Elasmosauridae is a derived clade of plesiosaurs known exclusively from Cretaceous strata. It represents the most taxonomically diverse radiation among plesiosauroids and achieved a near-global palaeogeographic distribution.
The earliest elasmosaurids diagnostic on genus level occur in the Hauterivian (Lower Cretaceous), including Jucha squalea and Lagenanectes richterae. A defining feature of most members of the clade is the hyper-elongate neck. In taxa preserving complete or nearly complete cervical series, more than 50 cervical vertebrae are typical. In extreme cases, such as Elasmosaurus platyurus and Albertonectes vanderveldei, the count exceeds 70 cervical vertebrae.
Among many Late Cretaceous elasmosaurids, a ventral notch on the articular faces of the cervical vertebrae is a recurring feature. This condition is absent in early forms such as Lagenanectes richterae and Callawayasaurus colombiensis, but is present in Cenomanian–Turonian taxa including Thalassomedon haningtoni and Libonectes morgani. Another common trait is a longitudinal lateral ridge on the cervical centra, although this also occurs in other long-necked plesiosaurs.
Recent phylogenetic studies subdivide Elasmosauridae into several subclades, including Styxosaurinae and Elasmosaurinae. Aristonectinae forms a well-supported lineage within the family and represents one of its latest radiations. Known primarily from Maastrichtian deposits of the Southern Hemisphere, aristonectines differ from non-aristonectine elasmosaurids in possessing a relatively large skull with numerous slender teeth and a comparatively short neck composed of anteroposteriorly abbreviated vertebrae. The stratigraphically youngest elasmosaurids are recorded from the upper Maastrichtian, including Zarafasaura oceanis and Chubutinectes carmeloi.