M. macropteus M. tournemirensis Microcleidus is an extinct genus of sauropterygian reptile belonging to the Plesiosauroidea.

The type species, M. homalospondylus, was the largest, measuring 5.1 m (17 ft) long and weighing 650 kg (1,430 lb).

Other species were smaller: M. tournemirensis was about 4 m (13 ft) long and weighed 300 kg (660 lb), and M. melusinae was about 3 m (9.8 ft) long and weighed 120 kg (260 lb).

Attenborosaurus conybeari advanced pliosaurids NHMUK OR 49202 [now Anningasaura lymense] advanced rhomaleosaurids "Plesiosaurus" macrocephalus Archaeonectrus rostratus Macroplata tenuiceps OUMNH J.10337 [now Stratesaurus taylori] Seeleyosaurus guilelmiimperatoris

Plesiosaurus dolichodeirus Elasmosauridae and Cryptoclidia Microcleidus homalospondylus Hydrorion brachypterygius Occitanosaurus tournemiensis