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Euryapsida is a polyphyletic (unnatural, as the various members are not closely related) group of sauropsids that are distinguished by a single temporal fenestra, an opening behind the orbit, under which the post-orbital and squamosal bones articulate. They are different from Synapsida, which also have a single opening behind the orbit, by the placement of the fenestra. In synapsids, this opening is below the articulation of the post-orbital and squamosal bones. It is now commonly believed that euryapsids (particularly sauropterygians) are in fact diapsids (which have two fenestrae behind the orbit) that lost the lower temporal fenestra. Euryapsids are usually considered entirely extinct, although turtles might be part of the sauropterygian clade while other authors disagree. Euryapsida may also be a synonym of Sauropterygia sensu lato.

The ichthyosaurian skull is sometimes described as having a metapsid (or parapsid) condition instead of a truly euryapsid one. In ichthyosaurs, the squamosal bone is never part of the fenestra's margin. Parapsida was originally a taxon consisting of ichthyosaurs, squamates, protorosaurs, araeoscelidans and pleurosaurs.

Historically, a variety of reptiles with upper fenestrae, either alone or with a lower emargination, have been considered euryapsid or parapsid, and to have had their patterns of fenestration originate separately from those of diapsids. This includes araeoscelidans, mesosaurs, squamates, pleurosaurids, weigeltisaurids, protorosaurs, and trilophosaurs. With the exception of mesosaurs, which only have the lower temporal opening, all of these are universally agreed to be diapsids which either secondarily closed the lower opening (araeoscelids, trilophosaurs) or lost the lower bar (squamates, pleurosaurs, protorosaurs).

Euryapsida was proposed by Edwin H. Colbert as a substitute for the earlier term Synaptosauria, originally created by Edward D. Cope for a taxon including sauropterygians, turtles and rhynchocephalians. Baur removed the rhynchocephalians from Synaptosauria and Williston later resurrected the taxon, including only Sauropterygia (Nothosauria and Plesiosauria) and Placodontia in it.

The terms Enaliosauria and Halisauria have also been used for a taxon including ichthyosaurs and sauropterygians.

Some 21st century studies have found that ichthyosaurs, thalattosaurs and sauropterygians were close relatives, either as stem-archosaurs or as stem-saurians.

See also

• Anapsida
• Diapsida
• Synapsida

References

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2. ^ Simões, Tiago R.; Kammerer, Christian F.; Caldwell, Michael W.; Pierce, Stephanie E. (2022-08-19). "Successive climate crises in the deep past drove the early evolution and radiation of reptiles". Science Advances. 8 (33): eabq1898. Bibcode:2022SciA....8.1898S. doi:10.1126/sciadv.abq1898. ISSN 2375-2548. PMC 9390993. PMID 35984885.
3. ^ Rieppel, Olivier (1989-01-04). "A new pachypleurosaur (Reptilia: Sauropterygia) from the Middle Triassic of Monte San Giorgio, Switzerland". Philosophical Transactions of the Royal Society of London. B, Biological Sciences. 323 (1212): 1–73. doi:10.1098/rstb.1989.0001. ISSN 0080-4622.
4. Maisch, M. (2010). "Phylogeny, systematics, and origin of the Ichthyosauria – the state of the art". Palaeodiversity. 3: 151–214. S2CID 87979321.
5. Williston, Samuel Wendell (1925). Osteology of the Reptiles.
6. Romer, Alfred Sherwood (1956). Osteology of the Reptiles.
7. Romer, Alfred Sherwood (1966). Vertebrate Paleontology (3rd ed.). Chicago: University of Chicago Press.
8. "Enaliosauria". Merriam-Webster. Retrieved 22 December 2021.
9. Haeckel, Ernst (1895). Die systematische Phylogenie (Dritter Theil/Systematische Phylogenie der Wirbelthiere (Vertebrata)). Vol. 3. Georg Reimer.
10. Wolniewicz, Andrzej S; Shen, Yuefeng; Li, Qiang; Sun, Yuanyuan; Qiao, Yu; Chen, Yajie; Hu, Yi-Wei; Liu, Jun (2023-08-08). Ibrahim, Nizar; Perry, George H; Benton, Michael (eds.). "An armoured marine reptile from the Early Triassic of South China and its phylogenetic and evolutionary implications". eLife. 12: e83163. doi:10.7554/eLife.83163. ISSN 2050-084X. PMC 10499374. PMID 37551884.
11. Neenan, James M.; Klein, Nicole; Scheyer, Torsten M. (2013-03-27). "European origin of placodont marine reptiles and the evolution of crushing dentition in Placodontia". Nature Communications. 4 (1): 1621. Bibcode:2013NatCo...4.1621N. doi:10.1038/ncomms2633. ISSN 2041-1723. PMID 23535642.
12. Jiang, Baoyu; He, Yiming; Elsler, Armin; Wang, Shengyu; Keating, Joseph N.; Song, Junyi; Kearns, Stuart L.; Benton, Michael J. (2023). "Extended embryo retention and viviparity in the first amniotes". Nature Ecology & Evolution. 7 (7): 1131–1140. Bibcode:2023NatEE...7.1131J. doi:10.1038/s41559-023-02074-0. ISSN 2397-334X. PMC 10333127. PMID 37308704.

v t e Sauropsida (Reptilia sensu lato)
Kingdom: Animalia Phylum: Chordata Superclass: Tetrapoda Clade: Amniota Tetrapodomorpha see Tetrapodomorpha Reptiliomorpha see Reptiliomorpha Sauropsida see below↓
Sauropsida Sauropsida †Australothyris †Erpetonyx †Microleter †Acleistorhinidae Acleistorhinus Carbonodraco Colobomycter Delorhynchus Feeserpeton Karutia Klastomycter †Bolosauridae Belebey Bolosaurus Eudibamus †Mesosauria Brazilosaurus Mesosaurus Stereosternum †Millerettidae Broomia Heleophilus Milleretta Millerosaurus Milleropsis Nanomilleretta †Procolophonia see Procolophonia Diapsida see below↓
Diapsida Diapsida †Adelosaurus? †Claudiosaurus †Cryptovaranoides †Dolerosaurus †Elachistosuchus †Eunotosaurus? †Feralisaurus †Heleosuchus †Kenyasaurus †Kudnu †Lanthanolania †Maiothisavros †Microzemiotes †Orovenator †Palacrodon †Palaeagama †Saurosternon †Sphodrosaurus †Younginidae Galesphyrus Thadeosaurus Youngina †Tangasauridae Acerosodontosaurus? Hovasaurus Tangasaurus †Weigeltisauridae Coelurosauravus Glaurung Rautiania Weigeltisaurus †Drepanosauromorpha Ancistronychus Avicranium Dolabrosaurus Drepanosaurus Hypuronector Kyrgyzsaurus Longisquama? Megalancosaurus Protoavis? Skybalonyx Unguinychus Vallesaurus †Kuehneosauridae Icarosaurus Idiosaura? Kuehneosaurus Kuehneosuchus Pamelina Rhabdopelix? †Choristodera see Choristodera †Ichthyosauromorpha see Ichthyosauromorpha †Sauropterygia see Sauropterygia †Thalattosauria see Thalattosauria Sauria (Crown-Reptilia) Lepidosauromorpha see Lepidosauromorpha Includes Lepidosauria includes rhynchocephalians and squamates Archelosauria Archosauromorpha see Archosauromorpha Includes Archosauria includes birds and crocodilians Pantestudines see Pantestudines Includes Testudines turtles
See also Possible sauropsids Araeoscelidia Captorhinidae Protorothyrididae Recumbirostra Varanopidae Other taxa Anapsida Captorhinida Eureptilia Parareptilia Lanthanosuchoidea Millerosauria Procolophonomorpha Ankyramorpha Neodiapsida Eosuchia Younginiformes Avicephala Eolacertilia Euryapsida
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