Plesiosauroidea (/ˈpliːsiəsɔːr/; Greek: πλησιος plēsios 'near, close to' and σαυρος sauros 'lizard') is an extinct clade of carnivorous marine reptiles.

Plesiosauroid remains were found by the Scottish geologist Hugh Miller in 1844 in the rocks of the Great Estuarine Group (then known as 'Series') of western Scotland.

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.

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".

[9] It is likely that they cruised slowly below the surface of the water, using their long flexible neck to move their head into position to snap up unwary fish or cephalopods.

Their four-flippered swimming adaptation may have given them exceptional maneuverability, so that they could swiftly rotate their bodies as an aid to catching prey.

Before this study, plesiosauroids had sometimes been portrayed crawling out of water to lay eggs in the manner of sea turtles, but experts had long suspected that their anatomy was not compatible with movement on land.