Eastern Water dragon, Physignathus lesueurii
Sauropsids are a diverse group of mostly egg-laying vertebrate animals. The Sauropsida includes all modern and most extinct "reptiles", but excludes synapsids. Living sauropsids include lizards, snakes, turtles, crocodiles, and birds. Extinct sauropsids include non-avian dinosaurs, pterosaurs, plesiosaurs, ichthyosaurs, and many others.
History of the Term "Sauropsida"
The terms "Sauropsida" ("Lizard Faces") and "Theropsida" ("Beast Faces") were coined in 1916 by E.S. Goodrich to distinguish between lizards, birds, and their relatives on one hand (Sauropsida) and mammals and their extinct relatives (Theropsida) on the other. This division is supported by the nature of the hearts and blood vessels in each group, and other features such as the structure of the forebrain. According to Goodrich both lineages evolved from an earlier stem group, the Protosauria ("First Lizards") which included some Paleozoic amphibians as well as early reptiles.
In 1956 D.M.S. Watson observed that the first two groups diverged very early in reptilian history, and so he divided Goodrich's Protosauria among them. He also reinterpreted the Sauropsida and Theropsida to exclude birds and mammals respectively. Thus his Sauropsida included Procolophonia, Eosuchia, Millerosauria, Chelonia (turtles), Squamata (lizards and snakes), Rhynchocephalia, Crocodilia, "thecodonts" (paraphyletic basal Archosauria), non-avian dinosaurs, pterosaurs, ichthyosaurs, and sauropyterygians.
This classification supplemented, but was never as popular as, the classification of the reptiles (according to Romer's classic Vertebrate Paleontology) into four subclasses according to the positioning of temporal fenestrae, openings in the sides of the skull behind the eyes. Those divisions were:
- Anapsida - no fenestrae
- Synapsida - one low fenestra (beneath the postorbital and squamosal bones)
- Euryapsida - one high fenestra (above the postorbital and squamosal)
- Diapsida - two fenestrae
All of the above but Synapsida fall under Sauropsida.
Sauropsida versus Reptilia
In the current cladistic system, the Linnean classification of the Class Reptilia into four subclasses has been replaced. Euryapsida is variously considered a subgroup of Diapsida or rejected altogether. Reptilia is often considered a clade including the most recent common ancestor of Crocodylia, Squamata, Rhynchocephalia, and Chelonia, plus all descendants of that ancestor. (Controversially, this includes birds and excludes all synapsids.) Clade Sauropsida includes all amniotes sharing more recent ancestry with Clade Reptilia than with Mammalia. The term "Theropsida" is usually replaced by Synapsida, which now refers to both the old subclass Synapsida plus the mammals.
Anapsids are generally thought to reflect the ancestral amniote condition. If so, they might not form a natural group, but might simply be a paraphyletic assemblage of primitive amniotes. It is also possible that some anapsids (notoriously turtles) are actually diapsids that lost their temporal fenestrae secondarily, which would make the group polyphyletic.
In the 2004 edition of his textbook, Michael J. Benton uses the term "Class Sauropsida" to refer to all non-synapsid, non-avian amniotes, although most systematists would include Aves (birds), as in the original sense of the taxon.
- CLASS SAUROPSIDA*
- Subclass Anapsida (= Parareptilia)
- Subclass Diapsida
- Order Araeoscelidia (Extinct)
- Order Avicephala (Extinct)
- Order Younginiformes (Extinct)
- Superorder Ichthyopterygia ("fish lizards") (Extinct)
- Infraclass Lepidosauromorpha
- Infraclass Archosauromorpha
- Order Choristodera (Extinct)
- Family Trilophosauridae (Extinct)
- Family Rhynchosauridae ("beaked lizards") (Extinct)
- Order Prolacertiformes (Extinct)
- Family Proterosuchidae (Extinct)
- Family Erythrosuchidae (Extinct)
- Family Euparkeriidae (Extinct)
- Division Archosauria
- Family Ornithosuchidae (Extinct)
- Order Phytosauria (Extinct)
- Order Aetosauria (Extinct)
- Order Rauisuchia (Extinct)
- Superorder Crocodylomorpha
- Order Pterosauria (flying reptiles) (Extinct)
- Superorder Dinosauria
- Division Archosauria
Sauropsida |--Anapsida | |--Mesosauridae (Extinct) | `--Parareptilia | |--Millerettidae (Extinct) | |--Bolosauridae (Extinct) | `--Procolophonomorpha | |--Procolophonia | | |--Procolophonidae (Extinct) | | `--Pareiasauridae (Extinct) | `--Testudines | `--Chelonia (turtles) `--Eureptilia |--Captorhinidae (Extinct) `--Romeriida |--Protorothyrididae (Extinct) `--Diapsida |--Araeoscelidia (Extinct) |--Avicephala (Extinct) `--Neodiapsida |-?Younginiformes (Extinct) |-?Ichthyopterygia (Extinct) `--Sauria |--Lepidosauromorpha | |-?Sauropterygia (Extinct) | `--Lepidosauriformes | `--Lepidosauria | |--Sphenodontida (tuatara) | `--Squamata (lizards and snakes) `--Archosauromorpha |-?Choristodera (Extinct) |--Trilophosauridae (Extinct) |--Rhynchosauridae (Extinct) |--Prolacertiformes (Extinct) `--Archosauriformes |--Proterosuchidae (Extinct) |-?Erythrosuchidae (Extinct) |--Euparkeriidae (Extinct) `--Avesuchia |--Proterochampsidae `--Archosauria |--Crurotarsi | `--Crocodylia `--Ornithodira `--Dinosauria `--Aves (birds)
- Tree of Life Describes the early amniote clades
- University of Bristol Taxonomic hierarchy of the vertebrates, according to Benton 2004
- Benton, M. J. (2004), Vertebrate Paleontology, 3rd ed. Blackwell Science Ltd
- Colbert, E.H. (1969), Evolution of the Vertebrates, 2nd ed., John Wiley and Sons Inc.
- Goodrich, E.S. (1916). On the classification of the Reptilia. Proceedings of the Royal Society of London 89B: 261–276.
- Romer, A.S., Vertebrate Paleontology University of Chicago Press, 1933; 3rd ed. 1966.
- Watson, D.M.S. (1957). On Millerosaurus and the early history of the sauropsid reptiles. Philosophical Transactions of the Royal Society of London, Series B, Biological Sciences, 1957, V.240, N.673, P.325-400.