Editing Synapsida (section)

{{Short description|Clade of tetrapods}}
{{Distinguish|synapse|synapsis|therapsid}}
{{Automatic taxobox
| fossil_range = {{fossilrange|312|0|[[Pennsylvanian (geology)|Pennsylvanian]]–[[Holocene]], 318？<ref name=Steen1934>{{cite journal | last=Steen | first=Margaret C. | title=The amphibian fauna from the South Joggins. Nova Scotia | journal=Journal of Zoology | volume=104 | issue=3 | pages=465–504 | year=1934 | doi=10.1111/j.1096-3642.1934.tb01644.x }}</ref>–0&nbsp;Ma| | earliest=312}}<small>Probably originated in the [[Carboniferous]] [[Mississippian]]<small><ref>{{cite journal|last1=Long |first1=J. A. |last2=Niedźwiedzki |first2=G. |last3=Garvey |first3=J. |last4=Clement |first4=A. M. |last5=Camens |first5=A. B. |last6=Eury |first6=C. A. |last7=Eason |first7=J. |last8=Ahlberg |first8=P. E. |year=2025 |title=Earliest amniote tracks recalibrate the timeline of tetrapod evolution |journal=Nature |volume=641 |issue=8065 |pages=1193–1200 |doi=10.1038/s41586-025-08884-5 |pmid=40369062 |pmc=12119326 |doi-access=free }}</ref>
| image = Synapsid diversity 3.jpg
| image_caption = Examples of synapsids (left to right, top to bottom): ''[[Cotylorhynchus]]'', ''[[Dimetrodon]]'', ''[[Inostrancevia]]'', ''[[Moschops]]'',  ''[[Castorocauda]]'', ''[[Adelobasileus]]'',  ''[[Tachyglossus aculeatus]]'', and ''[[Panthera tigris]]''
| image_upright = 1.3
| taxon = Synapsida
| display_parents = 3
| authority = [[Henry Fairfield Osborn|Osborn]], 1903
| subdivision_ranks = Subgroups
| subdivision = 
* {{extinct}}''[[Asaphestera]]''
* {{extinct}}''[[Protoclepsydrops]]''?
* {{extinct}}[[Diadectomorpha]]?<ref name=Berman>{{Cite journal|author=David S. Berman |year=2013 |title=Diadectomorphs, amniotes or not? |journal=New Mexico Museum of Natural History and Science Bulletin |volume=60 |pages=22–35 |url=https://books.google.com/books?id=5f4oCgAAQBAJ&pg=PA22}}</ref><ref name=Klembara>{{cite journal |first1=Jozef |last1=Klembara |first2=Miroslav |last2=Hain |first3=Marcello |last3=Ruta |first4=David S. |last4=Berman |first5=Stephanie E. |last5=Pierce |first6=Amy C. |last6=Henrici |year=2019 |title=Inner ear morphology of diadectomorphs and seymouriamorphs (Tetrapoda) uncovered by high-resolution x-ray microcomputed tomography, and the origin of the amniote crown group |journal=Palaeontology |volume=63 |pages= 131–154 |doi=10.1111/pala.12448 |doi-access=free }}</ref><ref name="Brocklehurst2021">{{cite journal |last1=Brocklehurst |first1=N. |year=2021 |title=The first age of reptiles? Comparing reptile and synapsid diversity, and the influence of lagerstätten, during the Carboniferous and early Permian |journal=Frontiers in Ecology and Evolution |volume=9 |pages=669765 |doi=10.3389/fevo.2021.669765 |doi-access=free }}</ref>
* {{extinct}}[[Caseasauria]]
** {{extinct}}[[Caseidae]]
** {{extinct}}[[Eothyrididae]]
** {{extinct}}''[[Phreatophasma]]''?
* [[Eupelycosauria]]
** {{extinct}}[[Ophiacodontidae]]
** {{extinct}}[[Varanopidae]]?<ref>{{cite journal |last1=Simões |first1=T. |last2=Kammerer |first2=C. |date=August 2022 |title=Successive climate crises in the deep past drove the early evolution and radiation of reptiles |journal=Science Advances |volume=08 |issue=33 |pages= eabq1898|doi=10.1126/sciadv.abq1898 |pmid=35984885 |pmc=9390993 |bibcode=2022SciA....8.1898S |s2cid=251694019 |doi-access=free }}</ref>
** [[Haptodontiformes]]
*** {{extinct}}[[Edaphosauridae]]
*** [[Sphenacodontia]]
| synonyms = Theropsida {{small|([[Harry Seeley|Seeley]], 1895)}}<ref>{{cite journal |last = Seeley |first = Harry Govier |year = 1895 |title = Researches on the structure, organisation, and classification of the fossil reptilia. Part&nbsp;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 |journal = Proceedings of the Royal Society of London |volume = 59 |issue = 353–358 |pages = 167–169 |doi = 10.1098/rspl.1895.0070 |doi-access = free |url = https://zenodo.org/record/1432087}}</ref>                                                                    
"[[Pelycosauria]]" (Cladistically including therapsids)
}}

'''Synapsida'''{{efn|{{langx|el|συν-|syn-|together}} + {{lang|el|ἁψίς}} ({{transliteration|el|apsís}}, 'arch') > {{lang|el|*συναψίδης}} ({{transliteration|el|synapsídes}}) "having a fused arch"; synonymous with '''''theropsids''''' (Greek, "beast-face")}} is a diverse group of [[tetrapod]] [[vertebrate]]s that includes all [[mammal]]s and their extinct relatives. It is one of the two major [[clade]]s of the group [[Amniota]], the other being the more diverse group [[Sauropsid]]a (which includes all extant [[reptile]]s and therefore, [[bird]]s). Unlike other amniotes, synapsids have a single [[temporal fenestra]], an opening low in the [[skull roof]] behind each [[eye socket]], leaving a [[zygomatic arch|bony arch]] beneath each; this accounts for the name "synapsid".<ref name=Romer>{{cite book |author1-link=Alfred Romer |author1=Romer, A.S |author2=Parsons, T.S. |year=1985 |title=The Vertebrate Body |edition=6th |publisher=Saunders |place=Philadelphia, PA}}</ref> The distinctive temporal fenestra developed about 318&nbsp;million years ago during the [[Late Carboniferous]] period,<ref name=Steen1934/> when synapsids and sauropsids diverged, but was subsequently merged with the orbit in early mammals.

The [[basal (phylogenetics)|basal]] amniotes ([[reptiliomorph]]s) 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 '''[[pelycosaur]]s''' or '''pelycosaur-[[evolutionary grade|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.<ref name=Carroll397>{{cite book |author-link=Robert L. Carroll |author=Carroll, Robert L. |year=1988 |title=Vertebrate Paleontology and Evolution |title-link=Vertebrate Paleontology and Evolution |place=New York, NY |publisher=W.H. Freeman & Co. |isbn=0-7167-1822-7 |page=397}}</ref><ref name="Dilkes1996">{{cite journal|author=D. W. Dilkes, R. R. Reisz|year=1996|url=https://www.researchgate.net/publication/238324333|title=First Record of a Basal Synapsid ('Mammal-Like Reptile') in Gondwana|journal=Proceedings of the Royal Society B|volume=263|issue=1374|pages=1165–1170|doi=10.1098/rspb.1996.0170}}</ref><ref name=Benton122>{{cite book |author-link=Michael J. Benton |author=Benton, Michael J. |year=2005 |title-link=Vertebrate Palaeontology (Benton) |title=Vertebrate Palaeontology |edition=3rd |place=Oxford, UK |publisher=[[Blackwell's|Blackwell Science Ltd]] |isbn=0-632-05637-1 |page=122}}</ref><ref>{{Cite web|url=https://evolution.berkeley.edu/evolibrary/article/evograms_05|title=Jaws to ears in the ancestors of mammals|website=evolution.berkeley.edu|access-date=2020-02-20}}</ref> 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'''.<ref>{{cite web|url=http://www.uchospitals.edu/news/2013/20130807-megaconus.html|title=New proto-mammal fossil sheds light on evolution of earliest mammals|publisher=University of Chicago |date=August 7, 2013}}</ref><ref>{{Cite web |last=Naish |first=Darren |title=The Stem-Mammals--a Brief Primer |url=https://blogs.scientificamerican.com/tetrapod-zoology/the-stem-mammals-a-brief-primer/ |access-date=2022-02-27 |website=Scientific American Blog Network |language=en}}</ref><ref name=M2021/> 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 animal|terrestrial]] vertebrates in the [[Permian]] period (299 to 251&nbsp;[[million years ago|mya]]), rivalled only by some large [[pareiasaurian]] [[parareptile]]s such as ''[[Scutosaurus]]''. They were the [[dominance (ecology)|dominant]] land [[predator]]s of the late [[Paleozoic]] and early [[Mesozoic]], with [[eupelycosaur]]s such as ''[[Dimetrodon]]'', ''[[Titanophoneus]]'' and ''[[Inostrancevia]]'' being the [[apex predator]]s during the Permian, and [[theriodont]]s 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 [[dicynodont]]s and [[eutheriodonts]] (consisting of [[therocephalian]]s and [[cynodont]]s) 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,<ref name=Benton-2006>{{cite book |first=M.J. |last=Benton |author-link=Michael J. Benton |year=2006 |title=When Life Nearly Died: The greatest mass extinction of all time |isbn=978-0-500-28573-2 |publisher=Thames & Hudson |location=London, UK }}</ref><ref name=Cokinos-2007>{{cite magazine |first=Christopher |last=Cokinos |title=The consolations of extinction |orig-date=May–June 2007 |date=12 October 2007 |magazine=[[Orion (magazine)|Orion]] |url=http://www.orionmagazine.org/index.php/articles/article/268/ |archive-url=https://web.archive.org/web/20071012225407/http://www.orionmagazine.org/index.php/articles/article/268/ |archive-date=2007-10-12}}</ref> but declined again after the [[Smithian–Spathian boundary event]] with their dominant [[ecological niche|niche]]s largely taken over by the rise of [[archosaurian]] sauropsids, first by the [[pseudosuchian]]s and then by the [[pterosaur]]s and [[dinosaur]]s. The cynodont group [[Probainognathia]], which includes the group [[Mammaliaformes]], were the only synapsids to survive beyond the Triassic,<ref>{{cite press release |title=Greatest mass extinction responsible for the making of modern mammals |date=19 September 2013 |place=Bloemfontein, ZA |publisher=The National Museum [of South Africa] |type=research publ. ann. |url=http://www.nasmus.co.za/museum/news/greatest-mass-extinction-responsible-making-modern-mammals |url-status=dead |access-date=2015-08-22 |archive-url=https://web.archive.org/web/20190328120431/http://www.nasmus.co.za/museum/news/greatest-mass-extinction-responsible-making-modern-mammals |archive-date=2019-03-28}}</ref> and mammals are the only synapsid lineage that have survived past the [[Jurassic]], having [[nocturnal bottleneck|lived mostly nocturnally]] to avoid [[competition (biology)|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]].<ref>{{cite web |author1=Laurin, Michel |author2=Reisz, Robert R. |year=2011 |title=Synapsida: Mammals and their extinct relatives |edition=Version&nbsp;14 |publisher=The [[Tree of Life Web Project]] |url=http://tolweb.org/Synapsida/14845}}</ref>