior synonyms of other genera. The following names are invalid:

  • ‘Woolungasaurus’ = Elasmosauridae indet.
  • ‘Alzadasaurus’ = ?
  • Aprosaurus ?
  • Ogmodirus ?
  • Fresnosaurus ?