View source for Synapsida

{{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 (geology)|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]]
**** other extinct groups
**** [[Therapsida]]
| 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>

==Linnaean and cladistic classifications==

At the turn of the 20th century, synapsids were thought to be one of the four main subclasses of [[reptile]]s. However, this notion was disproved upon closer inspection of skeletal remains, as synapsids are differentiated from reptiles by their distinctive temporal openings. These openings in the [[skull]] bones allowed the attachment of larger jaw muscles, hence a more efficient bite.

Synapsids were subsequently considered to be a later reptilian lineage that became mammals by gradually [[evolution|evolving]] increasingly mammalian features, hence the name "mammal-like reptiles" (also known as [[pelycosaur]]s). These became the traditional terms for all [[Paleozoic]] (early) synapsids. More recent studies have debunked this notion as well, and reptiles are now classified within [[Sauropsida]] (sauropsids), the sister group to synapsids, thus making synapsids their own [[taxonomy|taxonomic]] group.<ref name=Carroll397/><ref name=Benton122/>

As a result, the [[paraphyletic]] terms "mammal-like reptile" and "pelycosaur" are seen as outdated and disfavored in technical literature, and the term '''stem mammal''' (or sometimes '''protomammal''' or '''paramammal''') is used instead. [[Phylogenetically]], it is now understood that synapsids comprise an independent branch of the [[tree of life (biology)|tree of life]].<ref>{{cite journal |last = Angielczyk |first = Kenneth D. |year = 2009 |title = Dimetrodon is not a dinosaur: Using tree thinking to understand the ancient relatives of mammals and their evolution |journal = Evolution: Education and Outreach |volume = 2 |issue = 2 |pages = 257–271 |s2cid = 24110810 |doi = 10.1007/s12052-009-0117-4 |doi-access = free}}</ref> The [[monophyly]] of Synapsida is not in doubt, and the expressions such as "Synapsida contains the mammals" and "synapsids gave rise to the mammals" both express the same phylogenetic hypothesis. This terminology reflects the modern [[cladistic]] approach to animal relationships, according to which the only valid groups are those that include all of the descendants of a common ancestor: these are known as [[monophyletic]] groups, or [[clade]]s.

Additionally, [[Reptilia]] (reptiles) has been revised into a monophyletic group and is considered entirely distinct from Synapsida, falling within [[Sauropsida]], the sister group of Synapsida within Amniota.<ref name="modestoanderson2004">{{cite journal|last1=Modesto|first1=S.P.|last2=Anderson|first2=J.S.|year=2004|title=The phylogenetic definition of Reptilia|journal=Systematic Biology|volume=53|issue=5|pages=815–821|doi=10.1080/10635150490503026|pmid=15545258|doi-access=free}}</ref>

===Primitive and advanced synapsids===
The synapsids are traditionally divided for convenience, into [[therapsid]]s, an advanced group of synapsids and the branch within which mammals evolved, and stem mammals, (previously known as [[pelycosaur]]s), comprising the other six more primitive families of synapsids.<ref>[[Michael J. Benton|Benton, Michael J.]] (2005).'' [[Vertebrate Paleontology (Benton)|Vertebrate Paleontology]]'', 3rd ed. Oxford: [[Blackwell's|Blackwell Science Ltd]]. {{ISBN|0-632-05637-1}}. p. 120.</ref> Stem mammals were all rather lizard-like, with sprawling gait and possibly horny [[scute]]s, while therapsids tended to have a more erect pose and possibly hair, at least in some forms. In traditional taxonomy, the Synapsida encompasses two distinct [[evolutionary grade|grades]]: the low-slung stem mammals have given rise to the more erect therapsids, who in their turn have given rise to the mammals. In traditional vertebrate classification, the stem mammals and therapsids were both considered [[order (biology)|orders]] of the subclass Synapsida.<ref name=Romer/><ref name=Carroll397/>

===Practical versus phylogenetic usage of "synapsid" and "therapsid"===
In [[phylogenetic nomenclature]], the terms are used somewhat differently, as the daughter clades are included. Most papers published during the 21st century have treated "Pelycosaur" as an informal grouping of primitive members. Therapsida has remained in use as a clade containing both the traditional therapsid families and mammals.

Although Synapsida and Therapsida include modern mammals, in practical usage, those two terms are used almost exclusively when referring to the more [[basal (phylogenetics)|basal]] members that lie outside of [[Mammaliaformes]].

==Characteristics==

===Temporal openings===
[[File:Skull synapsida 1.svg|thumb|right|200px|The synapsids are distinguished by a single hole, known as the [[temporal fenestra]], in the skull behind each eye. This schematic shows the skull viewed from the left side. The middle opening is the orbit of the eye; the opening to the right of it is the temporal fenestra.]]
Synapsids evolved a [[temporal fenestra]] behind each eye [[orbit (anatomy)|orbit]] on the lateral surface of the skull. It may have provided new attachment sites for jaw muscles. A similar development took place in the [[diapsid]]s, which evolved two rather than one opening behind each eye. Originally, the openings in the skull left the inner cranium covered only by the jaw muscles, but in higher therapsids and mammals, the [[sphenoid bone]] has expanded to close the opening. This has left the lower margin of the opening as an arch extending from the lower edges of the braincase.

===Teeth===
[[File:Eothyris head.jpg|thumb|200px|right|''[[Eothyris]]'', an early synapsid with multiple canines]]
Synapsids are characterized by having differentiated teeth. These include the [[Canine tooth|canine]]s, [[molars]], and [[incisor]]s.<ref>Angielczch, Kennenth; Kammerer, Christian F.; Frobisch, Jorg. (2013). ''Early Evolutionary History of Synapsida''. Springer Science & Business Media. {{ISBN|978-94-007-6841-3}}, p. 11</ref> The trend towards differentiation is found in some [[labyrinthodont]]s and early [[anapsida|anapsid]] reptilians in the form of enlargement of the first teeth on the [[maxilla]], forming a sort of protocanines. This trait was subsequently lost in the [[diapsid]] line, but developed further in the synapsids. Early synapsids could have two or even three enlarged "canines", but in the therapsids, the pattern had settled to one canine in each upper jaw half. The lower canines developed later.

===Jaw===
The jaw transition is a good [[Scientific classification|classification]] tool, as most other fossilized features that make a chronological progression from a reptile-like to a mammalian condition follow the progression of the jaw transition. The [[mandible]], or lower jaw, consists of a single, tooth-bearing bone in mammals (the dentary), whereas the lower jaw of modern and prehistoric reptiles consists of a conglomeration of smaller bones (including the dentary, [[articular]], and others). As they evolved in synapsids, these jaw bones were reduced in size and either lost or, in the case of the articular, gradually moved into the ear, forming one of the middle ear bones: while modern mammals possess the [[malleus]], [[incus]] and [[stapes]], [[Basal (phylogenetics)|basal]] synapsids (like all other tetrapods) possess only a stapes. The malleus is derived from the articular (a lower jaw bone), while the incus is derived from the [[quadrate bone|quadrate]] (a cranial bone).<ref name="Salentijn">Salentijn, L. ''Biology of Mineralized Tissues: Prenatal Skull Development'', [[Columbia University College of Dental Medicine]] post-graduate dental lecture series, 2007</ref>

Mammalian jaw structures are also set apart by the dentary-squamosal [[temporomandibular joint|jaw joint]]. In this form of jaw joint, the dentary forms a connection with a depression in the [[squamosal]] known as the [[glenoid cavity]]. In contrast, all other jawed vertebrates, including reptiles and nonmammalian synapsids, possess a jaw joint in which one of the smaller bones of the lower jaw, the articular, makes a connection with a bone of the [[skull|cranium]] called the [[quadrate bone]] to form the articular-quadrate jaw joint. In forms transitional to mammals, the jaw joint is composed of a large, lower jaw bone (similar to the dentary found in mammals) that does not connect to the squamosal, but connects to the quadrate with a receding articular bone.

===Palate===
Over time, as synapsids became more mammalian and less 'reptilian', they began to develop a [[secondary palate]], separating the mouth and [[nasal cavity]]. In early synapsids, a secondary palate began to form on the sides of the [[maxilla]], still leaving the mouth and nostril connected.

Eventually, the two sides of the palate began to curve together, forming a U shape instead of a C shape. The palate also began to extend back toward the throat, securing the entire mouth and creating a full [[palatine bone]]. The maxilla is also closed completely. In fossils of one of the first [[eutheriodont]]s, the beginnings of a palate are clearly visible. The later ''[[Thrinaxodon]]'' has a full and completely closed palate, forming a clear progression.<ref>{{cite journal |last=Hopson |first=James A. |year=1987 |title=The Mammal-Like Reptiles: A Study of Transitional Fossils |journal=The American Biology Teacher |volume=49 |issue=1 |pages=16–26|doi=10.2307/4448410 |jstor=4448410 }}</ref>

===Skin and fur===
[[File:Sea Otter kuchang kushiro hokkaido.jpg|thumb|The [[sea otter]] has the densest fur of modern mammals.]]
In addition to the glandular skin covered in fur found in most modern mammals, modern and extinct synapsids possess a variety of modified skin coverings, including [[osteoderm]]s (bony armor embedded in the skin), [[scute]]s (protective structures of the dermis often with a horny covering), hair or fur, and [[Scale (anatomy)|scale]]-like structures (often formed from modified hair, as in [[pangolin]]s and some [[rodent]]s). While the skin of reptiles is rather thin, that of mammals has a thick [[dermis|dermal]] layer.<ref>{{cite book |last1=Hildebran |first1=M. |last2=Goslow |first2=G. |year=2001 |title=Analysis of Vertebrate Structure |edition=5th |publisher=John Wiley & Sons |location=New York |isbn=0-471-29505-1}}</ref>

The ancestral skin type of synapsids has been subject to discussion. The type specimen of the oldest known synapsid ''[[Asaphestera]]'' preserved [[Scale (zoology)|scale]]s.<ref name="Mann Gee Pardo Marjanović p. ">{{cite journal | last=Mann | first=Arjan | last2=Gee | first2=Bryan M. | last3=Pardo | first3=Jason D. | last4=Marjanović | first4=David | last5=Adams | first5=Gabrielle R. | last6=Calthorpe | first6=Ami S. | last7=Maddin | first7=Hillary C. | last8=Anderson | first8=Jason S. | year=2020 | title=Reassessment of historic ‘microsaurs’ from Joggins, Nova Scotia, reveals hidden diversity in the earliest amniote ecosystem | journal=Papers in Palaeontology |volume=6 |issue=4 |pages=605-625 |doi=10.1002/spp2.1316 |url=https://www.researchgate.net/publication/341164991_Reassessment_of_historic_'microsaurs'_from_Joggins_Nova_Scotia_reveals_hidden_diversity_in_the_earliest_amniote_ecosystem}}</ref> Impressions of [[epidermal scale]]s are preserved in the [[Early Permian]] ([[Sakmarian]]) synapsid trace fossil ''Bromackerichnus requiescens'' from the [[Tambach Formation]] ([[Germany]]), the only known early synapsid body impression most likely belonging to a [[sphenacodontid]] based on its association with ''[[Dimetropus]]''.<ref>{{cite journal |last1=Marchetti |first1=L. |last2=Logghe |first2=A. |last3=Buchwitz |first3=M. |last4=Fröbisch |first4=J. |year=2025 |title=Early Permian synapsid impressions illuminate the origin of epidermal scales and aggregation behavior |journal=Current Biology |doi=10.1016/j.cub.2025.04.077 |doi-access=free }}</ref> Among the early synapsids, only two species of small [[varanopid]]s have been found to possess [[osteoderm]]s;<ref>{{cite journal |author1=Vickaryous, Matthew K. |author2=Sire, Jean-Yves |name-list-style=amp |year=2009 |title=The integumentary skeleton of tetrapods: origin, evolution, and development |journal=Journal of Anatomy |volume=214 |issue= 4|pages=441–464 |doi=10.1111/j.1469-7580.2008.01043.x |pmid=19422424 |pmc=2736118}}</ref> fossilized rows of [[osteoderm]]s indicate bony armour on the neck and back. However, some recent studies have cast doubt on the placement of Varanopidae in Synapsida,<ref>{{Cite journal |last1=Ford |first1=David P. |last2=Benson |first2=Roger B. J. |date=May 2019 |editor-last=Mannion |editor-first=Philip |title=A redescription of Orovenator mayorum (Sauropsida, Diapsida) using high-resolution μ CT, and the consequences for early amniote phylogeny |journal=Papers in Palaeontology |language=en |volume=5 |issue=2 |pages=197–239 |doi=10.1002/spp2.1236 |issn=2056-2802|doi-access=free |bibcode=2019PPal....5..197F }}</ref><ref>{{Cite journal |last1=Ford |first1=David P. |last2=Benson |first2=Roger B. J. |date=January 2020 |title=The phylogeny of early amniotes and the affinities of Parareptilia and Varanopidae |url=https://www.nature.com/articles/s41559-019-1047-3 |journal=Nature Ecology & Evolution |language=en |volume=4 |issue=1 |pages=57–65 |doi=10.1038/s41559-019-1047-3 |pmid=31900445 |issn=2397-334X}}</ref> while others have countered and lean towards this traditional placement.<ref>{{Cite journal |last1=Maddin |first1=Hillary C. |last2=Mann |first2=Arjan |last3=Hebert |first3=Brian |date=January 2020 |title=Varanopid from the Carboniferous of Nova Scotia reveals evidence of parental care in amniotes |url=https://www.nature.com/articles/s41559-019-1030-z |journal=Nature Ecology & Evolution |language=en |volume=4 |issue=1 |pages=50–56 |doi=10.1038/s41559-019-1030-z |pmid=31900446 |issn=2397-334X|url-access=subscription }}</ref><ref>{{Cite journal |last1=Benoit |first1=Julien |last2=Ford |first2=David |last3=Miyamae |first3=Juri |last4=Ruf |first4=Irina |date=2021 |title=Can maxillary canal morphology inform varanopid phylogenetic affinities? |journal=Acta Palaeontologica Polonica |language=en |volume=66 |doi=10.4202/app.00816.2020 |issn=0567-7920|doi-access=free }}</ref> Skin impressions indicate some early synapsids basal possessed rectangular scutes on their undersides and tails.<ref>{{Cite journal |last=Reisz |first=Robert |date=1975 |title=Pennsylvanian Pelycosaurs from Linton, Ohio and Nýřany, Czechoslovakia |url=https://www.jstor.org/stable/1303422 |journal=Journal of Paleontology |volume=49 |issue=3 |pages=522–527 |jstor=1303422 |issn=0022-3360}}</ref><ref name=NB12>{{Cite journal | last1 = Niedźwiedzki | first1 = G. | last2 = Bojanowski | first2 = M. | doi = 10.1080/10420940.2012.702549 | title = A supposed eupelycosaur body impression from the early Permian of the Intra-Sudetic basin, Poland | journal = Ichnos | volume = 19 | issue = 3 | pages = 150–155 | year = 2012 | bibcode = 2012Ichno..19..150N | s2cid = 129567176 }}</ref> The pelycosaur scutes probably were nonoverlapping [[dermal]] structures with a horny overlay, like those found in modern [[crocodile]]s and [[turtle]]s. These differed in structure from the [[Reptile scale|scales of lizards and snakes]], which are an epidermal feature (like mammalian hair or avian feathers).<ref name="caroll1969">{{cite journal | last1 = Carroll | first1 = R.L. | year = 1969 | title = Problems of the origin of reptiles | journal = Biological Reviews | volume = 44 | issue = 3 | pages = 393–432 | doi = 10.1111/j.1469-185X.1969.tb01218.x | s2cid = 84302993 }}</ref> Recently, skin impressions from the genus ''[[Ascendonanus]]'' suggest that at least varanopsids developed scales similar to those of [[squamate]]s.<ref>{{cite journal |first1=Frederik |last1=Spindler |first2=Ralf |last2=Werneburg |first3=Joerg W. |last3=Schneider |first4=Ludwig |last4=Luthardt |first5=Volker |last5=Annacker |first6=Ronny |last6=Rößler |title=First arboreal 'pelycosaurs' (Synapsida: Varanopidae) from the early Permian Chemnitz Fossil Lagerstätte, SE Germany, with a review of varanopid phylogeny |journal=[[Paläontologische Zeitschrift|PalZ]] |year=2018 |volume=92 |issue=2 |pages=315–364 |doi=10.1007/s12542-018-0405-9 |bibcode=2018PalZ...92..315S |s2cid=133846070 }}</ref>

It is currently unknown exactly when mammalian characteristics such as [[fur|body hair]] and [[mammary gland]]s first appeared, as the fossils only rarely provide direct evidence for soft tissues. An exceptionally well-preserved skull of ''[[Estemmenosuchus]]'', a therapsid from the Upper Permian, preserves smooth skin with what appear to be glandular depressions,<ref>{{cite book |last=Kardong |first=K.V. |year=2002 |title=Vertebrates: Comparative Anatomy, Function, Evolution |url=https://archive.org/details/vertebratescompa00kard |url-access=registration |edition=3rd |publisher=McGraw-Hill |location=Boston |isbn=0-07-112235-4 }}</ref> an animal noted as being semi-[[aquatic animal|aquatic]].<ref>{{cite journal |title=The origin and early radiation of the therapsid mammal-like reptiles: A palaeobiological hypothesis |first=T.S. |last=Kemp |journal=[[Journal of Evolutionary Biology]] |volume=19 |issue=4 |year=2006 |pages=1231–1247 |doi=10.1111/j.1420-9101.2005.01076.x |pmid=16780524 |s2cid=3184629 |doi-access=free }}</ref> The oldest known fossil showing unambiguous imprints of hair is the [[Callovian]] (late middle [[Jurassic]]) ''[[Castorocauda]]'' and several contemporary [[haramiyida]]ns, both non-mammalian [[mammaliaform]]<ref name="JiLuoYuanTabrumCastorocauda">{{Cite journal |last1=Ji |first1=Q. |last2=Luo |first2=Z-X |last3=Yuan |first3=Chong-Xi |last4=Tabrum |first4=Alan R. |date=February 2006 |title=A swimming mammaliaform from the middle Jurassic and ecomorphological diversification of early mammals |journal=Science |volume=311 |issue=5764 |pmid=16497926 |doi=10.1126/science.1123026|pages=1123–7 |bibcode=2006Sci...311.1123J |s2cid=46067702|url=http://doc.rero.ch/record/13437/files/PAL_E249.pdf }}<br/>See also the news item at {{cite news |title=Jurassic "beaver" found; rewrites history of mammals |date=Feb 2006 |website=[[National Geographic]] |url=http://news.nationalgeographic.com/news/2006/02/0223_060223_beaver.html}}</ref><ref>{{cite journal | last1 = Meng | first1 = Qing-Jin | last2 = Grossnickle | first2 = David M. | last3 = Di | first3 = Liu | last4 = Zhang | first4 = Yu-Guang | last5 = Neander | first5 = April I. | last6 = Ji | first6 = Qiang | last7 = Luo | first7 = Zhe-Xi | year = 2017 | title = New gliding mammaliaforms from the Jurassic | journal = Nature | volume = 548 | issue = 7667 | pages = 291–296 | doi = 10.1038/nature23476 | pmid = 28792929 | bibcode = 2017Natur.548..291M | s2cid = 205259206 }}</ref> (see below, however). More primitive members of the [[Cynodontia]] are also hypothesized to have had fur or a fur-like covering based on their inferred warm-blooded metabolism.<ref name=Fur/> While more direct evidence of fur in early cynodonts has been proposed in the form of small pits on the snout possibly associated with [[whisker]]s, such pits are also found in some reptiles that lack whiskers.<ref name=Fur/> There is evidence that some other non-mammalian cynodonts more basal than ''Castorocauda'', such as ''[[Morganucodon]]'', had [[Harderian glands]], which are associated with the grooming and maintenance of fur. The apparent absence of these glands in non-mammaliaformes may suggest that fur did not originate until that point in synapsid evolution.<ref name=Fur/> It is possible that fur and associated features of true warm-bloodedness did not appear until some synapsids became extremely small and nocturnal, necessitating a higher metabolism.<ref name=Fur>{{cite journal| last1=Ruben|first1= J.A. |last2= Jones | first2= T.D. |year=2000 |title= Selective factors associated with the origin of fur and feathers |journal=Am. Zool. |volume= 40 | issue=4 |pages= 585–596 | doi = 10.1093/icb/40.4.585 |doi-access= free}}</ref> The oldest examples of nocturnality in synapsids is believed to have been in species that lived more than 300 million years ago.<ref>{{cite web |last1=Gaare |first1=Megan |title=An Early Nocturnal Ancestor |url=https://www.fieldmuseum.org/blog/early-nocturnal-ancestor |publisher=Field Museum of Natural History |access-date=11 March 2022 |date=7 October 2014}}</ref>

However, [[Late Permian]] [[coprolite]]s from Russia and possibly South Africa showcase that at least some synapsids did already have pre-mammalian hair in this epoch. These are the oldest impressions of hair-like structures on synapsids.<ref>{{cite journal |title=Microbiota and food residues including possible evidence of pre-mammalian hair in Upper Permian coprolites from Russia |first1=Piotr |last1=Bajdek |first2=Martin |last2=Qvarnström |first3=Krzysztof |last3=Owocki |first4=Tomasz |last4=Sulej |first5=Andrey G. |last5=Sennikov |first6=Valeriy K. |last6=Golubev |first7=Grzegorz |last7=Niedźwiedzki |journal=[[Lethaia]] |volume=49 |issue=4 |year=2016 |pages=455–477 |doi=10.1111/let.12156 |bibcode=2016Letha..49..455B }}</ref><ref>{{Cite journal| doi = 10.1016/j.palaeo.2011.09.006| issn = 0031-0182| volume = 312| issue = 1–2| pages = 40–53| last1 = Smith| first1 = Roger M.H.| last2 = Botha-Brink| first2 = Jennifer| title = Morphology and composition of bone-bearing coprolites from the Late Permian Beaufort Group, Karoo Basin, South Africa| journal = Palaeogeography, Palaeoclimatology, Palaeoecology| date = 2011| bibcode = 2011PPP...312...40S| url = https://linkinghub.elsevier.com/retrieve/pii/S0031018211004792| url-access = subscription}}</ref>

==== Mammary glands ====
Early synapsids, as far back as their known evolutionary debut in the Late Carboniferous period,<ref name="Oftedal-2002a"/> may have laid parchment-shelled (leathery) eggs,<ref name="Oftedal-2012"/> which lacked a calcified layer, as most modern reptiles and [[monotreme]]s do. This may also explain why there is no fossil evidence for synapsid eggs to date.<ref name="Oftedal-2002b">{{cite journal |last=Oftedal |first=Olav T. |date=2002-07-01 |title=The origin of lactation as a water source for parchment-shelled eggs |journal=Journal of Mammary Gland Biology and Neoplasia |volume=7 |issue=3 |pages=253–266 |issn=1083-3021 |pmid=12751890|doi=10.1023/A:1022848632125|s2cid=8319185}}</ref> Because they were vulnerable to desiccation, secretions from [[Apocrine sweat gland|apocrine]]-like glands may have helped keep the eggs moist.<ref name="Oftedal-2002a"/>

According to Oftedal, early synapsids may have buried the eggs into moisture laden soil, hydrating them with contact with the moist skin, or may have carried them in a moist pouch, similar to that of monotremes ([[echidna]]s carry their eggs and offspring via a temporary pouch<ref>{{Cite web |url=http://www.life.umd.edu/classroom/bsci338m/Lectures/Monotremes.html |title=Monotremes and marsupials |website=www.life.umd.edu |access-date=2018-08-23}}</ref><ref>{{cite web |title=Life History and Ecology of the Monotremata |website=www.ucmp.berkeley.edu |url=http://www.ucmp.berkeley.edu/mammal/monotremelh.html |access-date=2018-08-23}}</ref>), though this would limit the mobility of the parent. The latter may have been the primitive form of egg care in synapsids rather than simply burying the eggs, and the constraint on the parent's mobility would have been solved by having the eggs "parked" in nests during foraging or other activities and periodically be hydrated, allowing higher clutch sizes than could fit inside a pouch (or pouches) at once, and large eggs, which would be cumbersome to carry in a pouch, would be easier to care for. The basis of Oftedal's speculation is the fact that many species of [[Anura (frog)|anura]]ns can carry eggs or tadpoles attached to the skin, or embedded within cutaneous "pouches" and how most [[salamander]]s curl around their eggs to keep them moist, both groups also having glandular skin.<ref name="Oftedal-2002b"/>

The glands involved in this mechanism would later evolve into true mammary glands with multiple modes of secretion in association with hair follicles. Comparative analyses of the evolutionary origin of milk constituents support a scenario in which the secretions from these glands evolved into a complex, nutrient-rich milk long before true mammals arose (with some of the constituents possibly predating the split between the synapsid and [[sauropsid]] lines). [[Cynodont]]s were almost certainly able to produce this, which allowed a progressive decline of yolk mass and thus egg size, resulting in increasingly [[altricial]] hatchlings as milk became the primary source of nutrition, which is all evidenced by the small body size, the presence of [[epipubic bone]]s, and limited tooth replacement in advanced cynodonts, as well as in [[mammaliaforms]].<ref name="Oftedal-2002a">{{cite journal |last=Oftedal |first=Olav T. |date=2002-07-01 |title=The mammary gland and its origin during synapsid evolution |journal=Journal of Mammary Gland Biology and Neoplasia |volume=7 |issue=3 |pages=225–252 |issn=1083-3021 |pmid=12751889 |doi=10.1023/a:1022896515287 |s2cid=25806501}}</ref><ref name="Oftedal-2012">{{Cite journal |last=Oftedal |first=O.T. |date=2012-03-01 |title=The evolution of milk secretion and its ancient origins |journal=Animal |volume=6 |issue=3 |pages=355–368 |doi=10.1017/S1751731111001935 |doi-access=free |issn=1751-732X |pmid=22436214|bibcode=2012Anim....6..355O }}</ref>

====Patagia====
Aerial locomotion first began in non-mammalian [[haramiyida]]n cynodonts, with ''[[Arboroharamiya]]'', ''[[Xianshou]]'', ''[[Maiopatagium]]'' and ''[[Vilevolodon]]'' bearing exquisitely preserved, fur-covered wing membranes that stretch across the limbs and tail. Their fingers are elongated, similar to those of bats and [[colugo]]s and likely sharing similar roles both as wing supports and to hang on tree branches.<ref>{{cite journal | last1 = Luo | first1 = Zhe-Xi | last2 = Meng | first2 = Qing-Jin | last3 = Grossnickle | first3 = David M. | last4 = Di | first4 = Liu | last5 = Neander | first5 = April I. | last6 = Zhang | first6 = Yu-Guang | last7 = Ji | first7 = Qiang | year = 2017 | title = New evidence for mammaliaform ear evolution and feeding adaptation in a Jurassic ecosystem | journal = Nature | volume = 548| issue = 7667| pages = 326–329| doi = 10.1038/nature23483 | pmid = 28792934 | bibcode = 2017Natur.548..326L | s2cid = 4463476 }}</ref>

Within true mammals, aerial locomotion first occurs in [[volaticotheria]]n [[eutriconodont]]s. A fossil ''[[Volaticotherium]]'' has an exquisitely preserved furry [[patagium]] with delicate wrinkles and that is very extensive, "sandwiching" the poorly preserved hands and feet and extending to the base of the tail.<ref>{{cite journal | last1 = Meng | first1 = J. | last2 = Hu | first2 = Y.-M. | last3 = Wang | first3 = Y.-Q. | last4 = Wang | first4 = X.-L. | last5 = Li | first5 = C.-K. | year = 2007 | title = Corrigendum: A Mesozoic gliding mammal from northeastern China | journal = Nature | volume = 446 | issue = 7131| page = 102 | doi = 10.1038/nature05639 | bibcode = 2007Natur.446Q.102M | doi-access = free }}</ref> ''[[Argentoconodon]]'', a close relative, shares a similar femur adapted for flight stresses, indicating a similar lifestyle.<ref>{{cite journal | last1 = Gaetano | first1 = L.C. | last2 = Rougier | first2 = G.W. | year = 2011 | title = New materials of Argentoconodon fariasorum (Mammaliaformes, Triconodontidae) from the Jurassic of Argentina and its bearing on triconodont phylogeny | journal = Journal of Vertebrate Paleontology | volume = 31 | issue = 4| pages = 829–843 | doi = 10.1080/02724634.2011.589877 | bibcode = 2011JVPal..31..829G | s2cid = 85069761 | hdl = 11336/68497 | hdl-access = free }}</ref>

[[Theria]]n mammals would only achieve powered flight and gliding long after these early aeronauts became extinct, with the earliest-known gliding [[metatheria]]ns and [[bat]]s evolving in the [[Paleocene]].<ref>{{cite conference |last1=Szalay, FS |last2=Sargis, EJ |last3=Stafford, BJ |year=2000 |title=Small marsupial glider from the Paleocene of Itaboraí, Brazil |journal=Journal of Vertebrate Paleontology |volume=20 |series=Supplement 73A |conference=Meeting of the Society of Vertebrate Paleontology}}</ref>

===Metabolism===
Recently, it has been found that [[endothermy]] was developed as early as ''[[Ophiacodon]]'' in the late Carboniferous. The presence of fibrolamellar, a specialised type of bone that can grow quickly while maintaining a stable structure, shows that Ophiacodon would have used its high internal body temperature to fuel a fast growth comparable to modern endotherms.<ref>{{cite web |url=https://www.sciencedaily.com/releases/2015/10/151029134252.htm |title=Ancestry of mammalian 'warm-bloodedness' revealed |date=October 29, 2015 |website=www.sciencedaily.com |publisher=Society of Vertebrate Paleontology |access-date=October 29, 2015}}</ref>

==Evolutionary history==
{{main|Evolution of mammals}}
[[File:Archaeothyris florensis.jpg|thumb|left|''[[Archaeothyris]]'', one of the oldest synapsids found]]
[[Image:Ophiacomorphs2.jpg|thumb|left|''[[Cotylorhynchus]]'' (background), ''[[Ophiacodon]]'' and ''[[Varanops]]'' were early synapsids that lived until the [[Cisuralian|Early Permian]].]]
Over the course of synapsid evolution, progenitor taxa at the start of adaptive radiations have tended to be derived carnivores. Synapsid adaptive radiations have generally occurred after extinction events that depleted the biosphere and left vacant niches open to be filled by newly evolved taxa. In non-mammaliaform synapsids, those taxa that gave rise to rapidly diversifying lineages have been both small and large in body size, although after the Late Triassic, progenitors of new synapsid lineages have generally been small, unspecialised generalists.<ref>{{Cite journal |last1=Grossnickle |first1=David |last2=Hellert |first2=Spencer |last3=Kammerer |first3=Christian |last4=Angielczyk |first4=Kenneth D. |last5=Lloyd |first5=Graeme |date=1 October 2022 |title=Survival of the novel: derived faunivores are the forerunners of major synapsid radiations |url=https://par.nsf.gov/biblio/10405929-survival-novel-derived-faunivores-forerunners-major-synapsid-radiations |journal=[[Journal of Vertebrate Paleontology]] |language=en |issue=Program and Abstracts, 2022 |access-date=30 September 2023}}</ref>

The earliest known synapsid ''[[Asaphestera]]'' coexisted with the earliest known sauropsid ''[[Hylonomus]]'' which lived during the [[Bashkirian]] age of the [[Late Carboniferous]].<ref name="Mann Gee Pardo Marjanović p. "/><ref name=M2021>{{cite journal|last1=Marjanović|first1=D.|year=2021|title=The Making of Calibration Sausage Exemplified by Recalibrating the Transcriptomic Timetree of Jawed Vertebrates|journal=Frontiers in Genetics|volume=12|at=521693|doi=10.3389/fgene.2021.521693|doi-access=free |pmid=34054911 |pmc=8149952 }}</ref> It was one of many types of primitive synapsids that are now informally grouped together as stem mammals or sometimes as protomammals (previously known as [[pelycosaur]]s). The early synapsids spread and diversified, becoming the largest terrestrial animals in the latest Carboniferous and [[Cisuralian|Early Permian]] periods, ranging up to {{convert|6|m|0}} in length. They were sprawling, bulky, possibly cold-blooded, and had small brains. Some, such as ''Dimetrodon'', had large sails that might have helped [[thermoregulation|raise their body temperature]]. A few [[Relict (biology)|relict]] groups lasted into the later Permian but, by the middle of the Late Permian, all had either died off or evolved into their successors, the therapsids.<ref>{{cite journal |first1= Sean P. |last1=Modesto |first2=Roger M. H. |last2=Smith| first3=Nicolás E. |last3=Campione| first4=Robert R. |last4=Reisz |year=2011 |title=The last 'pelycosaur': a varanopid synapsid from the Pristerognathus Assemblage Zone, Middle Permian of South Africa |journal=Naturwissenschaften |volume=98 |issue=12 |pages=1027–34 |doi=10.1007/s00114-011-0856-2|pmid=22009069 |bibcode=2011NW.....98.1027M |s2cid=27865550 }}</ref>

[[File:Moschops BW.jpg|thumb|right|''[[Moschops]]'' was a tapinocephalian from the [[Middle Permian]] of South Africa.]]
The therapsids, a more advanced group of synapsids, appeared during the [[Middle Permian]] and included the largest terrestrial animals in the Middle and [[Late Permian]]. They included herbivores and carnivores, ranging from small animals the size of a rat (e.g.: ''[[Robertia]]''), to large, bulky herbivores a ton or more in weight (e.g.: ''[[Moschops]]''). After flourishing for many millions of years, these successful animals were all but wiped out by the [[Permian–Triassic extinction event|Permian–Triassic mass extinction]] about 250 mya, the largest known [[extinction event|extinction]] in [[History of Earth|Earth's history]], possibly related to the [[Siberian Traps]] volcanic event.
[[File:Nikkasaurus1DB.jpg|thumb|left|''[[Nikkasaurus]]'' was an enigmatic synapsid from the Middle Permian of Russia.]]

[[File:Lystr georg1DB.jpg|thumb|right|''[[Lystrosaurus]]'' was the most common synapsid shortly after the [[Permian–Triassic extinction event]].]]
Only a few therapsids went on to be successful in the new early [[Triassic]] landscape; they include ''[[Lystrosaurus]]'' and ''[[Cynognathus]]'', the latter of which appeared later in the Early Triassic. However, they were accompanied by the early [[archosaur]]s (soon to give rise to the [[dinosaur]]s). Some of these archosaurs, such as ''[[Euparkeria]]'', were small and lightly built, while others, such as ''[[Erythrosuchus]]'', were as big as or bigger than the largest therapsids.

After the Permian extinction, the synapsids did not count more than three surviving clades. The first comprised the [[therocephalia]]ns, which only lasted the first 20 million years of the Triassic period. The second were specialised, beaked herbivores known as [[dicynodont]]s (such as the [[Kannemeyeriidae]]), which contained some members that reached large size (up to a tonne or more). And finally there were the increasingly mammal-like carnivorous, herbivorous, and insectivorous cynodonts, including the [[eucynodonts]] from the [[Olenekian]] age, an early representative of which was ''Cynognathus''.

[[File:Cynognathus BW.jpg|thumb|''[[Cynognathus]]'' was the largest predatory cynodont of the [[Triassic]].]]
Unlike the dicynodonts, which were large, the cynodonts became progressively smaller and more mammal-like as the Triassic progressed, though some forms like ''[[Trucidocynodon]]'' remained large. The first mammaliaforms evolved from the cynodonts during the early [[Norian]] age of the Late Triassic, about 225 mya.

During the evolutionary succession from early therapsid to cynodont to eucynodont to mammal, the main lower jaw bone, the dentary, replaced the adjacent bones. Thus, the lower jaw gradually became just one large bone, with several of the smaller jaw bones migrating into the [[inner ear]] and allowing sophisticated hearing.

[[File:Repenomamus BW.jpg|thumb|right|''[[Repenomamus]]'' was the largest mammal of the [[Mesozoic]].]]
Whether through climate change, vegetation change, ecological competition, or a combination of factors, most of the remaining large cynodonts (belonging to the [[Traversodontidae]]) and dicynodonts (of the family Kannemeyeriidae) had disappeared by the [[Rhaetian]] age, even before the [[Triassic–Jurassic extinction event]] that killed off most of the large [[Crurotarsi|non-dinosaurian archosaurs]]. The remaining Mesozoic synapsids were small, ranging from the size of a shrew to the badger-like mammal ''[[Repenomamus]]''.

[[File:Tritylodon BW.jpg|thumb|left|''[[Tritylodon]]'' was a cynodont that lived in the [[Early Jurassic]].]]
During the Jurassic and Cretaceous, the remaining non-mammalian cynodonts were small, such as ''[[Tritylodon]]''. No cynodont grew larger than a cat. Most Jurassic and Cretaceous cynodonts were [[herbivorous]], though some were [[carnivorous]]. The family [[Tritheledontidae]], which first appeared near the end of the Triassic, was carnivorous and persisted well into the [[Middle Jurassic]]. The other, [[Tritylodontidae]], first appeared at the same time as the tritheledonts, but was herbivorous. This group became extinct at the end of the Early Cretaceous epoch. Dicynodonts are generally thought to have become extinct near the end of the Triassic period, but there was evidence this group survived, in the form of six fragments of fossil bone that were found in Cretaceous rocks of [[Queensland]], Australia.<ref name="Thulborn">{{cite journal |last1=Thulborn |first1=T. |last2=Turner |first2=S. |year=2003 |title=The last dicynodont: an Australian Cretaceous relict |journal=Proceedings of the Royal Society B: Biological Sciences |volume=270 |issue=1518 |pages=985–993 |doi=10.1098/rspb.2002.2296 |jstor=3558635 |pmc=1691326 |pmid=12803915}}</ref> If true, it would mean there is a significant [[ghost lineage]] of Dicynodonts in [[Gondwana]]. However, these fossils were re-described in 2019 as being [[Pleistocene]] in age, and possibly belonging to a [[Diprotodontidae|diprotodontid]] [[marsupial]].<ref>{{cite journal |author1=Espen M. Knutsen |author2=Emma Oerlemans |year=2019 |title=The last dicynodont? Re-assessing the taxonomic and temporal relationships of a contentious Australian fossil |journal=Gondwana Research |volume=77 |pages=184–203 |doi=10.1016/j.gr.2019.07.011 |s2cid=202908716 |doi-access=}}</ref>

Today, the 5,500 species of living synapsids, known as the [[mammal]]s, include both aquatic ([[cetacean]]s) and flying ([[bat]]s) species, and the largest animal ever known to have existed (the [[blue whale]]). Humans are synapsids, as well. Most mammals are [[viviparous]] and give birth to live young rather than laying eggs with the exception being the [[monotreme]]s.

Triassic and Jurassic ancestors of living mammals, along with their close relatives, had high metabolic rates. This meant consuming food (generally thought to be insects) in much greater quantity. To facilitate rapid [[digestion]], these synapsids evolved [[mastication]] (chewing) and specialized teeth that aided chewing. Limbs also evolved to move under the body instead of to the side, allowing them to breathe more efficiently during locomotion.<ref>{{cite journal |last=Bramble |first=D. M. |author2=Jenkins, F. A. |year=1993 |title=Mammalian locomotor-respiratory integration: Implications for diaphragmatic and pulmonary design |journal=[[Science (journal)|Science]] |volume=262 |issue=5131 |pages=235–240 |doi=10.1126/science.8211141|pmid=8211141|bibcode=1993Sci...262..235B }}
</ref> This helped make it possible to support their higher metabolic demands.

==Relationships==
Below is a [[cladogram]] of the most commonly accepted [[phylogeny]] of synapsids, showing a long stem lineage including Mammalia and successively more basal clades such as Theriodontia, Therapsida and Sphenacodontia:<ref name=TOL>{{cite web |url=http://tolweb.org/Synapsida/14845 |title=Synapsida. Mammals and their extinct relatives |last=Laurin |first=M. |author2=Reisz, R.R. |year=2011 |work=The Tree of Life Web Project |access-date=26 April 2012}}</ref><ref name=KTS11>{{cite book |last=Kemp |first=T.S. |year=2011 |chapter=The origin and radiation of therapsids |title=Forerunners of Mammals |editor=Chinsamy-Turan, A. |publisher=Indiana University Press |location=Bloomington |isbn=978-0-253-35697-0 |pages=3–30 |chapter-url=https://books.google.com/books?id=cp26-CA2CDUC&q=forerunners%20of%20the%20mammals&pg=PA3}}</ref>
{{clade| style=font-size:100%;line-height:100%
|label1='''Synapsida'''
|1={{clade
   |1=†[[Caseasauria]] [[File:Ennatosaurus BW.jpg|50px]]
   |label2=[[Eupelycosauria]]
   |2={{clade
    |1=†[[Varanopidae]] [[File:Varanops brevirostris.jpg|50px]]
     |2={{clade
      |1=†[[Ophiacodontidae]] [[File:Archaeothyris BW.jpg|50px]]
      |2={{clade
      |1=†[[Edaphosauridae]] [[File:Ianthasaurus BW.jpg|50px]]
           |label2=[[Sphenacodontia]]
            |2={{clade
             |1=†[[Sphenacodontidae]] [[File:Palaeohatteria DB.jpg|50px]]
               |label2=[[Therapsida]]
               |2={{clade
                  |1=†[[Biarmosuchia]] [[File:Biarmosuchus.jpg|50px]]
                  |label2=[[Eutherapsida]]
                  |2={{clade
                     |1=†[[Dinocephalia]] [[File:Struthiocephalus DB.jpg|50px]]
                     |label2=[[Neotherapsida]]
                     |2={{clade
                        |1=†[[Anomodontia]] [[File:Eodicynodon BW.jpg|50px]]
                        |label2=[[Theriodontia]]
                        |2={{clade
                          |1=†[[Gorgonopsia]] [[File:Gorgonops whaitsii1.jpg|50px]]
                           |label2=[[Eutheriodontia]]
                           |2={{clade
                             |1=†[[Therocephalia]] [[File:Moschorhinus DB.jpg|50px]]
                              |label2=[[Cynodontia]]
                              |2={{clade
                              |1=† [[Cynognathia]] [[File:Cynognathus BW.jpg|50px]]
                               |label2=[[Probainognathia]]
                              |2={{clade
                                    |1='''[[Mammalia]]'''[[File:Ruskea rotta.png|50 px]]
}} }} }} }} }} }} }} }} }} }} }} }} }}

Most uncertainty in the phylogeny of synapsids lies among the earliest members of the group, including forms traditionally placed within Pelycosauria. As one of the earliest phylogenetic analyses, Brinkman & Eberth (1983) placed the family [[Varanopidae]] with [[Caseasauria]] as the most basal offshoot of the synapsid lineage. Reisz (1986) removed Varanopidae from Caseasauria, placing it in a more derived position on the stem. While most analyses find Caseasauria to be the most basal synapsid clade, Benson's analysis (2012) placed a clade containing [[Ophiacodontidae]] and Varanopidae as the most basal synapsids, with Caseasauria occupying a more derived position. Benson attributed this revised phylogeny to the inclusion of postcranial characteristics, or features of the skeleton other than the skull, in his analysis. When only cranial or skull features were included, Caseasauria remained the most basal synapsid clade. Below is a [[cladogram]] modified from Benson's analysis (2012):<ref name=BRJ12>{{cite journal |last=Benson |first=R.J. |year=2012 |title=Interrelationships of basal synapsids: cranial and postcranial morphological partitions suggest different topologies |journal=[[Journal of Systematic Palaeontology]] |volume= 10|doi=10.1080/14772019.2011.631042 |issue=4 |pages=601–624|bibcode=2012JSPal..10..601B |s2cid=84706899 }}</ref>

{{clade| style=font-size:100%;line-height:100%
|1={{clade
   |1=''[[Tseajaia campi]]''
   |2=''[[Limnoscelis paludis]]''
   |label3=[[Amniota]]
   |3={{clade
      |1={{clade
         |1=''[[Captorhinus]]'' spp.
         |2=''[[Protorothyris archeri]]''}}
      |label2='''Synapsida'''
      |2={{clade
         |1={{clade
            |label1=[[Ophiacodontidae]]
            |1={{clade
               |1=''[[Archaeothyris florensis]]''
               |2={{clade
                  |1=''[[Varanosaurus acutirostris]]''
                  |2={{clade
                     |1=''[[Ophiacodon]]'' spp.
                     |2=''[[Stereophallodon ciscoensis]]''}} }} }}
            |label2=[[Varanopidae]]
            |2={{clade
               |1=''[[Archaeovenator hamiltonensis]]''
               |2={{clade
                  |1=''[[Pyozia mesenensis]]''
                  |2={{clade
                     |1={{clade
                        |1=''[[Mycterosaurus longiceps]]''
                        |2={{clade
                           |1=?''[[Elliotsmithia longiceps]]'' (BP/1/5678)
                           |2=''[[Heleosaurus scholtzi]]''
                           |3=''[[Mesenosaurus romeri]]''}} }}
                     |2={{clade
                        |1={{clade
                           |1=''[[Varanops brevirostris]]''
                           |2={{clade
                              |1=''[[Watongia meieri]]''
                              |2=''[[Varanodon agilis]]''}} }}
                        |2={{clade
                           |1=''[[Ruthiromia elcobriensis]]''
                           |2={{clade
                              |1=''[[Aerosaurus wellesi]]''
                              |2=''[[Aerosaurus greenleorum]]''}} }} }} }} }} }} }}
         |2={{clade
            |label1=[[Caseasauria]]
            |1={{clade
               |label1=[[Eothyrididae]]
               |1={{clade
                  |1=''[[Eothyris parkeyi]]''
                  |2=''[[Oedaleops campi]]''}}
               |label2=[[Caseidae]]
               |2={{clade
                  |1=''[[Oromycter dolesorum]]''
                  |2={{clade
                     |1=''[[Casea broilii]]''
                     |2={{clade
                        |1=''[[Trichasaurus texensis]]''
                        |2={{clade
                           |1=''[[Euromycter rutenus]]'' (=''"Casea" rutena'')
                           |2={{clade
                              |1=''[[Ennatosaurus tecton]]''
                              |2={{clade
                                 |1=''[[Angelosaurus romeri]]''
                                 |2={{clade
                                    |1=''[[Cotylorhynchus romeri]]''
                                    |2={{clade
                                       |1=''[[Cotylorhynchus bransoni]]''
                                       |2=''[[Cotylorhynchus hancocki]]''}} }} }} }} }} }} }} }} }}
            |2={{clade
               |1=''[[Ianthodon schultzei]]''
               |2={{clade
                  |label1=[[Edaphosauridae]]
                  |1={{clade
                     |1=''[[Ianthasaurus hardestiorum]]''
                     |2={{clade
                        |1=''[[Glaucosaurus megalops]]''
                        |2=''[[Lupeosaurus kayi]]''
                        |3={{clade
                           |1=''[[Edaphosaurus boanerges]]''
                           |2=''[[Edaphosaurus novomexicanus]]''}} }} }}
                  |label2=[[Sphenacodontia]]
                  |2={{clade
                     |1=''[[Haptodus garnettensis]]''
                     |2={{clade
                        |1=''[[Pantelosaurus saxonicus]]''
                        |2={{clade
                           |label1='''[[Therapsida]]'''
                           |1={{clade
                              |1=''[[Raranimus dashankouensis]]''
                              |2={{clade
                                 |1=''[[Biarmosuchus tener]]''
                                 |2={{clade
                                    |1=''[[Biseridens qilianicus]]''
                                    |2=''[[Titanophoneus potens]]''}} }} }}
                           |label2=[[Sphenacodontidae]]
                           |2={{clade
                              |1=''[[Cutleria wilmarthi]]''
                              |2={{clade
                                 |1=''[[Secodontosaurus obtusidens]]''
                                 |2={{clade
                                    |1=''[[Cryptovenator hirschbergeri]]''
                                    |2={{clade
                                       |1=''[[Dimetrodon]]'' spp.
                                       |2=''[[Sphenacodon]]'' spp. }} }} }} }} }} }} }} }} }} }} }} }} }} }}

However, more recent examination of the phylogeny of basal synapsids, incorporating newly described basal caseids and eothyridids,<ref name="ReferenceA">{{cite journal |author1=Neil Brocklehurst |author2=Robert Reisz |author3=Vincent Fernandez |author4=Jörg Fröbisch |year=2016 |title=A Re-Description of 'Mycterosaurus' smithae, an Early Permian Eothyridid, and Its Impact on the Phylogeny of Pelycosaurian-Grade Synapsids |journal=PLOS ONE |volume=11 |issue=6 |pages=e0156810 |doi=10.1371/journal.pone.0156810 |pmid=27333277 |bibcode=2016PLoSO..1156810B |pmc=4917111 |doi-access=free }}</ref> returned Caseasauria to its position as the sister to all other synapsids. Brocklehurst et al. (2016)<ref name="ReferenceA"/> demonstrated that many of the postcranial characters used by Benson (2012) to unite Caseasauria with [[Sphenacodontidae]] and [[Edaphosauridae]] were absent in the newly discovered postcranial material of eothyridids, and were therefore acquired convergently.

==See also==
* [[Anapsid]]
* [[Diapsid]]
* [[Euryapsida]]
* [[List of prehistoric mammals]]
* [[Lists of synapsids]]
* [[Mammal classification]]
* [[Timeline of the evolutionary history of life]]
* [[Vertebrate paleontology]]

==Notes==
{{Notelist}}

==References==
{{Reflist|30em}}

==Further reading==
* {{cite book|last=Colbert|first=E. H.|author-link=Edwin H. Colbert|title=Evolution of the Vertebrates|edition=2nd|publisher=[[John Wiley & Sons|John Wiley & Sons Inc]]|location=New York|year=1969|isbn=0-471-16466-6|title-link=Evolution of the Vertebrates}}

==External links==
{{Wikispecies|Synapsida}}
* [https://archive.today/20060313/http://www.palaeos.com/Vertebrates/Units/Unit390/000.html Synapsida - Pelycosauria] - at Palaeos
* [http://www.talkorigins.org/faqs/faq-transitional/part1b.html Transitional Vertebrate Fossils] - includes description of important [[transitional fossils|transitional]] genera in the evolutionary sequence linking primitive synapsids to mammals

{{Early tetrapods|state=autocollapse|A.}}
{{Synapsida|S.|state=autocollapse}}
{{Taxonbar|from=Q189069}}
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[[Category:Synapsida| ]]
[[Category:Tetrapod unranked clades]]
[[Category:Extant Pennsylvanian first appearances]]
[[Category:Taxa named by Henry Fairfield Osborn]]