Ma
- Machaeroprosopus,
- Machimosaurus †
- Macronychoptera,
- Macroplacus Schubert-Klempnauer, 1975 †
- Macroplacus,
- Macroplata Swinton, 1930 †
- Macrorhineura,
- Mahajangasuchidae Sereno & Larsson, 2009 †
- Mahajangasuchus Buckley & Brochu, 1999 †
- Malerisaurus Chatterjee, 1980 †
- Malutinisuchus,
- Marasuchus,
- Maresaurus Gasparini, 1997 †
- Marmoretta,
- Massaliasuchus Martin & Buffetaut, 2008 †
- Mauisaurus Hector, 1874 †
Me
- Mecistops Gray, 1844
- Mecistotrachelos,
- Megachirella
- Megacnemus,
- Megadontosuchus †
- Megalancosauridae,
- Megalancosaurinae
- Megalneusaurus Knight, 1898 †
- Megalosaurus terquemi,
- Melanosuchus Gray, 1825
†Klada Merriamosauria Motani 1999
- †Besanosauridae McGowan and Motani 2003
- †Californosauridae Von Huene 1948
- †Euichthyosauria Motani 1999
- †Guanlingsauridae Yin et al. 2000
- †Merriamosauridae Maisch and Matzke 2003
- †Parvipelvia Motani 1999
- †Shastasauria Motani 1999
- †Shastasauridae Merriam 1902
- †Shonisauridae Camp 1980
- †Thalattoarchon Fröbisch et al. 2013
Diapsida → Ichthyopterygia → Ichthyosauria → Hueneosauria → Longipinnati
- Mesadactylus Jensen & Padian, 1989 †
- Mesenosauridae †
- Mesenosauridae,
- Mesodapedon,
- Mesosuchia †
- Mesosuchidia,
- Metanothosaurus Yabe & Shikama, 1948 †
- Metanothosaurus,
- Metriorhynchidae Fitzinger, 1843 †
- Metriorhynchus Meyer, 1830 †
Mi
- Microchampsa †
- Microcleidus Watson, 1909 †
- Microcnemus Huene, 1940 †
- Microleptosaurus,
- Microlophus,
- Microsuchus †
- Mimeosaurini,
- Mimeosaurus,
- Miodentosaurus Cheng et al., 2007 †
- Mixosauridae Baur, 1887 †
- Mixosaurus Baur, 1887 †
Mo
- Montanazdarcho Padian, de Ricqlès & Horner, 1995 †
- Monjurosuchidae,
- Monjurosuchus,
- Morenosaurus Welles, 1943 †
- Morunasius
Mu
- Muraenosauridae,
- Muraenosaurus Seeley, 1847 †
My
- Mycterosuchus †
- Myrmecodaptria,
- Mystriosuchidae,
- Mythunga Molnar & Thulborn, 2008 †