Middle Jurassic plesiosaurs

Maresaurus is an extinct genus of plesiosaur from the Middle Jurassic (Bajocian) Los Molles Formation of Argentina. The type species, Maresaurus coccai, was named by Gasparini in 1997. Recent phylogenetic analysis found Maresaurus to be a rhomaleosaurid.

Borealonectes is a genus of rhomaleosaurid pliosauroid, a type of plesiosaur. Its fossils were found in the Callovian-age (Middle Jurassic, about 165-161 million years ago) Hiccles Cove Formation of Melville Island, Canada, one of the islands in the Canadian Arctic Archipelago. It is based on a skull, neck vertebrae, and the right forelimb of one individual. Named in 2008 by Sato and Wu, Borealonectes is one of the few plesiosaurs known from the Jurassic of North America, and the first marine reptile from the Canadian Arctic with a well-preserved skull. The type species is B. russelli.

Yuzhoupliosaurus is an extinct genus of plesiosaur from the Middle Jurassic of China. The genus is known only from a lower jaw, eighteen vertebrae, fragments of a pectoral girdle and parts of the right hindlimb. It is believed this genus lived in fresh water.