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Synapsida

Not to be confused with synapse, synapsis, or therapsid.

Synapsida
Temporal range:
Examples of synapsids (left to right, top to bottom): Cotylorhynchus, Dimetrodon, Inostrancevia, Moschops, Castorocauda, Adelobasileus, Tachyglossus aculeatus, and Panthera tigris
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Superclass: Tetrapoda
Clade: Reptiliomorpha
Clade: Amniota
Clade: Synapsida
Osborn, 1903
Subgroups
Synonyms

Theropsida (Seeley, 1895)[6] "Pelycosauria" (Cladistically including therapsids)

Synapsida[a] is a diverse group of tetrapod vertebrates that includes all mammals and their extinct relatives. It is one of the two major clades of the group Amniota, the other being the more diverse group Sauropsida (which includes all extant reptiles and birds). Unlike other amniotes, synapsids have a single temporal fenestra, an opening low in the skull roof behind each eye socket, leaving a bony arch beneath each; this accounts for the name "synapsid".[7] The distinctive temporal fenestra developed about 318 million years ago during the Late Carboniferous period,[1] when synapsids and sauropsids diverged, but was subsequently merged with the orbit in early mammals.

The basal amniotes (reptiliomorphs) from which synapsids evolved were historically simply called "reptiles". Therefore, stem group synapsids were then described as mammal-like reptiles in classical systematics, and non-therapsid synapsids were also referred to as pelycosaurs or pelycosaur-grade synapsids. These paraphyletic terms have now fallen out of favor and are only used informally (if at all) in modern literature, as it is now known that all extant reptiles are more closely related to each other and birds than to synapsids, so the word "reptile" has been re-defined to mean only members of Sauropsida or even just an under-clade thereof. In a cladistic sense, synapsids are in fact a monophyletic sister taxon of sauropsids, rather than a part of the sauropsid lineage.[8][9][10][11] Therefore, calling synapsids "mammal-like reptiles" is incorrect under the new definition of "reptile", so they are now referred to as stem mammals, proto-mammals, paramammals or pan-mammals.[12][13][14] Most lineages of pelycosaur-grade synapsids were replaced by the more advanced therapsids, which evolved from sphenacodontoid pelycosaurs, at the end of the Early Permian during the so-called Olson's Extinction.

Synapsids were the largest terrestrial vertebrates in the Permian period (299 to 251 mya), rivalled only by some large pareiasaurian parareptiles such as Scutosaurus. They were the dominant land predators of the late Paleozoic and early Mesozoic, with eupelycosaurs such as Dimetrodon, Titanophoneus and Inostrancevia being the apex predators during the Permian, and theriodonts such as Moschorhinus during the Early Triassic. Synapsid population and diversity were severely reduced by the Capitanian mass extinction event and the Permian–Triassic extinction event, and only two groups of therapsids, the dicynodonts and eutheriodonts (consisting of therocephalians and cynodonts) are known to have survived into the Triassic. These therapsids rebounded as disaster taxa during the early Mesozoic, with the dicynodont Lystrosaurus making up as much as 95% of all land species at one time,[15][16] but declined again after the Smithian–Spathian boundary event with their dominant niches largely taken over by the rise of archosaurian sauropsids, first by the pseudosuchians and then by the pterosaurs and dinosaurs. The cynodont group Probainognathia, which includes the group Mammaliaformes, were the only synapsids to survive beyond the Triassic,[17] and mammals are the only synapsid lineage that have survived past the Jurassic, having lived mostly nocturnally to avoid competition with dinosaurs. After the Cretaceous-Paleogene extinction wiped out all non-avian dinosaurs and pterosaurs, synapsids (as mammals) rose to dominance once again during the Cenozoic.[18]

  1. ^ a b Steen, Margaret C. (1934). "The amphibian fauna from the South Joggins. Nova Scotia". Journal of Zoology. 104 (3): 465–504. doi:10.1111/j.1096-3642.1934.tb01644.x.
  2. ^ David S. Berman (2013). "Diadectomorphs, amniotes or not?". New Mexico Museum of Natural History and Science Bulletin. 60: 22–35.
  3. ^ Klembara, Jozef; Hain, Miroslav; Ruta, Marcello; Berman, David S.; Pierce, Stephanie E.; Henrici, Amy C. (2019). "Inner ear morphology of diadectomorphs and seymouriamorphs (Tetrapoda) uncovered by high‐resolution x‐ray microcomputed tomography, and the origin of the amniote crown group". Palaeontology. 63: 131–154. doi:10.1111/pala.12448.
  4. ^ Brocklehurst, N. (2021). "The first age of reptiles? Comparing reptile and synapsid diversity, and the influence of lagerstätten, during the Carboniferous and early Permian". Frontiers in Ecology and Evolution. 9: 669765. doi:10.3389/fevo.2021.669765.
  5. ^ Simões, T.; Kammerer, C. (August 2022). "Successive climate crises in the deep past drove the early evolution and radiation of reptiles". Science Advances. 08 (33): eabq1898. Bibcode:2022SciA....8.1898S. doi:10.1126/sciadv.abq1898. PMC 9390993. PMID 35984885. S2CID 251694019.
  6. ^ Seeley, Harry Govier (1895). "Researches on the structure, organisation, and classification of the fossil reptilia. Part X. On the complete skeleton of an anomodont reptile (Aristodesmus rutimeyeri, Wiedersheim), from the Bunter sandstone of Reihen, near Basel, giving new evidence of the relation of the Anomodontia to the Monotremata". Proceedings of the Royal Society of London. 59: 167–169. doi:10.1098/rspl.1895.0070.
  7. ^ Romer, A.S; Parsons, T.S. (1985). The Vertebrate Body (6th ed.). Philadelphia, PA: Saunders.
  8. ^ Carroll, Robert L. (1988). Vertebrate Paleontology and Evolution. New York, NY: W.H. Freeman & Co. p. 397. ISBN 0-7167-1822-7.
  9. ^ D. W. Dilkes, R. R. Reisz (1996). "First Record of a Basal Synapsid ('Mammal-Like Reptile') in Gondwana". Proceedings of the Royal Society B. 263 (1374): 1165–1170. doi:10.1098/rspb.1996.0170.
  10. ^ Benton, Michael J. (2005). Vertebrate Palaeontology (3rd ed.). Oxford, UK: Blackwell Science Ltd. p. 122. ISBN 0-632-05637-1.
  11. ^ "Jaws to ears in the ancestors of mammals". evolution.berkeley.edu. Retrieved 2020-02-20.
  12. ^ "New proto-mammal fossil sheds light on evolution of earliest mammals". University of Chicago. August 7, 2013.
  13. ^ Naish, Darren. "The Stem-Mammals--a Brief Primer". Scientific American Blog Network. Retrieved 2022-02-27.
  14. ^ Cite error: The named reference M2021 was invoked but never defined (see the help page).
  15. ^ Benton, M.J. (2006). When Life Nearly Died: The greatest mass extinction of all time. London, UK: Thames & Hudson. ISBN 978-0-500-28573-2.
  16. ^ Cokinos, Christopher (12 October 2007) [May-June 2007]. "The consolations of extinction". Orion. Archived from the original on 2007-10-12.
  17. ^ "Greatest mass extinction responsible for the making of modern mammals" (research publ. ann.). Bloemfontein, ZA: The National Museum [of South Africa]. 19 September 2013. Archived from the original on 2019-03-28. Retrieved 2015-08-22.
  18. ^ Laurin, Michel; Reisz, Robert R. (2011). "Synapsida: Mammals and their extinct relatives" (Version 14 ed.). The Tree of Life Web Project.


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Synapsida

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