Editing Synapsida (section)

==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 | last1=Mann | first1=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. | editor-last=Sansom | editor-first=Robert | title=Reassessment of historic 'microsaurs' from Joggins, Nova Scotia, reveals hidden diversity in the earliest amniote ecosystem | journal=Papers in Palaeontology | publisher=Wiley | date=5 May 2020 | volume=6 | issue=4 | pages=605–625 | issn=2056-2802 | doi=10.1002/spp2.1316 | bibcode=2020PPal....6..605M }}</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>