Plesiosauroids evolved from earlier, similar forms such as
pistosaurs. There are a number of
families of plesiosauroids, which retain the same general appearance and are distinguished by various specific details. These include the
Plesiosauridae, unspecialized types which are limited to the
Early Jurassic period;
Cryptoclididae, (e.g.
Cryptoclidus), with a medium-long neck and somewhat stocky build;
Elasmosauridae, with very long, flexible necks and tiny heads; and the
Cimoliasauridae, a poorly known group of small Cretaceous forms. According to traditional classifications, all plesiosauroids have a small head and long neck but, in recent classifications, one short-necked and large-headed Cretaceous group, the
Polycotylidae, are included under the Plesiosauroidea, rather than under the traditional Pliosauroidea. Size of different plesiosaurs varied significantly, with an estimated length of
Trinacromerum being three meters and
Mauisaurus growing to twenty meters.
Relationships '', a cryptoclidid '', an elasmosaurid '', a polycotylid Within Plesiosauroidea, there is a more exclusive group,
Cryptoclidia. Cryptoclidia was named and defined as a node clade in 2010 by
Hilary Ketchum and
Roger Benson: the group consisting of the
last common ancestor of
Cryptoclidus eurymerus and
Polycotylus latipinnis; and all its descendants. The smaller group within Cryptoclidia was erected prior, in 2007 under the name "Leptocleidoidea". Although established as a clade, the name Leptocleidoidea implies that it is a
superfamily. Leptocleidoidea is placed within the superfamily Plesiosauroidea, so it was renamed
Leptocleidia by Hilary F. Ketchum and Roger B. J. Benson (2010) to avoid confusion with ranks. Leptocleidia is a
node-based taxon which was defined by Ketchum and Benson as "
Leptocleidus superstes,
Polycotylus latipinnis, their most recent common ancestor and all of its descendants". ==Behavior==