After Bardet, Godefroit & Sciau, 1999, O'Keefe, 2001, and Benson, Evans & Druckenmiller, 2012

<==o †Plesiosauria de Blainville, 1835 sensu O'Keefe, 2001 [Streptosauria Cope, 1869] (plesiosaurs; joutsenliskot)
   |-- †Pliosauroidea Welles, 1943 sensu O'Keefe, 2001 (lyhytkaulaiset joutsenliskot; pliosaurs, short-necked plesiosaurs)
   `--o †Plesiosauroidea (Gray, 1825) Welles, 1943 sensu O'Keefe, 2001 (pitkäkaulaiset joutsenliskot; plesiosaurs, long-necked plesiosaurs)
      |-- †Eoplesiosaurus antiquior Benson, Evans & Druckenmiller, 2012
      `--+--o †Plesiosauridae Gray, 1825 sensu O'Keefe, 2001
         |  `--o †Plesiosaurus De la Beche & Conybeare, 1821 [Pentatarsostinus Hawkins, 1840]
         |     |?- †P. macrocephalus Conybeare, in Buckland, 1836
         |     `-- †P. dolichodirus Conybeare, 1824 [P. macromus Owen, 1840]
         `--o †Euplesiosauria O'Keefe, 2001
            |?- †Sthenarosaurus dawkinsi Watson, 1909 [a Rhomaleosaurid sensu Smith & Dyke, 2008]
            |?- †Leurospondylus ultimus Brown, 1913
            |-- †Cryptoclidia Williston, 1925 sensu Ketchum & Benson, 2010 [Cryptoclidoidea Williston, 1925 sensu O'Keefe, 2001]
            `--+--+-- †Djupedalia engeri Knutsen, Druckenmiller & Hurum, 2012b
               |  `--o †Spitrasaurus Knutsen, Druckenmiller & Hurum, 2012a
               |     |-- †S. larseni Knutsen, Druckenmiller & Hurum, 2012a
               |     `-- †S. wensaasi Knutsen, Druckenmiller & Hurum, 2012a
               `--o †Elasmosauridae Cope, 1869 sensu Bardet, Godefroit & Sciau, 1999
                  |-- †Elasmosauridae Cope, 1869 sensu O'Keefe, 2001
                  `?-o †Microcleididae Benson, Evans & Druckenmiller, 2012
                     |-- †Morenosaurus stocki Welles, 1943
                     |-- †Eretmosaurus rugosus (Owen, 1840) Seeley, 1874 [Plesiosaurus rugosus Owen, 1840]
                     `--+-- †Westphaliasaurus simonsensii Schwermann & Sander, 2011
                        `--+-- †Seelyosaurus guilelmiimperatoris (Dames, 1895) White, 1940 [Plesiosaurus guilelmiimperatoris Dames, 1895; incl.? Seeleyosaurus holzmadensis White, 1940; Plesiosaurus tournemirensis Sciau, Crochet & Mattieu 1990]
                           `--o †Microcleidus Watson, 1909 sensu Benson, Evans & Druckenmiller, 2012
                              |-- †Occitanosaurus tournemirensis (Sciau, Crochet & Mattei, 1990) Bardet, Godefroit & Sciau, 1999 [Plesiosaurus tournemirensis Sciau, Crochet & Mattieu 1990]
                              `--o †Microcleidus Watson, 1909
                                 |-- †M. (Hydrorion) brachypterygius von Huene, 1923 [jr. synon. of Plesiosaurus [= Seelyosaurus] guilelmiimperatoris sensu Storrs, 1997]
                                 `--o †(Microcleidus Watson, 1909)
                                    |-- †M. homalospondylus (Owen, 1865) Watson, 1909
                                    `-- †M. macropterus (Seeley, 1865) Watson, 1911

Plesiosauria incertae sedis:


    †Aptychodon Reuss, 1855
    †“Plesiosaurus” ichthyspondylus
    †“Plesiosaurus” balticus
    †“Cimoliasaurus” cantabrigiensis
    †Discosaurus Leidy, 1852
    †Dravidosaurus blanfordi Yadagiri & Ayyasami, 1979 [regarded as a stegosaur by the original descriptors]
    †Opallionectes andamookaensis Kear, 2006; E.Cret. Aust.
    †Orophosaurus Cope, 1887
    †Pantosaurus Marsh, 1893
    †Piptomerus Cope, 1887 [nomen dubium]
    †Piratosaurus Leidy, 1865 [nomen dubium]

Alternative phylogenies of basal plesiosaurs

After Carpenter, 1999 (via Ketchum & Benson, 2010)

<==o †Plesiosauroidea
   |-- †Cryptoclididae
   `--+-- †Plesiosauridae
      `--+-- †Brancasaurus
         `--+-- †Microcleidus
            `--+?- †Mureanosaurus (sedis mutabilis)
               `--+-- †Leptocleidus
                  `--o †Elasmosauridae
                     |-- †Liponectes
                     `--+-- †Elasmosaurus
                        `--+-- †Styxosaurus
                           |-- †Hydrotherosaurus
                           |-- †Hydralmosaurus
                           |-- †Callawayasaurus
                           `--+-- †Thalassomedon
                              `-- †Tuarangisaurus

After Bardet, Godefroit & Sciau, 1999 (via Ketchum & Benson, 2010)

<==o †Plesiosauroidea
   |-- †Hydrorion
   |-- †Plesiosaurus
   `--+-- †Seeleyosaurus
      `--+--o †Cryptoclididae
         |  |-- †Tricleidus
         |  `-- †Cryptocleidus
         `--o †Elasmosauridae
            |--+-- †Mureanosaurus
            |  `--+-- †Microcleidus
            |     `-- †Occitanosaurus
            `--+-- †Alzadasaurus
               `--+-- †Hydrotherosaurus
                  `-- †Thalassomedon

Reference:

  • Bardet, N., Godefroit, P. & Sciau, J., 1999: A new elasmosaurid plesiosaur from the Lower Jurassic of southern France.
    –Palaeontology: Vol. 42, #5, pp. 927-952

After O'Keefe, 2001

<==o †Plesiosauria
   |--o †Pliosauroidea
   |  |-- †Thalassiodracon
   |  `--+-- †Eurycleidus
   |     |-- †Attemborosaurus
   |     |-- †Rhomaleosauridae
   |     `-- †Pliosauridae
   `--o †Plesiosauroidea
      |-- †Plesiosaurus
      `--o †Euplesiosauria O'Keefe, 2001
         |-- †Microcleidus
         |-- †Elasmosauridae
         `--o †Cryptoclidoidea
            |-- †Cryptoclididae
            `--o †Tricleidia
               |-- †Tricleidus
               |-- †Cimoliasauridae [= Aristonectidae]
               `-- †Polycotylidae

Reference:

  • O'Keefe, F. R., 2001: A cladistic analysis and taxonomic revision of the Plesiosauria (Reptilia: Sauropterygia).
    –Acta Zoologica Fennica: Vol. #213, pp. 1-63

After Gasparini et al, 2002 (via Ketchum & Benson, 2010)

<==o †Plesiosauroidea
   |-- †Plesiosaurus
   `--+-- †Mureanosaurus
      `--+-- †Tricleidus
         `--+-- †Vinialesaurus
            `--+-- †Kimmerosaurus
               `-- †Cryptocleidus

After Gasparini et al, 2003 (via Ketchum & Benson, 2010)

<==o †Plesiosauroidea
   |-- †Plesiosaurus
   `--+--o †Cryptocleididae
      |  |-- †Tricleidus
      |  `-- †Cryptocleidus
      `--o †Elasmosauridae
         |-- †Microcleidus
         `--+-- †Mureanosaurus
            |-- †Occitanosaurus
            `--+-- †Aristonectes
               |-- †Hydrotherosaurus
               |-- †Libonectes
               `-- †Callawayasaurus

After Großmann, 2007 (via Ketchum & Benson, 2010)

<==o †Plesiosauroidea
   |-- †Thalassiodraco
   `--+-- †Plesiosaurus
      `--+--o †Cryptocleididae
         |  |-- †Cryptocleidus
         |  `-- †Kimmerosaurus
         `--o †Elasmosauridae
            |-- †Seeleyosaurus
            |-- SMNS 16812 [Plesiopterys]
            `--+-- †Mureanosaurus
               `--+--+-- †Libonectes
                  |  `-- †Bracasaurus
                  `--+-- †Occitanosaurus
                     `--+-- †Microcleidus
                        `-- †Hydrorion

After Ketchum & Benson, 2010 (fig. 6)

<==o †Plesiosauria
   |--+-- BMNH 49202
   |  `--+-- †Archaeonectrus
   |     `-- †Macroplata
   `--+-- †“Plesiosaurus” macrocephalus
      `--+-- †Attemborosaurus
         `--o †Neoplesiosauria Ketchum & Benson, 2010
            |-- †Pliosauroidea
            `--o †Plesiosauroidea
               |-- OUMNH J.10337
               `--+--+-- †Hauffiosaurus
                  |  `--+-- MMUM LL 8004
                  |     `-- †“Macroplata” longirostris
                  `--+-- †Thalassiodragon
                     `--+--o Plesiosauridae
                        |  |--+-- †Plesiosaurus
                        |  |  `-- †Seelyosaurus
                        |  `--+-- †Microcleidus
                        |     `--+-- †Hydrorion
                        |        `-- †Occitanosaurus
                        `--+-- †Elasmosauridae [incl. Aristonectidae]
                           `--o †Cryptoclidia
                              |--o †Cryptocleididae
                              |  |--+-- †Cryptoclidus
                              |  |  `-- †Kimmerosaurus
                              |  `--+-- †Mureanosaurus
                              |     `-- †Tricleidus
                              `--o †Leptocleidia
                                 |-- †Leptocleididae
                                 `-- †Polycotylidae

Reference(s):

  • Ketchum, H. F. & Benson, R. B. J., 2010: Global interrelationships of Plesiosauria (Reptilia, Sauropterygia) and the pivotal role of taxon sampling in determining the outcome of phylogenetic analyses.
    –Biological Reviews: Vol. 85, #2, pp. 361-392 [doi: 10.1111/j.1469-185X.2009.00107.x]

After Vincent, Bardet, Pereda Suberbiola, Bouya, Amaghzaz & Meslouh, 2011:

<==o †Plesiosauroidea
   |-- †Thalassiodraco
   `--+-- †Plesiosaurus
      `--+-- †Seeleyosaurus
         `--+-- †Occitanosaurus
            `--+--+-- †Eromangasaurus
               |  `--+--+-- †Kaiwhekea
               |     |  `-- †Tuarangisaurus
               |     `--+-- †Nichollssaura
               |        `--+-- †Thililua
               |           `-- †Dolichorhynchops
               `--o †Elasmosauridae
                  |-- †Microcleidus
                  `--+-- †Callawayasaurus
                     `--+--+-- †Thalassomedon
                        |  `--+-- †Hydralmosaurus
                        |     `-- †Styxosaurus
                        `--+-- †Hydrotherosaurus
                           `--+-- †Terminonatator
                              `--+-- †Libonectes
                                 |-- †Futabasaurus
                                 `-- †Zarafasaura

Reference(s):

  • Vincent, P., Bardet, N., Pereda Suberbiola, X., Bouya, B., Amaghzaz, M. & Meslouh, S., 2011: Zarafasaura oceanis, a new elasmosaurid (Reptilia: Sauropterygia) from the Maastrichtian Phosphates of Morocco and the palaeobiogeography of latest Cretaceous plesiosaurs.
    –Gondwana Research: Vol. 19, #4, pp. 1062-1073 [doi: 10.1016/j.gr.2010.10.005]

After Benson, Evans & Druckenmiller, 2012

<==o †Plesiosauria
   |-- †Rhomaleosauridae
   `--o †Neoplesiosauria
      |-- †Pliosauridae
      `--o †Plesiosauroidea
         |-- †Eoplesiosaurus
         `--+-- †Plesiosaurus
            `--o †Euplesiosauria
               |--+-- †Plesiospterys
               |  `-- †Cryptoclidoidea
               `--o †Microcleididae
                  |-- †Eretmosaurus
                  |-- †Westphaliasaurus
                  `--+-- †Seelyosaurus
                     `--o †Microcleidus
                        |-- †M. (Occitanosaurus) tournemirensis
                        `--+-- †M. (Hydrorion) brachypterygius
                           `-- †M. (Microcleidus) homalospondylus

Reference:

  • Benson, R. B. J., Evans, M. & Druckenmiller, P. S., 2012: High Diversity, Low Disparity and Small Body Size in Plesiosaurs (Reptilia, Sauropterygia) from the Triassic–Jurassic Boundary.
    –Plos One: Vol. 7, #3, pp. e31838 [doi: 10.1371/journal.pone.0031838]

Reference(s):