A euryapsid skull.

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[1] while other authors disagree.[2]

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,[3] weigeltisaurids, protorosaurs, and trilophosaurs.[4][5] 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).

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.[6]

A parapsid skull.

The terms Enaliosauria and Halisauria are also used for a taxon including ichthyosaurs and sauropterygians.[7][8]

See also

References

  1. ↑ Lee, M. S. Y. (2013). "Turtle origins: insights from phylogenetic retrofitting and molecular scaffolds". Journal of Evolutionary Biology. 26 (12): 2729–2738. doi:10.1111/jeb.12268.
  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. doi:10.1126/sciadv.abq1898. ISSN 2375-2548. PMC 9390993. PMID 35984885.
  3. ↑ Williston, Samuel Wendell (1925). Osteology of the Reptiles.
  4. ↑ Romer, Alfred Sherwood (1956). Osteology of the Reptiles.
  5. ↑ Romer, Alfred Sherwood (1966). Vertebrate Paleontology.
  6. ↑ Maisch, M. (2010). "Phylogeny, systematics, and origin of the Ichthyosauria – the state of the art". Palaeodiversity. 3: 151–214. S2CID 87979321.
  7. ↑ "Enaliosauria". Merriam-Webster. Retrieved 22 December 2021.
  8. ↑ Haeckel, Ernst (1895). Die systematische Phylogenie (Dritter Theil/Systematische Phylogenie der Wirbelthiere (Vertebrata)). Vol. 3. Georg Reimer.
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