The Elasmosauridae sre characteristically 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
- 'Woolungasaurus' = Elasmosauridae indet.
- 'Alzadasaurus' = ?
- Aprosaurus ?
- Ogmodirus ?
- Fresnosaurus ?