Microcleidus is an extinct genus of sauropterygian reptile belonging to the Plesiosauroidea. The species has 40 neck vertebrae and a short tail of 28 vertebrae. Fossils of the genus have been found in France, the Posidonia Shale in Germany and Luxembourg, and the Alum Shale Formation of England.

Quick Facts Scientific classification, Type species ...

Microcleidus Temporal range: Toarcian ~182–175 Ma PreꞒ Ꞓ O S D C P T J K Pg N
Fossil M. homalospondylus
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Superorder:	†Sauropterygia
Order:	†Plesiosauria
Family:	†Microcleididae
Genus:	†Microcleidus Watson, 1909
Type species
Plesiosaurus homalospondylus (Owen, 1865)
Other species
M. macropterus (Seeley, 1865) M. tournemirensis (Sciau et al., 1990) M. melusinae Vincent et al., 2019
Synonyms
M. macropteus Plesiosaurus macropteus Seeley, 1865 Eretmosaurus macropterus Benton and Spencer, 1995 M. tournemirensis Plesiosaurus tournemirensis Sciau et al., 1990 Occitanosaurus tournemirensis Bardet et al., 1999

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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).^[1][2][3]

Species include: Microcleidus homalospondylus (Owen 1865) and Microcleidus macropterus (Seeley 1865).

Occitanosaurus tournemirensis (originally "Plesiosaurus" tournemirensis), was named by Sciau et al. in 1990, based on a nearly complete skeleton of an animal approximately 4 meters (13 ft) long.^[1] It was later found to be a species of Microcleidus.

The following cladogram follows an analysis by Ketchum & Benson, 2011.^[4]

• List of plesiosaur genera
• Timeline of plesiosaur research