Editing Synapsida (section)

==Evolutionary history==
{{main|Evolution of mammals}}
[[File:Archaeothyris BW.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.