Editing Dimetrodon

{{Short description|Genus of carnivorous synapsids from the Permian}}
{{Good article}}
{{Italic title}}
{{Automatic taxobox
| fossil_range = [[Cisuralian]] to [[Guadalupian]] ([[Asselian]] to [[Roadian]]), {{fossil range|295|270}}
| image = Dimetrodon incisivum 01.jpg
| image_upright = 1.15
| image_caption = Skeleton of ''D. limbatus'', [[Staatliches Museum für Naturkunde Karlsruhe]]
| taxon = Dimetrodon
| authority = [[Edward Drinker Cope|Cope]], [[1878 in paleontology|1878]] ([[conserved name]])
| type_species = {{extinct}}'''''Dimetrodon limbatus'''''
| type_species_authority = Cope, 1877
| subdivision_ranks = Species
| subdivision = See [[List of Dimetrodon species|below]]
| synonyms = {{collapsible list|bullets = true
 |title=<small>Genus synonymy</small>
|''[[Dimetrodon borealis|Bathygnathus]]''<br /><small>Leidy, 1854</small>
|''Embolophorus''<br /><small>Cope, 1878</small>
|''Theropleura''<br /><small>Cope, 1880</small>
|''Bathyglyptus''<br /><small>Case, 1911</small>
|''Eosyodon''<br /><small>Olson, 1962</small>
}}
{{collapsible list|bullets = true
 |title=<small>Species synonymy</small>
|''[[Dimetrodon borealis|Bathygnathus borealis]]''<br /><small>Leidy, 1854</small>
|''[[Clepsydrops]] limbatus''<br /><small>Cope, 1877</small>
|''Clepsydrops gigas''<br /><small>Cope, 1878</small>
|''Clepsydrops natalis''<br /><small>Cope, 1878</small>
|''Dimetrodon gigas''<br /><small>Cope, 1878</small>
|''Dimetrodon incisivus''<br /><small>Cope, 1878</small>
|''Dimetrodon rectiformis''<br /><small>Cope, 1878</small>
|''Embolophorus dollovianus''<br /><small>Cope, 1878</small>
|''Dimetrodon semiradicatus''<br /><small>Cope, 1881</small>
|''Clepsydrops macrospondylus''<br /><small>Cope 1884</small>
|''Dimetrodon platycentrus''<br /><small>Case, 1907</small>
|''Theropleura grandis''<br /><small>Case, 1907</small>
|''Bathyglyptus theodori''<br /><small>Case, 1911</small>
|''Dimetrodon maximus''<br /><small>Romer, 1936</small>
|''Eosyodon hudsoni''<br /><small>Olson, 1962</small>
}}
}}

'''''Dimetrodon''''' ({{IPAc-en|d|aɪ|ˈ|m|iː|t|r|ə|ˌ|d|ɒ|n}}{{refn|{{cite dictionary |url=https://lexico.com/definition/Dimetrodon |archive-url=https://web.archive.org/web/20200322182731/https://www.lexico.com/definition/dimetrodon |url-status=dead |archive-date=2020-03-22 |title=Dimetrodon |dictionary=[[Lexico]] UK English Dictionary |publisher=[[Oxford University Press]]}}}} or {{IPAc-en|d|aɪ|ˈ|m|ɛ|t|r|ə|ˌ|d|ɒ|n|audio=En-us-Dimetrodon.ogg}};<ref>{{cite Dictionary.com |dimetrodon |access-date=2018-02-12}}</ref> {{lit|two measures of teeth}}) is an extinct [[genus]] of [[sphenacodontid]] [[synapsid]] that lived during the [[Cisuralian]] (Early Permian) [[Epoch (geology)|epoch]] of the [[Permian]] period, around 295–272&nbsp;million years ago.<ref name=PaleoDB>{{cite web |url=https://paleobiodb.org/classic/checkTaxonInfo?taxon_no=38904 |title=Dimetrodon |work=Paleobiology Database |access-date=23 August 2012 |archive-date=2 October 2013 |archive-url=https://web.archive.org/web/20131002102758/http://paleodb.org/cgi-bin/bridge.pl?a=checkTaxonInfo&taxon_no=38904 |url-status=live }}</ref><ref name=AKD09>{{cite journal | last1 = Angielczyk | first1 = K. D. | title = Dimetrodon is Not a Dinosaur: Using Tree Thinking to Understand the Ancient Relatives of Mammals and their Evolution | doi = 10.1007/s12052-009-0117-4 | journal = Evolution: Education and Outreach | volume = 2 | issue = 2 | pages = 257–271 | year = 2009 | doi-access = free }}</ref><ref>{{cite book |last1=Huttenlocker |first1=A. K. |first2=E. |last2=Rega |year=2012 |chapter=The Paleobiology and Bone Microstructure of Pelycosauriangrade Synapsids |pages=90–119 |editor-first=A. |editor-last=Chinsamy |title=Forerunners of Mammals: Radiation, Histology, Biology |publisher=Indiana University Press |isbn=978-0-253-35697-0 }}</ref> With most species measuring {{convert|1.7|-|4.6|m|ft|abbr=on}} long and weighing {{convert|28|-|250|kg|lb|abbr=on}}, the most prominent feature of ''Dimetrodon'' is the large [[neural spine sail]] on its back formed by elongated spines extending from the [[vertebra]]e. It was an obligate [[Quadrupedalism|quadruped]] (it could walk only on four legs) and had a tall, curved skull with large teeth of different sizes set along the jaws. Most fossils have been found in the [[Southwestern United States]], the majority of these coming from a geological deposit called the [[Red Beds of Texas and Oklahoma]]. More recently, its fossils have also been found in [[Germany]] and over a dozen species have been named since the [[genus]] was first erected in 1878.

''Dimetrodon'' is often mistaken for a [[dinosaur]] or portrayed as a contemporary of dinosaurs in [[popular culture]], but it became extinct by the middle [[Permian]], some 40 million years before the advent of dinosaurs.<ref name=":0">{{Cite web|date=Feb 17, 2021|title=Famous Prehistoric Animals That Weren't Actually Dinosaurs|url=https://medium.com/prehistoric-tales/famous-prehistoric-animals-that-werent-actually-dinosaurs-730bb941ed50|url-status=live|archive-url=https://web.archive.org/web/20210407023624/https://medium.com/prehistoric-tales/famous-prehistoric-animals-that-werent-actually-dinosaurs-730bb941ed50 |archive-date=2021-04-07 }}</ref><ref>{{Cite web|last1=Black|first1=Riley|title=The Dimetrodon in Your Family Tree|url=https://www.smithsonianmag.com/science-nature/the-dimetrodon-in-your-family-tree-54302176/|access-date=2021-12-05|website=Smithsonian Magazine|language=en}}</ref> Although [[reptile]]-like in appearance and physiology, ''Dimetrodon'' is much more closely related to [[mammal]]s, as it belongs to the closest [[sister taxon|sister]] [[family (biology)|family]] to [[therapsid]]s, the latter of which contains the direct ancestor of mammals.<ref name=AKD09 /> ''Dimetrodon'' is traditionally assigned to the [[paraphyletic]] group "[[pelycosaur]]s", a term now considered obsolete and replaced by terms such as "primitive synapsids" or "basal synapsids"; another name "[[mammal-like reptile]]s" is also used traditionally but incorrectly for non-mammalian synapsids<ref name=AKD09 /> due to some of the features shared with modern mammals such as [[heterodont|tooth specialization]] and [[endothermy]], but that term is now also defunct. ''Dimetrodon'' [[skull]] has a single opening ([[temporal fenestra]]) behind each eye, a feature shared among all synapsids, unlike the skulls of reptiles and birds, both of which belonging to the clade [[Sauropsida]], which diverged from the synapsids at least since the [[Late Carboniferous]].

''Dimetrodon'' was probably one of the [[apex predator]]s of the Cisuralian ecosystems, feeding on fish and [[tetrapod]]s, including reptiles and [[amphibian]]s. Smaller ''Dimetrodon'' species may have had different [[ecological niche|ecological roles]]. The sail of ''Dimetrodon'' may have been used to stabilize its spine or to heat and cool its body as a form of [[thermoregulation]].<ref>{{Cite book|last=Cloudsley-Thompson|first=J. L.|url=https://books.google.com/books?id=7PN7y4YjczYC&q=Dimetrodon+Sail+Heat&pg=PA102|title=Ecology and Behaviour of Mesozoic Reptiles|date=2005-01-19|publisher=Springer Science & Business Media|isbn=978-3-540-22421-1|language=en}}</ref> Some recent studies argue that the sail would have been ineffective at removing heat from the body, due to large species being discovered with small sails and small species being discovered with large sails, essentially ruling out heat regulation as its main purpose. The sail was most likely used in [[courtship display]], including threatening away rivals or showing off to potential mates.<ref>{{Cite book|last1=Fiesta|first1=Enrique|url=https://books.google.com/books?id=9q0gBgAAQBAJ&q=Dimetrodon+Sail+Heat&pg=PA8|title=Dimetrodon - Permian Predator|last2=Davidson|first2=John|date=2015-01-10|publisher=Mendon Cottage Books|isbn=978-1-310-19617-1|language=en}}</ref><ref>{{Cite book|last1=Zachos|first1=Frank|url=https://books.google.com/books?id=dQd2DwAAQBAJ&q=Dimetrodon+Sail+Heat&pg=PA141|title=Mammalian Evolution, Diversity and Systematics|last2=Asher|first2=Robert|date=2018-10-22|publisher=Walter de Gruyter GmbH & Co KG|isbn=978-3-11-034155-3|language=en}}</ref>
{{TOClimit|3}}

== Description ==
[[File:Dimetrodon NT2 small.jpg|thumb|Restoration of ''D. giganhomogenes'' with exposed neural spine tips]]
''Dimetrodon'' was a [[Quadrupedalism|quadrupedal]], sail-backed synapsid that most likely had a semi-sprawling posture between that of a mammal and a lizard and also could walk in a more upright stance with its body and the majority or all of its tail off the ground.<ref>{{Cite journal |last1=Gônet |first1=Jordan |last2=Bardin |first2=Jérémie |last3=Girondot |first3=Marc |last4=Hutchinson |first4=John R. |last5=Laurin |first5=Michel |year=2023 |title=Unravelling the postural diversity of mammals: Contribution of humeral cross-sections to palaeobiological inferences |url=https://www.researchgate.net/publication/368391458 |journal=[[Journal of Mammalian Evolution]] |volume=30 |issue=2 |pages=321–337 |doi=10.1007/s10914-023-09652-w |s2cid=256788973 }}</ref> Most ''Dimetrodon'' species ranged in length from {{convert|1.7|to|4.6|m|ft|0|abbr=on}} and are estimated to have weighed between {{convert|28|and|250|kg|lb|-1|abbr=on}}.<ref name=BRMH01>{{cite journal |last=Berman |first=D.S. |author2=Reisz, R.R. |author3=Martens, T. |author4= Henrici, A.C.  |year=2001 |title=A new species of ''Dimetrodon'' (Synapsida: Sphenacodontidae) from the Lower Permian of Germany records the first occurrence of the genus outside of North America |journal=Canadian Journal of Earth Sciences |volume=38|issue=5 |pages=803–812 |doi=10.1139/cjes-38-5-803 |url=http://www.stuartsumida.com/BIOL680-09/BermanEtAl2001.pdf|bibcode=2001CaJES..38..803B }}</ref> The smallest known species ''D. teutonis'' was about {{convert|60|cm|in|abbr=on}} long and weighed {{convert|14|kg|lb}}.<ref name=BRMH01/><ref name=FSMSS11>{{cite journal |last=Fröbisch |first=J. |author2=Schoch, R.R. |author3=Müller, J. |author4=Schindler, T. |author5= Schweiss, D.  |year=2011 |title=A new basal sphenacodontid synapsid from the Late Carboniferous of the Saar-Nahe Basin, Germany |journal=Acta Palaeontologica Polonica |volume=56 |issue=1 |pages=113–120 |doi=10.4202/app.2010.0039 |s2cid=45410472 |url=http://app.pan.pl/archive/published/app56/app20100039.pdf|doi-access=free }}</ref> The larger species of ''Dimetrodon'' were among the largest predators of the Early Permian, although the closely related ''[[Tappenosaurus]]'', known from skeletal fragments in slightly younger rocks, may have been even larger at an estimated {{convert|18|ft|m|order=flip}} long.<ref name=OB53>{{cite journal |last=Olson |first=E.C. |author2=Beerbower, J.R. |year=1953 |title=The San Angelo Formation, Permian of Texas, and its vertebrates |journal=[[The Journal of Geology]] |volume=61 |issue=5 |pages=389–423 |doi=10.1086/626109 |bibcode=1953JG.....61..389O|s2cid=128681671 }}</ref><ref name=OEC55>{{cite journal |last=Olson |first=E.C. |year=1955 |title=Parallelism in the evolution of the Permian reptilian faunas of the Old and New Worlds |journal=Fieldiana |volume=37 |issue=13 |pages=385–401 |url=https://archive.org/stream/parallelisminevo3713olso#page/n5/mode/2up}}</ref> Although some ''Dimetrodon'' species could grow very large, many juvenile specimens are known.<ref name=SCW42>{{cite journal |last=Sternberg |first=C.W. |year=1942 |title=The skeleton of an immature pelycosaur, ''Dimetrodon ''cf. ''grandis'', from the Permian of Texas |journal=Journal of Paleontology |volume=16 |issue=4 |pages=485–486|jstor=1298848}}</ref>

=== Skull ===
{|  style="margin:0.46em 0.2em"
|- style="font-size: 75%;"
| style="vertical-align: top;" |[[File:Dimetrodon skull dorsal.svg|300px]]<br /> || style="vertical-align: top;" |[[File:Dimetrodon skull lateral.svg|300px]]<br />
|- style="font-size: 75%;"
| style="vertical-align: top;" |[[File:Dimetrodon skull ventral.svg|300px]]<br /> || style="vertical-align: top;" |[[File:Dimetrodon skull occipital.svg|300px]]<br />
|}
A [[temporal fenestra|single large opening]] on either side of the back of the skull links ''Dimetrodon'' to mammals and distinguishes it from most of the earliest sauropsids, which either [[anapsid|lack openings]] or have [[diapsid|two openings]]. Features such as ridges on the inside of the [[nasal cavity]] and a ridge at the back of the lower jaw are thought to be part of an evolutionary progression from early [[tetrapod|four-limbed land-dwelling vertebrates]] to [[mammal]]s.

The skull of ''Dimetrodon'' is tall and compressed [[Lateral (anatomy)|laterally]], or side-to-side. The eye sockets are positioned high and far back in the skull. Behind each eye socket is a single hole called an [[infratemporal fenestra]]. An additional hole in the skull, the [[supratemporal fenestra]], can be seen when viewed from above. The back of the skull (the [[occiput]]) is oriented at a slight upward angle, a feature that it shares with all other early [[synapsid]]s.<ref name=TOL1>{{cite web |url=http://tolweb.org/Synapsida/14845 |title=Synapsida: mammals and their extinct relatives |last=Laurin |first=M. |author2=Reisz, R.R. |year=2012 |work=Tree of Life Web Project |access-date=24 August 2012 |archive-date=7 December 2012 |archive-url=https://web.archive.org/web/20121207131908/http://tolweb.org/Synapsida/14845 |url-status=dead }}</ref> The upper margin of the skull slopes downward in a convex arc to the tip of the snout. The tip of the upper jaw, formed by the [[premaxilla]] bone, is raised above the part of the jaw formed by the [[maxilla]] bone to form a maxillary "step". Within this step is a [[Diastema (dentistry)|diastema]], a gap in the tooth row. Its [[skull]] was more heavily built than a dinosaur's skull.

==== Teeth ====
The size of the teeth varies greatly along the length of the jaws, lending ''Dimetrodon'' its name, which means "two measures of tooth" in reference to sets of small and large teeth.<ref name=AMNH>{{cite web|url=http://www.amnh.org/exhibitions/permanent/fossilhalls/vertebrate/specimens/dimetrodon.php |title=Exhibit Specimens: Dimetrodon |work=American Museum of Natural History |access-date=2 July 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120704182723/http://www.amnh.org/exhibitions/permanent/fossilhalls/vertebrate/specimens/dimetrodon.php |archive-date=4 July 2012 }}</ref> One or two pairs of caniniforms (large, pointed, [[Canine tooth|canine]]-like teeth) extend from the maxilla. Large incisor teeth are also present at the tips of the upper and lower jaws, rooted in the premaxillae and [[dentary bone]]s. Small teeth are present around the maxillary "step" and behind the caniforms, becoming smaller further back in the jaw.<ref name=BC99>{{cite journal |last=Baur |first=G. |author2=Case, E.C. |year=1899 |title=The history of the Pelycosauria, with a description of the genus ''Dimetrodon'', Cope |journal=Transactions of the American Philosophical Society |volume=20 |issue=1 |pages=5–62 |jstor=1005488|doi=10.2307/1005488 |hdl=2027/uc1.32106020416696 |hdl-access=free }}</ref>

[[File:Dimetrodon grandis Exhibit Museum of Natural History.JPG|thumb|left|A skull of ''D.&nbsp;grandis'']]

Many teeth are widest at their midsections and narrow closer to the jaws, giving them the appearance of a teardrop. Teardrop-shaped teeth are unique to ''Dimetrodon'' and other closely related [[sphenacodontid]]s, which helps to distinguish them from other early synapsids.<ref name=FSMSS11 /> As in many other early [[synapsid]]s, the teeth of most ''Dimetrodon'' species are serrated at their edges.<ref name=FSMSS11 /> The serrations of ''Dimetrodon'' teeth were so fine that they resembled tiny cracks.<ref name="alberto-tooth-abs-84">Abler, W.L. 2001. A kerf-and-drill model of tyrannosaur tooth serrations. p.&nbsp;84-89. In: ''Mesozoic Vertebrate Life''. Ed.s Tanke, D. H., Carpenter, K., Skrepnick, M. W. Indiana University Press.</ref> The dinosaur ''[[Albertosaurus]]'' had similarly crack-like serrations, but, at the base of each serration was a round [[Vacuum|void]], which would have functioned to distribute force over a larger [[surface area]] and prevent the stresses of feeding from causing the crack to spread through the tooth. Unlike ''Albertosaurus'', ''Dimetrodon'' teeth lacked adaptations that would stop cracks from forming at their serrations.<ref name="alberto-tooth-abs-84" /> The teeth of ''D.&nbsp;teutonis'' lack serrations, but still have sharp edges.<ref name=FSMSS11 />

A 2014 study shows that ''Dimetrodon'' was in an arms race against its prey. The smaller species, ''D. milleri'', had no tooth serrations because it ate small prey. As prey grew larger, several ''Dimetrodon'' species started developing serrations on their teeth and increasing in size. For instance, ''D. limbatus'' had enamel serrations that helped it cut through flesh (which were similar to the serrations that can be found on ''[[Secodontosaurus]]''). The second-largest species, ''D. grandis'', has denticle serrations similar to those of sharks and [[Theropoda|theropod]] dinosaurs, making its teeth even more specialized for slicing through flesh. As ''Dimetrodon'''s prey grew larger, the various species responded by growing to larger sizes and developing ever-sharper teeth.<ref>{{Cite web|url=https://www.nationalgeographic.com/environment/topic/great-energy-challenge|title=Great Energy Challenge|website=Environment}}</ref> The thickness and mass of the teeth of ''Dimetrodon'' may also have been an adaptation for increasing dental longevity.<ref>{{cite journal |last1=Maho |first1=Tea |last2=Maho |first2=Sigi |last3=Scott |first3=Dianne |last4=Reisz |first4=Robert R. |date=19 August 2022 |title=Permian hypercarnivore suggests dental complexity among early amniotes |journal=[[Nature Communications]] |volume=13 |issue=1 |page=4882 |doi=10.1038/s41467-022-32621-5 |pmid=35986022 |pmc=9391490 |bibcode=2022NatCo..13.4882M }}</ref>

==== Nasal cavity ====
On the inner surface of the nasal section of the skull are ridges called [[Nasoturbinal concha|nasoturbinals]], which may have supported cartilage that increased the area of the [[olfactory epithelium]], the layer of tissue that detects odors. These ridges are much smaller than those of later synapsids from the Late Permian and Triassic, whose large nasoturbinals are taken as evidence for warm-bloodedness because they may have supported mucous membranes that warmed and moistened incoming air. Thus, the nasal cavity of ''Dimetrodon'' is [[Transitional fossil|transitional]] between those of early land [[vertebrate]]s and mammals.<ref name=KTS06>{{cite journal |last=Kemp |first=T.S. |s2cid=3184629 |year=2006 |title=The origin and early radiation of the therapsid mammal-like reptiles: a palaeobiological hypothesis |journal=Journal of Evolutionary Biology |volume=19 |issue=4 |pages=1231–1247 |doi=10.1111/j.1420-9101.2005.01076.x |pmid=16780524|doi-access=free }}</ref>

==== Jaw joint and ear ====
Another transitional feature of ''Dimetrodon'' is a ridge in the back of the jaw called the reflected lamina, which is found on the [[articular]] bone, which connects to the [[quadrate bone]] of the skull to form the jaw joint. In later mammal ancestors, the articular and quadrate separated from the jaw joint, while the articular developed into the [[malleus]] bone of the [[middle ear]]. The reflected lamina became part of a ring called the tympanic annulus that supports the [[ear drum]] in all living mammals.<ref name=TOL2>{{cite web |url=http://tolweb.org/notes/?note_id=466 |title=Autapomorphies of the main clades of synapsids |last=Laurin |first=M. |author2=Reisz, R.R. |year=1997 |work=Tree of Life Web Project |access-date=24 August 2012}}</ref>

=== Tail ===
[[File:PSM V73 D566 Restoration of dimetrodon.png|thumb|An outdated restoration of ''Dimetrodon'' from 1908 showing a short tail, made before the discovery of skeletons with complete tails]]
The tail of ''Dimetrodon'' makes up a large portion of its total body length and includes around 50 [[caudal vertebra]]e. Tails were missing or incomplete in the first described skeletons of ''Dimetrodon''. The only caudal vertebrae known were the 11 closest to the hip. Since these first few caudal vertebrae narrow rapidly as they progress farther from the hip, many paleontologists in the late 19th and early 20th centuries thought that ''Dimetrodon'' had a very short tail. A largely complete tail of ''Dimetrodon'' was not described until 1927.<ref name=RAS27>{{cite journal |last=Romer |first=A.S. |year=1927 |title=Notes on the Permo-Carboniferous reptile ''Dimetrodon'' |journal=The Journal of Geology |volume=35 |issue=8 |pages=673–689 |jstor=30060393 |doi=10.1086/623462 |bibcode=1927JG.....35..673R|s2cid=140679339 }}</ref>

=== Sail ===
[[File:Dimetrodon Royal Tyrrell Museum.jpg|thumb|left|Two ''D. grandis'' skeletons, [[Royal Tyrrell Museum]]]]
The sail of ''Dimetrodon'' is formed by elongated [[neural spine]]s projecting from the vertebrae. Each spine varies in cross-sectional shape from its base to its tip in what is known as "dimetrodont" differentiation.<ref name=Retal12>{{cite journal | last1 = Rega | first1 = E. A. | last2 = Noriega | first2 = K. | last3 = Sumida | first3 = S. S. | last4 = Huttenlocker | first4 = A. | last5 = Lee | first5 = A. | last6 = Kennedy | first6 = B. | doi = 10.3158/2158-5520-5.1.104 | title = Healed Fractures in the Neural Spines of an Associated Skeleton of Dimetrodon: Implications for Dorsal Sail Morphology and Function | journal = Fieldiana Life and Earth Sciences | volume = 5 | pages = 104–111 | year = 2012 | s2cid = 108887164 }}</ref> Near the vertebra body, the spine cross section is laterally compressed into a rectangular shape and, closer to the tip, it takes on a figure-eight shape as a groove runs along either side of the spine. The figure-eight shape is thought to reinforce the spine, preventing bending and fractures.<ref name=RSNPL05>{{cite journal |last=Rega |first=E. |author2=Sumida, S. |author3=Noriega, K. |author4=Pell, C. |author5= Lee, A.  |year=2005 |title=Evidence-based paleopathology I: Ontogenetic and functional implications of dorsal sails in ''Dimetrodon'' |journal=[[Journal of Vertebrate Paleontology]] |volume=25 |issue=S3 |pages=103A|doi=10.1080/02724634.2005.10009942|s2cid=220413556 }}</ref> A cross-section of the spine of one specimen of ''Dimetrodon giganhomogenes'' is rectangular in shape but preserves figure-eight shaped rings close to its center, indicating that the shape of spines may change as individuals age.<ref name=SRN05>{{cite journal |last=Sumida |first=S. |author2=Rega, E. |author3= Noriega, K.  |year=2005 |title=Evidence-based paleopathology II: Impact on phylogenetic analysis of the genus ''Dimetrodon'' |journal=Journal of Vertebrate Paleontology |volume=25 |issue=S3 |pages=120A|doi=10.1080/02724634.2005.10009942|s2cid=220413556 }}</ref> The microscopic anatomy of each spine varies from base to tip, indicating where it was embedded in the muscles of the back and where it was exposed as part of a sail. The lower or [[proximal]] portion of the spine has a rough surface that would have served as an anchoring point for the [[Epaxial and hypaxial muscles|epaxial muscles]] of the back and also has a network of connective tissues called [[Sharpey's fibers]] that indicate it was embedded within the body. Higher up on the [[distal]] (outer) portion of the spine, the bone surface is smoother. The [[periosteum]], a layer of tissue surrounding the bone, is covered in small grooves that presumably supported the blood vessels that vascularized the sail.<ref name=HRS10>{{cite journal |last=Huttenlocker |first=A.K. |author2=Rega, E. |author3= Sumida, S.S.  |year=2010 |title=Comparative anatomy and osteohistology of hyperelongate neural spines in the sphenacodontids ''Sphenacodon'' and ''Dimetrodon'' (Amniota: Synapsida) |journal=Journal of Morphology |volume=271 |issue=12 |pages=1407–1421 |doi=10.1002/jmor.10876 |pmid=20886514 |s2cid=40899700 }}</ref>

The large groove that runs the length of the spine was once thought to be a channel for blood vessels, but since the bone does not contain vascular canals, the sail is not thought to have been as highly vascularized as once thought. Some specimens of ''Dimetrodon'' preserve deformed areas of the neural spines that appear to be healed-over fractures. The [[cortical bone]] that grew over these breaks is highly vascularized, suggesting that soft tissue must have been present on the sail to supply the site with [[blood vessel]]s.<ref name=RSNPL05 /> Layered [[lamellar bone]] makes up most of the neural spine's cross-sectional area, and contains lines of arrested growth that can be used to determine the age of each individual at death.<ref name=RNSL04>{{cite journal |last=Rega |first=E.A. |author2=Noriega, K. |author3=Sumida, S. |author4= Lee, A.  |year=2004 |title=Histological analysis of traumatic injury to multiple neural spines of an associated skeleton of ''Dimetrodon'': Implications for healing response, dorsal sail morphology and age-at-death in a Lower Permian synapsid |journal=Integrative and Comparative Biology |volume=44 |page=628}}</ref> In many specimens of ''D. gigashomogenes'', the distal portions of spines bend sharply, indicating that the sail would have had an irregular profile in life. Their crookedness suggests that soft tissue may not have extended all the way to the tips of the spines, meaning that the sail's webbing may not have been as extensive as it is commonly imagined.<ref name=Retal12 />

=== Skin ===
[[File:Dimetrodon grandis 3D Model Reconstruction.png|thumb|''Dimetrodon grandis'' in an upright posture based on ''Dimetropus'' tracks, restored hypothetically with scaleless skin and scutes on its underside—fossil impressions described in 2025 indicate that ''Dimetrodon'' in fact had scaly skin similar to reptiles and did not have ventral scutes]]
Scaly body impressions that likely were made by ''Dimetrodon teutonis'' were described in 2025 from the Early Permian Bromacker site in Germany. Given the ichnogenus name ''Bromackerichnus'', the impressions left by animals resting on mud show a scaly epidermis pattern on the belly, and on the underside of the forelimbs and the tail, supporting the idea that early synapsids in general had a scaly body covering similar to reptiles.<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> The fossilized bodies of the [[Varanopidae|varanopid]] ''[[Ascendonanus]]'' with preserved soft tissues from the Early Permian of Germany also indicate some early synapsids may have had squamate-like scales.<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 |year=2018 |title=First arboreal 'pelycosaurs' (Synapsida: Varanopidae) from the early Permian Chemnitz Fossil Lagerstätte, SE Germany, with a review of varanopid phylogeny |journal=[[PalZ]] |volume= 92 |issue=2 |pages=315–364 |doi=10.1007/s12542-018-0405-9 |bibcode=2018PalZ...92..315S |s2cid=133846070 }}</ref> However, the taxonomic placement of varanopids has been debated between synapsids or closer to diapsid reptiles.<ref>{{Cite journal|last=Modesto|first=Sean P.|date=January 2020|title=Rooting about reptile relationships|url=https://www.nature.com/articles/s41559-019-1074-0|journal=Nature Ecology & Evolution|language=en|volume=4|issue=1|pages=10–11|doi=10.1038/s41559-019-1074-0|pmid=31900449|s2cid=209672518|issn=2397-334X|url-access=subscription}}</ref><ref>{{Cite journal|last1=Ford|first1=David P.|last2=Benson|first2=Roger B. J.|date=2019|title=A redescription of Orovenator mayorum (Sauropsida, Diapsida) using high-resolution μCT, 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|s2cid=92485505 |issn=2056-2802|url=https://ora.ox.ac.uk/objects/uuid:3f9d9f16-aa8b-4910-a967-9a6a645e4d74|doi-access=free|bibcode=2019PPal....5..197F |url-access=subscription}}</ref> Some synapsid groups later developed bare, glandular skin, as indicated by the fossils of the dinocephalian therapsid ''[[Estemmenosuchus]]'' from the middle Permian of Russia, which show its skin would have been smooth and well-provided with glands.  ''Estemmenosuchus'' also had osteoderms embedded in its skin. Later synapsids evolved hair and whiskers that became characteristics of [[mammals]]. 

== Classification history ==

=== Earliest discoveries ===
[[File: Dimetrodon borealis (Bathygnathus borealis) jaw - Redpath Museum - McGill University - Montreal, Canada - DSC08069.jpg|thumb|Maxilla of ''[[Dimetrodon borealis]]'', the first ''Dimetrodon'' fossil to be described]]
The earliest discovery of ''Dimetrodon'' fossils were of a [[maxilla]] recovered in 1845 by a man named Donald McLeod, living in the British colony of [[Prince Edward Island]].<ref name="SDAE95">{{cite book|last=Spalding|first=D.A.E.|title=Vertebrate Fossils and the Evolution of Scientific Concepts|publisher=Taylor & Francis US|year=1995|isbn=2881249965|editor=Sarjeant, W.A.S.|pages=245–254|chapter=''Bathygnathus'', Canada's first "dinosaur"}}</ref> These fossils were purchased by John William Johnson, a Canadian geologist, and then described by [[Joseph Leidy]] in 1854 as the [[mandible]] of ''[[Dimetrodon borealis|Bathygnathus borealis]]'', a large [[carnivore]] related to ''[[Thecodontosaurus]],''<ref name="SDAE952">{{cite book|last=Spalding|first=D.A.E.|title=Vertebrate Fossils and the Evolution of Scientific Concepts|publisher=Taylor & Francis US|year=1995|isbn=2881249965|editor=Sarjeant, W.A.S.|pages=245–254|chapter=''Bathygnathus'', Canada's first "dinosaur"}}</ref> although it was later reclassified as a species of ''Dimetrodon'' in 2015, as ''Dimetrodon borealis''.<ref name="brink15"/> Although ''Bathygnathus'' was named first, a petition to conserve the genus ''Dimetrodon'' and suppress the genus ''Bathygnathus'' was submitted to the [[International Commission on Zoological Nomenclature]] (ICZN) in 2015,<ref>{{cite journal|author=Brink, K.S.|year=2015|title=Case 3695 — Dimetrodon Cope, 1878 (Synapsida, Sphenacodontidae): proposed conservation by reversal of precedence with Bathygnathus Leidy, 1853|journal=The Bulletin of Zoological Nomenclature|volume=72|issue=4|pages=297–299|doi=10.21805/bzn.v72i4.a17}}</ref> which was approved in 2019.<ref>{{cite journal|title=Opinion 2446 (Case 3695) – Dimetrodon Cope, 1878 (Synapsida, Sphenacodontidae): name conserved|journal=The Bulletin of Zoological Nomenclature|volume=76|issue=1|pages=200–201|doi=10.21805/bzn.v76.a063|date=December 2019}}</ref>

=== First descriptions by Cope ===
Fossils now attributed to ''Dimetrodon'' were first studied by American paleontologist [[Edward Drinker Cope]] in the 1870s. Cope had obtained the fossils along with those of many other Permian [[tetrapod]]s from several collectors who had been exploring a group of rocks in Texas called the [[Red Beds of Texas and Oklahoma|Red Beds]]. Among these collectors were Swiss naturalist [[Jacob Boll]], Texas geologist [[W. F. Cummins]], and amateur paleontologist [[Charles Hazelius Sternberg]].<ref name=MWD08>{{cite journal |last=Matthew |first=W.D. |year=1908 |title=Review of Case's 'Revision of the Pelycosauria of North America' |journal=Science |volume=27 |issue=699 |pages=816–818 |url=https://books.google.com/books?id=5kAbAAAAYAAJ |doi=10.1126/science.27.699.816|bibcode=1908Sci....27..816M }}</ref> Most of Cope's specimens went to the [[American Museum of Natural History]] or to the [[University of Chicago]]'s Walker Museum (most of the Walker fossil collection is now housed in the [[Field Museum of Natural History]]).

Sternberg sent some of his own specimens to German paleontologist [[Ferdinand Broili]] at [[Munich University]], although Broili was not as prolific as Cope in describing specimens. Cope's rival [[Othniel Charles Marsh]] also collected some bones of ''Dimetrodon'', which he sent to the Walker Museum.<ref name=CEC07>{{cite book |last=Case |first=E.C. |year=1907 |title=Revision of the Pelycosauria of North America |publisher=Carnegie Institution of Washington |location=Washington, D.C.|pages=[https://archive.org/details/bub_gb_7QMDAAAAIAAJ/page/n6 1]–176 |url=https://archive.org/details/bub_gb_7QMDAAAAIAAJ}}</ref> The first use of the name ''Dimetrodon'' came in 1878 when Cope named the species ''Dimetrodon incisivus'', ''Dimetrodon rectiformis'', and ''Dimetrodon gigas'' in the scientific journal ''[[Proceedings of the American Philosophical Society]]''.<ref name=CED78>{{cite journal |last=Cope |first=E.D. |year=1878 |title=Descriptions of extinct Batrachia and Reptilia from the Permian formation of Texas |journal=Proceedings of the American Philosophical Society |volume=17 |issue=101 |pages=505–530 |jstor=982652}}</ref>

The first description of a ''Dimetrodon'' fossil came a year earlier, though, when Cope named the species ''Clepsydrops limbatus'' from the Texas Red Beds.<ref name=CED77>{{cite journal |last=Cope |first=E.D. |year=1877 |title=Descriptions of extinct vertebrata from the Permian and Triassic Formations of the United States |journal=Proceedings of the American Philosophical Society |volume=17 |issue=100 |pages=182–193 |jstor=982295}}</ref> (The name ''Clepsydrops'' was first coined by Cope in 1875 for [[Sphenacodontidae|sphenacodontid]] remains from [[Vermilion County, Illinois]], and was later employed for many sphenacontid specimens from Texas; many new species of sphenacodontids from Texas were assigned to either ''Clepsydrops'' or ''Dimetrodon'' in the late 19th and early 20th centuries.) ''C.&nbsp;limbatus'' was reclassified as a species of ''Dimetrodon'' in 1940, meaning that Cope's 1877 paper was the first record of ''Dimetrodon''.

Cope was the first to describe a sail-backed synapsid with the naming of ''C. natalis'' in his 1878 paper, although he called the sail a fin and compared it to the crests of the modern [[Basiliscus (genus)|basilisk lizard]] (''Basilicus''). Sails were not preserved in the specimens of ''D.&nbsp; incisive'' and ''D.&nbsp;gigas'' that Cope described in his 1878 paper, but elongated spines were present in the ''D.&nbsp;rectiformis'' specimen he described.<ref name=CED78 /> Cope commented on the purpose of the sail in 1886, writing, "The utility is difficult to imagine. Unless the animal had aquatic habits and swam on its back, the crest or fin must have been in the way of active movements... The limbs are not long enough nor the claws acute enough to demonstrate [[Arboreal locomotion|arboreal]] habits, as in the existing genus ''Basilicus'', where a similar crest exists."<ref name=BC99 />

=== Early 20th century descriptions ===
[[File:DimetrodonKnight.jpg|thumb|Restoration of ''Dimetrodon'' and ''[[Edaphosaurus]]'' (background), [[Charles R. Knight]], 1897]]
In the first few decades of the 20th century, American paleontologists [[E. C. Case]] authored many studies on ''Dimetrodon'' and described several new species. He received funding from the [[Carnegie Institution]] for his study of many ''Dimetrodon'' specimens in the collections of the [[American Museum of Natural History]] and several other museums.<ref name=CEC07 /> Many of these fossils had been collected by Cope but had not been thoroughly described, as Cope was known for erecting new species on the basis of only a few bone fragments.

Beginning in the late 1920s, paleontologist [[Alfred Romer]] restudied many ''Dimetrodon'' specimens and named several new species. In 1940, Romer coauthored a large study with [[Llewellyn Ivor Price]] called "Review of the Pelycosauria" in which the species of ''Dimetrodon'' named by Cope and Case were reassessed.<ref name=RP40 /> Most of the species names considered valid by Romer and Price are still used today.<ref name=HRS10 />

=== New specimens ===
In the decades following Romer and Price's monograph, many ''Dimetrodon'' specimens were described from localities outside [[Texas]] and [[Oklahoma]]. The first was described from the [[Four Corners]] region of Utah in 1966<ref name=VPP66>{{cite journal |last=Vaughn |first=P.P. |year=1966 |title=Comparison of the Early Permian vertebrate faunas of the Four Corners region and north-central Texas |publisher=Los Angeles County Museum of Natural History |series=Contributions in Science |volume=105 |pages=1–13 |url=http://www.nhm.org/site/sites/default/files/pdf/contrib_science/CS105.pdf |access-date=2016-04-21 |archive-url=https://web.archive.org/web/20170110065602/http://www.nhm.org/site/sites/default/files/pdf/contrib_science/CS105.pdf |archive-date=2017-01-10 |url-status=dead }}</ref> and another was described from Arizona in 1969.<ref name=VPP69>{{cite journal |last=Vaughn |first=P.P. |year=1969 |title=Early Permian vertebrates from southern New Mexico and their paleozoogeographic significance |publisher=Los Angeles County Museum of Natural History |series=Contributions in Science |volume=166 |pages=1–22 |url=http://www.nhm.org/site/sites/default/files/pdf/contrib_science/CS166.pdf |access-date=2016-04-21 |archive-url=https://web.archive.org/web/20170110065545/http://www.nhm.org/site/sites/default/files/pdf/contrib_science/CS166.pdf |archive-date=2017-01-10 |url-status=dead }}</ref> In 1975, Olson reported ''Dimetrodon'' material from the [[Washington Formation]] of Ohio, which has been given a tentative assignment of ''D.'' cf. ''limbatus''.<ref>{{Cite journal |last1=Olson |first1=Everett C |last2=Olson |first2=Everett C. |date=1970-02-05 |title=Trematops Stonei sp. nov. (Temnospondyli Amphibia) from the Washington Formation, Dunkard Group, Ohio |url=https://www.biodiversitylibrary.org/part/202348 |journal=Kirtlandia |volume=8 |pages=1––12}}</ref><ref name=OEC75>{{cite journal |last=Olson |first=E.C. |year=1975 |title=Permo-Carboniferous paleoecology and morphotypic series |journal=American Zoologist |volume=15 |issue=2 |pages=371–389 |doi=10.1093/icb/15.2.371|doi-access=free }}</ref><ref name=":1">{{Cite journal |last=Lucas |first=Spencer G. |date=2013-11-01 |title=Vertebrate biostratigraphy and biochronology of the upper Paleozoic Dunkard Group, Pennsylvania–West Virginia–Ohio, USA |url=https://www.sciencedirect.com/science/article/abs/pii/S0166516213001249 |journal=International Journal of Coal Geology |series=SI:Dunkard Group Geology |volume=119 |pages=79–87 |doi=10.1016/j.coal.2013.04.007 |bibcode=2013IJCG..119...79L |issn=0166-5162|url-access=subscription }}</ref> A new species of ''Dimetrodon'' called ''D.&nbsp;occidentalis'' (meaning "western ''Dimetrodon''") was named in 1977 from New Mexico.<ref name=BDS77>{{cite journal |last=Berman |first=D.S. |year=1977 |title=A new species of ''Dimetrodon'' (Reptilia, Pelycosauria) from a non-deltaic facies in the Lower Permian of north-central New Mexico |journal=Journal of Paleontology |volume=51 |issue=1 |pages=108–115|jstor=1303466}}</ref> The specimens found in Utah and Arizona probably also belong to ''D.&nbsp;occidentalis''.<ref name=MSZR07>{{cite journal |last=Madalena |first=K. |author2=Sumida, S. |author3=Zeigler, K. |author4= Rega, E.  |year=2007 |title=A new record of the Early Permian pelycosaurian-grade synapsid ''Dimetrodon'' (Eupelycosauria: Sphenacodontidae) from the Lower Cutler Group (Early Permian) of Jemez Pueblo, north-central New Mexico |journal=Journal of Vertebrate Paleontology |volume=27 |issue=3, Suppl |pages=110A |doi=10.1080/02724634.2007.10010458}}</ref>

Before these discoveries, a theory existed that a midcontinental seaway separated what is now Texas and Oklahoma from more western lands during the Early Permian, isolating ''Dimetrodon'' to a small region of North America, while a smaller sphenacodontid called ''[[Sphenacodon]]'' dominated the western area. While this seaway probably did exist, the discovery of fossils outside Texas and Oklahoma show that its extent was limited and that it was not an effective barrier to the distribution of ''Dimetrodon''.<ref name=BDS77 /><ref name=VPP73>{{cite book |last=Vaughn |first=P.P. |year=1973 |title=Vertebrates from the Cutler Group of Monument Valley and vicinity |series=New Mexico Geological Society Guidebook |volume=24 |publisher=New Mexico Geological Society |pages=99–105 |url=http://nmgs.nmt.edu/publications/guidebooks/downloads/24/24_p0099_p0105.pdf |archive-date=2016-04-12 |access-date=2012-08-24 |archive-url=https://web.archive.org/web/20160412174344/http://nmgs.nmt.edu/publications/guidebooks/downloads/24/24_p0099_p0105.pdf |url-status=dead }}</ref>

In 2001, a new species of ''Dimetrodon'' called ''D. teutonis'' was described from the Lower Permian Bromacker locality at the Thuringian Forest of Germany, extending the geographic range of ''Dimetrodon'' outside North America for the first time.<ref name=BRMH01 />

== Species ==
[[File:Dimetrodon species2DB15.jpg|thumb|300px|Artist impressions of various species to scale]] 
Twenty '''[[species]] of ''Dimetrodon''''' have been named since the [[genus]] was first described in 1878. Many have been [[Synonym (taxonomy)|synonymized]] with older named species, and some now belong to different genera.

===Summary===
{| class="wikitable sortable"
|-
! Species !! Authority !! Location !! Status !! Synonyms !! class="unsortable" | Images
|-
| ''Dimetrodon angelensis'' || Olson, 1962 ||
* Texas
| Valid || || [[File:Dimetrodon angelensisDB.jpg|125px]]
|-
| ''[[Dimetrodon borealis]]''|| Leidy, 1854 ||
* Prince Edward Island
| Valid || Previously known as the [[dinosaur]] ''Bathygnathus borealis''|| [[File:Bathygnathus head1DB.jpg|125px]]
|-
| ''Dimetrodon booneorum'' || Romer, 1937 ||
* Texas
| Valid || ||
|-
| ''Dimetrodon dollovianus'' || Case, 1907 ||
* Texas
| Valid || ''Embolophorus dollovianus'' Cope, 1888 ||
|-
| ''Dimetrodon gigahomogenes'' || Case, 1907 ||
* Texas
| Valid || || [[File:Dimetrodon gigashomog DB.jpg|125px]]
|-
| ''Dimetrodon grandis'' || Romer and Price, 1940 ||
* Oklahoma
* Texas
| Valid || ''Clepsydrops gigas'' Cope, 1878<br /> ''Dimetrodon gigas'' Cope, 1878<br /> ''Theropleura grandis'' Case, 1907<br /> ''Bathyglyptus theodori'' Case, 1911<br /> ''Dimetrodon maximus'' Romer 1936||  [[File:Dimetrodon grandis in "high walk" pose.png|125px]]
|-
| ''Dimetrodon kempae'' || Romer, 1937 ||
* Texas
| Possible ''[[nomen dubium]]'' || ||
|-
| ''Dimetrodon limbatus'' || Romer and Price, 1940 ||
* Oklahoma
* Texas
* Ohio (tentatively)<ref name=":1" />
| Valid || ''Clepsydrops limbatus'' Cope, 1877<br /> ''Dimetrodon incisivus'' Cope, 1878<br /> ''Dimetrodon rectiformis'' Cope, 1878<br /> ''Dimetrodon semiradicatus'' Cope, 1881 ||
|-
| ''Dimetrodon loomisi'' || Romer, 1937 ||
* Texas
* Oklahoma
| Valid || || [[File:Dimetrodon loomisi.jpg|125px]]
|-
| ''Dimetrodon macrospondylus'' || Case, 1907 ||
* Texas
| Valid || ''Clepsydrops macrospondylus'' Cope, 1884<br /> ''Dimetrodon platycentrus'' Case, 1907 ||
|-
| ''Dimetrodon milleri'' || Romer, 1937 ||
* Texas
| Valid || || [[File:Dimetrodon milleri.jpg|125px]]
|-
| ''Dimetrodon natalis'' || Romer, 1936 ||
* Texas
| Valid || ''Clepsydrops natalis'' Cope, 1878 || [[File:D natalisDB.jpg|125px]]
|-
| ''Dimetrodon occidentalis'' || Berman, 1977 ||
* Arizona
* New Mexico
* Utah
| Valid || ||
|-
| ''Dimetrodon teutonis'' || Berman ''et al.'', 2001 ||
* Germany
| Valid || ||
|}

===''Dimetrodon limbatus''===
[[File:Dimetr incis22DB.jpg|thumb|Restoration of ''Dimetrodon limbatus'' feeding on ''[[Varanosaurus acutirostris]]'']]
''Dimetrodon limbatus'' was first described by Edward Drinker Cope in 1877 as ''Clepsydrops limbatus''.<ref name=CED77/> (The name ''Clepsydrops'' was first coined by Cope in 1875 for sphenacodontid remains from [[Vermilion County, Illinois]], and was later employed for many sphenacontid specimens from Texas; many new species of sphenacodontids from Texas were assigned to either ''Clepsydrops'' or ''Dimetrodon'' in the late nineteenth and early twentieth centuries.) Based on a specimen from the [[Red Beds of Texas and Oklahoma|Red Beds of Texas]], it was the first known sail-backed synapsid. In 1940, paleontologists [[Alfred Romer]] and [[Llewellyn Ivor Price]] reassigned ''C. limbatus'' to the genus ''Dimetrodon'', making ''D. limbatus'' the [[type species]] of ''Dimetrodon''.<ref name=RP40>{{cite journal |last=Romer |first=A.S. |author2=Price, L.I.  |year=1940 |title=Review of the Pelycosauria |journal=Geological Society of America Special Paper |volume=28 |pages=1–538 |doi=10.1130/spe28-p1|series=Geological Society of America Special Papers }}</ref> Remains tentatively assigned to this species are also known from [[Washington County, Ohio]], which correspond to a relatively large individual. These remains are slightly older than others assigned to ''D. limbatus'' from the west, although potential ''D. limbatus'' remains from New Mexico may be concurrent with it.<ref name=":1" />

====''Dimetrodon incisivus''====
The first use of the name ''Dimetrodon'' came in 1878 when Cope named the species ''Dimetrodon incisivus'' along with ''Dimetrodon rectiformis'' and ''Dimetrodon gigas''.<ref name=CED78/>

====''Dimetrodon rectiformis''====
''Dimetrodon rectiformis'' was named alongside ''Dimetrodon incisivus'' in Cope's 1878 paper, and was the only one of the three named species to preserve elongated neural spines.<ref name=CED78/> In 1907, paleontologist [[E. C. Case]] moved ''D. rectiformis'' into the species ''D. incisivus''.<ref name=CEC07/> ''D. incisivus'' was later synonymous with the type species ''Dimetrodon limbatus'', making ''D. rectiformis'' a synonym of ''D. limbatus''.<ref name=HRS10/>

====''Dimetrodon semiradicatus''====
Described in 1881 on the basis of upper jaw bones, ''Dimetrodon semiradicatus'' was the last species named by Cope.<ref name=CED81>{{cite journal |last=Cope |first=E.D. |year=1881 |title=On some new Batrachia and Reptilia from the Permian Red Beds of Texas |journal=Bulletin of the United States Geological Survey |volume=6 |pages=79–82}}</ref> In 1907, E. C. Case synonymized ''D. semiradicatus'' with ''D. incisivus'' based on similarities in the shape of the teeth and skull bones.<ref name=CEC07/> ''D. incisivus''' and ''D. semiradicatus'' are now considered synonyms of ''D. limbatus''.<ref name=HRS10/>

===''Dimetrodon dollovianus''===
''Dimetrodon dollovianus'' was first described by Edward Drinker Cope in 1888 as ''Embolophorus dollovianus''. In 1903, E. C. Case published a lengthy description of ''E. dollovianus'', which he later referred to ''Dimetrodon''.<ref name=CEC03>{{cite journal |last=Case |first=E.C. |year=1903 |title=The osteology of ''Embolophorus dollovianus'', Cope, with an attempted restoration |journal=The Journal of Geology |volume=11 |issue=1 |pages=1–28 |jstor=30056735 |doi=10.1086/621055 |bibcode=1903JG.....11....1C|doi-access=free }}</ref>

===''Dimetrodon grandis''===
[[File:Dimetrodon grandis in "high walk" pose.png|thumb|Restoration of ''Dimetrodon grandis'']]
Paleontologist E. C. Case named a new species of sail-backed synapsid, ''Theropleura grandis'', in 1907.<ref name=CEC07/> In 1940, Alfred Romer and Llewellyn Ivor Price reassigned ''Theropleura grandis'' to ''Dimetrodon'', erecting the species ''D. grandis''.<ref name=RP40/>

====''Dimetrodon gigas''====
In his 1878 paper on fossils from Texas, Cope named ''Clepsydrops gigas'' along with the first named species of ''Dimetrodon'', ''D. limbatus'', ''D. incisivus'', and ''D. rectiformis''.<ref name=CED78/> Case reclassified ''C. gigas'' as a new species of ''Dimetrodon'' in 1907.<ref name=CEC07/> Case also described a very well preserved skull of ''Dimetrodon'' in 1904, attributing it to the species ''Dimetrodon gigas''.<ref name=CEC04b>{{cite journal |last=Case |first=E.C. |year=1904 |title=The osteology of the skull of the pelycosaurian genus, ''Dimetrodon'' |journal=The Journal of Geology |volume=12 |issue=4 |pages=304–311 |jstor=30055825 |doi=10.1086/621157 |bibcode=1904JG.....12..304C|url=https://zenodo.org/record/1431443 |doi-access=free }}</ref> In 1919, [[Charles W. Gilmore]] attributed a nearly complete specimen of ''Dimetrodon'' to ''D. gigas''.<ref name=GCW19>{{cite journal |last=Gilmore |first=C.W. |year=1919 |title=A mounted skeleton of ''Dimetrodon gigas'' in the United States National Museum, with notes on the skeletal anatomy |journal=Proceedings of the U.S. National Museum |volume=56 |issue=2300 |pages=525–539 |doi=10.5479/si.00963801.56-2300.525|url=http://repository.si.edu/bitstream/handle/10088/14741/1/USNMP-56_2300_1919.pdf }}</ref> ''Dimetrodon gigas'' is now recognized as a synonym of ''D. grandis''.<ref name=Hetal05>{{cite book |last=Henrici |first=A.C. |author2=Berman, D.S. |author3=Lucas, S.G. |author4=Heckert, A.B. |author5=Rinehart, L.F. |author6= Zeigler, K.E.  |year=2005 |chapter=The carpus and tarsus of the Early Permian synapsid ''Sphenacodon ferox'' (Eupelycosauria: Sphenacodontidae) |title=The Nonmarine Permian |editor=Lucas, S.G. |editor2=Zeigler, K.E.|series=New Mexico Museum of Natural History and Science Bulletin |volume=30 |publisher=New Mexico Museum of Natural History and Science |location=Albuquerque |pages=106–110 |chapter-url=http://libres.uncg.edu/ir/asu/f/Heckert_A_2005_30_the_Carpus.pdf}}</ref>

===''Dimetrodon giganhomogenes''===
[[File:Dimetrodon gigashomog DB.jpg|thumb|Restoration of ''Dimetrodon giganhomogenes'']]
''Dimetrodon giganhomogenes'' was named by E. C. Case in 1907 and is still considered a valid species of ''Dimetrodon''.<ref name=CEC07/><ref name=HRS10/>

===''Dimetrodon macrospondylus''===
''Dimetrodon macrospondylus'' was first described by Cope in 1884 as ''Clepsydrops macrospondylus''. In 1907, Case reclassified it as ''Dimetrodon macrospondylus''.<ref name=CEC07/>

====''Dimetrodon platycentrus''====
''Dimetrodon platycentrus'' was first described by Case in his 1907 monograph. It is now considered a synonym of ''Dimetrodon macrospondylus''.<ref name=HRS10/>

===''Dimetrodon natalis''===
[[File:D natalisDB.jpg|thumb|Restoration of ''Dimetrodon natalis'']]
Paleontologist Alfred Romer erected the species ''Dimetrodon natalis'' in 1936, previously described as ''Clepsydrops natalis''. ''D. natalis'' was the smallest known species of ''Dimetrodon'' at that time, and was found alongside remains of the larger-bodied ''D. limbatus''.<ref name=RAS37>{{cite journal |last=Romer |first=A.S. |year=1937 |title=New genera and species of pelycosaurian reptiles |journal=Proceedings of the New England Zoological Club |volume=16 |pages=89–97 |url=http://www.stuartsumida.com/BIOL680-09/Romer1937.pdf}}</ref>

===''Dimetrodon booneorum''===
''Dimetrodon booneorum'' was first described by Alfred Romer in 1937 on the basis of remains from Texas.<ref name=RAS37/>

===''"Dimetrodon" kempae''===
''Dimetrodon kempae'' was named by Romer in 1937, in the same paper as ''D. booneorum'', ''D. loomisi'', and ''D. milleri''.<ref name=RAS37/> ''Dimetrodon kempae'' was named on the basis of a single humerus and a few vertebrae, and may therefore be a ''[[nomen dubium]]'' that cannot be distinguished as a unique species of ''Dimetrodon''.<ref name=BRMH01/> In 1940, Romer and Price raised the possibility that ''D. kempae'' may not fall within the genus ''Dimetrodon'', preferring to classify it as Sphenacodontidae ''[[incertae sedis]]''.<ref name=RP40/>

===''Dimetrodon loomisi''===
[[File:Dimetrodon loomisi.jpg|thumb|Restoration of ''Dimetrodon loomisi'']]
''Dimetrodon loomisi'' was first described by Alfred Romer in 1937 along with ''D. booneorum'', ''D. kempae'', and ''D. milleri''.<ref name=RAS37/> Remains have been found in Texas and Oklahoma.

===''Dimetrodon milleri''===
[[File:Dimetrodon milleri.jpg|thumb|Restoration of ''Dimetrodon milleri'']]
''Dimetrodon milleri'' was described by Romer in 1937.<ref name=RAS37/> It is one of the smallest species of ''Dimetrodon'' in North America and may be closely related to ''D. occidentalis'', another small-bodied species.<ref name=MSZR07/> ''D. milleri'' is known from two skeletons, one nearly complete (MCZ 1365) and another less complete but larger (MCZ 1367). ''D. milleri'' is the oldest known species of ''Dimetrodon''.

Besides its small size, ''D. milleri'' differs from other species of ''Dimetrodon'' in that its neural spines are circular rather than figure-eight shaped in cross-section. Its vertebrae are also shorter in height relative to the rest of the skeleton than those of other ''Dimetrodon'' species. The skull is tall and the snout is short relative to the temporal region. A short vertebrae and tall skull are also seen in the species ''D. booneorum'', ''D. limbatus'' and ''D. grandis'', suggesting that ''D. milleri'' may be the first of an evolutionary progression between these species.

===''Dimetrodon angelensis''===
[[File:Dimetrodon angelensisDB.jpg|thumb|Restoration of ''Dimetrodon angelensis'']]
''Dimetrodon angelensis'' was named by paleontologist [[Everett C. Olson]] in 1962.<ref name=OEC62>{{cite journal |last=Olson |first=E.C. |year=1962 |title=Late Permian terrestrial vertebrates, USA and USSR |journal=Transactions of the American Philosophical Society |volume=52 |issue=2 |pages=1–224 |doi=10.2307/1005904|jstor=1005904 }}</ref> Specimens of the species were reported from the [[San Angelo Formation]] of Texas.<ref name=BB00>{{Cite journal | last1 = Battail | first1 = B. | doi = 10.1016/S0899-5362(00)00081-6 | title = A comparison of Late Permian Gondwanan and Laurasian amniote faunas | journal = Journal of African Earth Sciences | volume = 31 | pages = 165–174 | year = 2000 | issue = 1 | bibcode = 2000JAfES..31..165B }}</ref> It is also the largest species of ''Dimetrodon''.

===''Dimetrodon occidentalis''===
''Dimetrodon occidentalis'' was named in 1977 from New Mexico.<ref name=BDS77/> Its name means "western ''Dimetrodon''" because it is the only North American species of ''Dimetrodon'' known west of Texas and Oklahoma. It was named on the basis of a single skeleton belonging to a relatively small individual. The small size of ''D. occidentalis'' is similar to that of ''D. milleri'', suggesting a close relationship. ''Dimetrodon'' specimens found in Utah and Arizona probably also belong to ''D. occidentalis''.<ref name=MSZR07/>

===''Dimetrodon teutonis''===
''Dimetrodon teutonis'' was named in 2001 from the [[Thuringian Forest]] of Germany and was the first species of ''Dimetrodon'' to be described outside North America. It is also the smallest species of ''Dimetrodon''.<ref name=BRMH01/>

===Species assigned to different genera===

====''Dimetrodon cruciger''====
In 1878, Cope published a paper called "The Theromorphous Reptilia" in which he described ''Dimetrodon cruciger''.<ref name=CED78b>{{cite journal |last=Cope |first=E.D. |year=1878 |title=The Theromorphous Reptilia |journal=The American Naturalist |volume=12 |issue=12 |pages=829–830| doi = 10.1086/272251|doi-access=free }}</ref> ''D. cruciger'' was distinguished by the small projections that extended from either side of each neural spine like the branches of a tree.<ref name=CED80>{{cite journal |last=Cope |first=E.D. |year=1880 |title=Second contribution to the history of the vertebrata of the Permian Formation of Texas |journal=Proceedings of the American Philosophical Society |volume=19 |issue=107 |pages=38–58 |jstor=982605}}</ref> In 1886, Cope moved ''D. cruciger'' to the genus ''[[Naosaurus]]'' because he considered its spines so different from those of other ''Dimetrodon'' species that the species deserved its own genus.<ref name=CED86>{{cite journal |last=Cope |first=E.D. |year=1886 |title=The long-spined Theromorpha of the Permian Epoch |journal=The American Naturalist |volume=20 |issue=6 |pages=544–545 | doi = 10.1086/274275 |doi-access=free }}</ref> ''Naosaurus'' would later be synonymized with ''[[Edaphosaurus]]'', a genus which Cope named in 1882 on the basis of skulls that evidently belonged to herbivorous animals given their blunt crushing teeth.<ref name=CEC82>{{cite journal |last=Cope |first=E.D. |year=1882 |title=Third contribution to the history of the vertebrata of the Permian Formation of Texas |journal=Proceedings of the American Philosophical Society |volume=20 |issue=112 |pages=447–461 |jstor=982692}}</ref>

====''Dimetrodon longiramus''====
E. C. Case named the species ''Dimetrodon longiramus'' in 1907 on the basis of a scapula and elongated mandible from the [[Belle Plains Formation]] of Texas.<ref name=CEC07/> In 1940, Romer and Price recognized that the ''D. longiramus'' material belonged to the same taxon as another specimen described by paleontologist [[Samuel Wendell Williston]] in 1916, which included a similarly elongated mandible and a long maxilla.<ref name=RP40/> Williston did not consider his specimen to belong to ''Dimetrodon'' but instead classified it as an [[ophiacodontid]].<ref name=WSW16 >{{cite journal |last=Williston |first=S.W. |year=1916 |title=The osteology of some American Permian vertebrates, II |journal=Contribution from the Walker Museum |volume=1 |pages=165–192}}</ref> Romer and Price assigned Case and Williston's specimens to a newly erected genus and species, ''[[Secodontosaurus longiramus]]'', that was closely related to ''Dimetrodon''.<ref name=RP40/><ref name=RBS92>{{Cite journal | last1 = Reisz | first1 = R. R. | last2 = Berman | first2 = D. S. | last3 = Scott | first3 = D. | s2cid = 56425294 | doi = 10.1111/j.1096-3642.1992.tb00920.x | title = The cranial anatomy and relationships of Secodontosaurus, an unusual mammal-like reptile (Synapsida: Sphenacodontidae) from the early Permian of Texas | journal = Zoological Journal of the Linnean Society | volume = 104 | issue = 2 | pages = 127–184 | year = 1992 }}</ref>

== Phylogenetic classification ==
{{See also|Synapsid#Linnaean and cladistic classifications}}
''Dimetrodon'' is an early member of a group called [[synapsid]]s, which include mammals and many of their extinct relatives, though it is not an ancestor of any mammal (which appeared millions of years later<ref>[[Mammal#The mammals appear|The mammals appear]]</ref>). It is often mistaken for a dinosaur in popular culture, despite having become extinct some 40&nbsp;million years (Ma) before the first appearance of dinosaurs in the [[Triassic]] period. As a synapsid, ''Dimetrodon'' is more closely related to mammals than to dinosaurs or any living reptile. By the early 1900s most paleontologists called ''Dimetrodon'' a reptile in accordance with [[Linnean taxonomy]], which ranked Reptilia as a [[class (biology)|class]] and ''Dimetrodon'' as a genus within that class. Mammals were assigned to a separate class, and ''Dimetrodon'' was described as a "mammal-like reptile". Paleontologists theorized that mammals evolved from this group in (what they called) a reptile-to-mammal transition.

=== Phylogenetic taxonomy of Synapsida ===
[[File:Dimetrodon skeleton.JPG|thumb|left|''D. grandis'' skeleton, [[North American Museum of Ancient Life]]]]
Under [[phylogenetic systematics]], the descendants of the [[last common ancestor]] of ''Dimetrodon'' and all living reptiles would include all mammals because ''Dimetrodon'' is more closely related to mammals than to any living reptile. Thus, if it is desired to avoid the clade that contains both mammals and the living reptiles, then ''Dimetrodon'' must not be included in that clade—nor any other "mammal-like reptile". Descendants of the last common ancestor of mammals and reptiles (which appeared around 310&nbsp;Ma in the [[Late Carboniferous]]) are therefore split into two clades: Synapsida, which includes ''Dimetrodon'' and mammals, and [[Sauropsida]], which includes living reptiles and all extinct reptiles more closely related to them than to mammals.<ref name=AKD09 />

Within clade Synapsida, ''Dimetrodon'' is part of the clade [[Sphenacodontia]], which was first proposed as an early synapsid group in 1940 by paleontologists Alfred Romer and Llewellyn Ivor Price, along with the groups [[Ophiacodontia]] and [[Edaphosauria]].<ref name=RP40 /> All three groups are known from the Late Carboniferous and Early Permian. Romer and Price distinguished them primarily by [[postcranial]] features such as the shapes of limbs and vertebrae. Ophiacodontia was considered the most primitive group because its members appeared the most reptilian, and Sphenacodontia was the most advanced because its members appeared the most like a group called [[Therapsida]], which included the closest relatives to mammals. Romer and Price placed another group of early synapsids called [[varanopid]]s within Sphenacodontia, considering them to be more primitive than other sphenacodonts like ''Dimetrodon''.<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|issue=2 |pages=601–624 |doi=10.1080/14772019.2011.631042 |bibcode=2012JSPal..10..601B |s2cid=84706899 }}</ref> They thought varanopids and ''Dimetrodon''-like sphenacodonts were closely related because both groups were carnivorous, although varanopids are much smaller and more lizard-like, lacking sails.

The modern view of synapsid relationships was proposed by paleontologist [[Robert R. Reisz]] in 1986, whose study included features mostly found in the skull rather than in the postcranial skeleton.<ref name=RRR86>{{cite encyclopedia |last=Reisz |first=R. R. |year=1986 |editor=Sues, H.-D.|encyclopedia=Handbuch der Paläoherpetologie |title=Pelycosauria |publisher=Gustav Fischer Verlag |volume=17A |pages=1–102|isbn=978-3-89937-032-4|ol=12985656M}}</ref> ''Dimetrodon'' is still considered a sphenacodont under this [[phylogeny]], but varanodontids are now considered more [[basal (phylogenetics)|basal]] synapsids, falling outside clade Sphenacodontia. Within Sphenacodontia is the group [[Sphenacodontoidea]], which in turn contains [[Sphenacodontidae]] and [[Therapsida]]. Sphenacodontidae is the group containing ''Dimetrodon'' and several other sail-backed synapsids like ''[[Sphenacodon]]'' and ''[[Secodontosaurus]]'', while [[Therapsid]]a includes mammals and their mostly Permian and [[Triassic]] relatives.

Below is the [[cladogram]] Clade Synapsida, which follows this phylogeny of [[Synapsida]] as modified from the analysis of Benson&nbsp;(2012).<ref name=BRJ12 />
{{clade| style=font-size:85%;line-height:85%
|label1=[[Amniota]]
|1={{clade
  |1=[[Sauropsida]] (including dinosaurs, living reptiles and birds) [[File:Protorothyris.jpg|50px]]
  |label2=[[Synapsida]]
  |2={{clade
  |1={{clade
  |1={{extinct}}[[Ophiacodontidae]] [[File:Archaeothyris BW.jpg|50px]]
  |2={{extinct}}[[Varanopidae]] [[File:Varanops brevirostris.jpg|50px]]}}
  |2={{clade
  |1={{extinct}}[[Caseasauria]] [[File:Ennatosaurus BW.jpg|50px]]
  |2={{clade
  |1={{extinct}}''[[Ianthodon schultzei]]''
  |2={{clade
  |1={{extinct}}[[Edaphosauridae]] [[File:Ianthasaurus BW.jpg|50px]]
  |label2=[[Sphenacodontia]]
  |2={{clade
  |1={{extinct}}''[[Haptodus garnettensis]]'' [[File:Haptodus BW.jpg|50px]]
  |2={{clade
  |1={{extinct}}''[[Pantelosaurus saxonicus]]''
  |2={{clade
  |label1={{extinct}}[[Sphenacodontidae]]
  |1={{clade
  |1={{extinct}}''[[Cutleria wilmarthi]]'' [[File:Cutleria wilmarthi reconstruction.jpg|50px]]
  |2={{clade
  |1={{extinct}}''[[Secodontosaurus obtusidens]]'' [[File:Secodontosaurus BW.jpg|50px]]
  |2={{clade
  |1={{extinct}}''[[Cryptovenator hirschbergeri]]''
  |2={{clade
  |1={{extinct}}'''''Dimetrodon''''' spp. [[File:Dimetrodon grandis.jpg|50px]]
  |2={{extinct}}''[[Sphenacodon]]'' spp. [[File:Sphenacodon222.jpg|50px]]}} }} }} }}
  |2=[[Therapsida]] (including mammals)[[File:Biarmosuchus.jpg|50px]]
}} }} }} }} }} }} }} }} }}

The below cladogram shows the relationships of a few ''Dimetrodon'' species, from Brink et al., (2015).<ref name="brink15">{{cite journal |doi=10.1139/cjes-2015-0100|title=Re-evaluation of the historic Canadian fossil ''Bathygnathus'' borealisfrom the Early Permian of Prince Edward Island|journal=Canadian Journal of Earth Sciences|volume=52|issue=12|pages=1109–1120|year=2015|last1=Brink|first1=Kirstin S.|last2=Maddin|first2=Hillary C.|last3=Evans|first3=David C.|last4=Reisz|first4=Robert R.|last5=Sues|first5=Hans-Dieter|bibcode=2015CaJES..52.1109B|doi-access=free}}</ref>

{{clade| style=font-size:85%; line-height:85%
|label1=[[Sphenacodontidae]]
|1={{clade
 |1=''[[Secodontosaurus]]''
 |2={{clade
  |1={{clade
  |1=''[[Sphenacodon]]''
  |2=''[[Ctenospondylus]]'' }}
  |2={{clade
  |1='''''Dimetrodon milleri'''''
  |2={{clade
  |1='''''Dimetrodon limbatus'''''
  |2={{clade
  |1='''''Dimetrodon borealis'''''
  |2='''''Dimetrodon grandis''''' }} }} }} }} }} }}

== Paleobiology ==

=== Function of neural spines ===
[[File:Dimetrodon8DB.jpg|thumb|''D. grandis'' warming up at sunrise]]
Paleontologists have proposed many ways in which the sail could have functioned in life. Some of the first to think about its purpose suggested that the sail may have served as camouflage among reeds while ''Dimetrodon'' waited for prey, or as an actual boat-like sail to catch the wind while the animal was in the water.<ref name=FKTW01>{{cite journal |last=Florides |first=G.A. |author2=Kalogirou, S.A. |author3=Tassou, S.A. |author4= Wrobel, L.  |year=2001 |title=Natural environment and thermal behaviour of ''Dimetrodon limbatus'' |journal=Journal of Thermal Biology |volume=26 |issue=1 |pages=15–20 |doi=10.1016/S0306-4565(00)00019-X|pmid=11070340 |bibcode=2001JTBio..26...15F }}</ref> Another is that the long neural spines could have stabilized the trunk by restricting up-and-down movement, which would allow for a more efficient side-to-side movement while walking.<ref name=RSNPL05 />

==== Thermoregulation ====
In 1940, [[Alfred Romer]] and [[Llewellyn Ivor Price]] proposed that the sail served a thermoregulatory function, allowing individuals to warm their bodies with the Sun. In the following years, many models were created to estimate the effectiveness of thermoregulation in ''Dimetrodon''. For example, in a 1973 article in the journal ''[[Nature (journal)|Nature]]'', paleontologists C.&nbsp;D.&nbsp;Bramwell and P.&nbsp;B.&nbsp;Fellgett estimated that it took a {{convert|200|kg|lb}} individual about one and a half hours for its body temperature to rise from {{convert|26|to|32|°C|°F}}.<ref name=BF73>{{cite journal |last=Bramwell |first=C.D. |author2=Fellgett, P.B. |year=1973 |title=Thermal regulation in sail lizards |journal=Nature |volume=242 |pages=203–205 |doi=10.1038/242203a0 |issue=5394|bibcode=1973Natur.242..203B |s2cid=4159825 }}</ref> In 1986, Steven C. Haack concluded that the warming was slower than previously thought and that the process probably took four hours. Using a model based on a variety of environmental factors and hypothesized physiological aspects of ''Dimetrodon'', Haack found that the sail allowed ''Dimetrodon'' to warm faster in the morning and reach a slightly higher body temperature during the day, but that it was ineffective in releasing excess heat and did not allow ''Dimetrodon'' to retain a higher body temperature at night.<ref name=HSC86>{{cite journal |last=Haack |first=S.C. |year=1986 |title=A thermal model of the sailback pelycosaur |journal=Paleobiology |volume=12 |issue=4 |pages=450–458|doi=10.1017/S009483730000316X |bibcode=1986Pbio...12..450H |s2cid=124339088 }}</ref> In 1999, a group of mechanical engineers created a computer model to analyze the ability of the sail to regulate body temperature during different seasons, and concluded that the sail was beneficial for capturing and releasing heat at all times in the year.<ref name=FWKT99>{{cite journal |last=Florides |first=G.A. |author2=Wrobel, L.C. |author3=Kalogirou, S.A. |author4= Tassou, S.A.  |year=1999 |title=A thermal model for reptiles and pelycosaurs |journal=Journal of Thermal Biology |volume=24 |issue=1 |pages=1–13 |doi=10.1016/S0306-4565(98)00032-1|bibcode=1999JTBio..24....1F }}</ref>
[[File:Dimetrodon milleri (1).jpg|thumb|left|The comparatively small ''D. milleri'']]
Most of these studies give two thermoregulatory roles for the sail of ''Dimetrodon'': one as a means of warming quickly in the morning, and another as a way to cool down when body temperature becomes high. ''Dimetrodon'' and all other Early Permian land vertebrates are assumed to have been cold-blooded or [[poikilotherm]]ic, relying on the sun to maintain a high body temperature. Because of its large size, ''Dimetrodon'' had high [[thermal inertia]], meaning that changes in body temperature occurred more slowly in it than in smaller-bodied animals. As temperatures rose in the mornings, the small-bodied prey of ''Dimetrodon'' could warm their bodies much faster than could something the size of ''Dimetrodon''. Many paleontologists including Haack have proposed that the sail of ''Dimetrodon'' may have allowed it to warm quickly in the morning in order to keep pace with its prey.<ref name=HSC86 /> The sail's large surface area also meant heat could dissipate quickly into the surroundings, useful if the animal needed to release excess heat produced by metabolism or absorbed from the sun. ''Dimetrodon'' may have angled its sail away from the sun to cool off or restricted blood flow to the sail to maintain heat at night.<ref name=FKTW01 />

In 1986, J.&nbsp;Scott Turner and C.&nbsp;Richard Tracy proposed that the evolution of a sail in ''Dimetrodon'' was related to the evolution of warm-bloodedness in mammal ancestors. They thought that the sail of ''Dimetrodon'' enabled it to be [[homeothermic]], maintaining a constant, albeit low, body temperature. Mammals are also homeothermic, although they differ from ''Dimetrodon'' in being [[endothermic]], controlling their body temperature internally through heightened metabolism. Turner and Tracy noted that early therapsids, a more advanced group of synapsids closely related to mammals, had long limbs which can release heat in a manner similar to that of the sail of ''Dimetrodon''. The homeothermy that developed in animals like ''Dimetrodon'' may have carried over to therapsids through a modification of body shape, which would eventually develop into the warm-bloodedness of mammals.<ref name=TT86>{{cite book |last=Turner |first=J.S. |author2=Tracy, C.R. |year=1986 |chapter=Body size, homeothermy and the control of heat exchange in mammal-like reptiles |title=The Ecology and Biology of Mammal-Like Reptiles |editor=Hotton, N. III. |editor2=MacLean, P.D. |editor3=Roth, J.J. |editor4=Roth, E.C. |publisher=Smithsonian Institution Press |location=Washington, D.C. |pages=185–194 |chapter-url=http://www.esf.edu/efb/turner/publication%20pdfs/mammal-like%20reptiles%201.pdf |access-date=2012-07-26 |archive-date=2016-04-12 |archive-url=https://web.archive.org/web/20160412174317/http://www.esf.edu/efb/turner/publication%20pdfs/mammal-like%20reptiles%201.pdf |url-status=dead }}</ref>
[[File:Sphenacodon ferox 2.jpg|thumb|''[[Sphenacodon]]'' had a low crest along its back]]
Recent studies on the sail of ''Dimetrodon'' and other sphenacodontids support Haack's 1986 contention that the sail was poorly adapted to releasing heat and maintaining a stable body temperature. The presence of sails in small-bodied species of ''Dimetrodon'' such as ''D.&nbsp;milleri'' and ''D.&nbsp;teutonis'' does not fit the idea that the sail's purpose was thermoregulation because smaller sails are less able to transfer heat and because small bodies can absorb and release heat easily on their own. Moreover, close relatives of ''Dimetrodon'' such as ''[[Sphenacodon]]'' have very low crests that would have been useless as thermoregulatory devices.<ref name=HRS10 /> The large sail of ''Dimetrodon'' is thought to have developed gradually from these smaller crests, meaning that over most of the sail's evolutionary history, thermoregulation could not have served an important function.<ref name=Tetal10>{{cite journal |last=Tomkins |first=J.L. |author2=LeBas, N.R. |author3=Witton, M.P. |author4=Martill, D.M. |author5=Humphries, S. |year=2010 |title=Positive allometry and the prehistory of sexual selection |journal=The American Naturalist |volume=176 |issue=2 |pages=141–148 |doi=10.1086/653001 |url=http://www.alternativetactics.org/PDF/Tomkins%20et%20al%20Am%20Nat%202010.pdf |pmid=20565262 |bibcode=2010ANat..176..141T |s2cid=36207 |url-status=dead |archive-url=https://web.archive.org/web/20160412174318/http://www.alternativetactics.org/PDF/Tomkins%20et%20al%20Am%20Nat%202010.pdf |archive-date=2016-04-12 }}</ref>

Although the function of its sail remains uncertain, ''Dimetrodon'' and other [[Sphenacodontidae|Sphenacodontids]] were likely to have been whole-body endotherms, characterised by a high energy metabolism ([[Warm-blooded|tachymetabolism]]) and probably a capacity for maintaining a high and stable body temperature. This conclusion was part of an [[amniote]]-wide study that found tachymetabolic endothermy to have been widespread throughout, and likely [[Plesiomorphy and symplesiomorphy|plesiomorphic]] to both [[Synapsid|synapsids]] and [[Sauropsida|sauropsids]]. For ''Dimetrodon'' the evidence was the endothermy-indicative size of the foramina through which blood was delivered to their long bones and the high blood pressure that would have been necessary to provide blood to the tops of the well-vascularised spines supporting the sail.<ref>{{Cite journal |last1=Grigg |first1=Gordon |last2=Nowack |first2=Julia |last3=Bicudo |first3=José Eduardo Pereira Wilken |last4=Bal |first4=Naresh Chandra |last5=Woodward |first5=Holly N. |last6=Seymour |first6=Roger S. |date=2022 |title=Whole-body endothermy: ancient, homologous and widespread among the ancestors of mammals, birds and crocodylians |journal=Biological Reviews |language=en |volume=97 |issue=2 |pages=766–801 |doi=10.1111/brv.12822 |pmid=34894040 |pmc=9300183 |hdl=2440/134060 |s2cid=245021195 |issn=1464-7931}}</ref>

Larger bodied specimens of ''Dimetrodon'' have larger sails relative to their size, an example of [[Allometric scaling|positive allometry]]. Positive allometry may benefit thermoregulation because it means that, as individuals get larger, surface area increases faster than mass. Larger-bodied animals generate a great deal of heat through metabolism, and the amount of heat that must be dissipated from the body surface is significantly greater than what must be dissipated by smaller-bodied animals. Effective heat dissipation can be predicted across many different animals with a single relationship between mass and surface area. However, a 2010 study of allometry in ''Dimetrodon'' found a different relationship between its sail and body mass: the actual scaling exponent of the sail was much larger than the exponent expected in an animal adapted to heat dissipation. The researchers concluded that the sail of ''Dimetrodon'' grew at a much faster rate than was necessary for thermoregulation, and suggested that [[sexual selection]] was the primary reason for its evolution.<ref name="Tetal10" />

==== Sexual selection ====
The allometric exponent for sail height is similar in magnitude to the scaling of interspecific antler length to shoulder height in [[Cervidae|cervids]]. Furthermore, as Bakker (1970) observed in the context of ''Dimetrodon'', many lizard species raise a dorsal ridge of skin during threat and courtship displays, and positively allometric, sexually dimorphic frills and dewlaps are present in extant lizards (Echelle et al. 1978; Christian et al. 1995). There is also evidence of sexual dimorphism both in the robustness of the skeleton and in the relative height of the spines of ''D. limbatus'' (Romer and Price 1940).<ref name=Tetal10 />

=== Sexual dimorphism ===
''Dimetrodon'' may have been [[sexually dimorphic]], meaning that males and females had slightly different body sizes. Some specimens of ''Dimetrodon'' have been hypothesized as males because they have thicker bones, larger sails, longer skulls, and more pronounced maxillary "steps" than others. Based on these differences, the mounted skeletons in the [[American Museum of Natural History]] (AMNH 4636) and the [[Field Museum of Natural History]] may be males and the skeletons in the [[Denver Museum of Nature and Science]] ([[Museum of Comparative Zoology|MCZ]] 1347) and the [[University of Michigan Museum of Natural History]] may be females.<ref name=RP40/>
{{multiple image
| align    = center
| image1   = Dimetrodon limbatus AMNH 4636.JPG
| width1   = {{#expr: (200 * 3072 / 2304) round 0}}
| caption1 = The mounted ''D. limbatus'' skeleton [[AMNH]] 4636 may represent the male type.
| image2   = Dimetrodon incisivus Exhibit Museum of Natural History.JPG
| width2   = {{#expr: (200 * 3264 / 2448) round 0}}
| caption2 = The ''D. incisivus'' skeleton in the [[University of Michigan Museum of Natural History]] may represent the female type.
}}

== Paleoecology ==
[[File:Dimetrodon track.jpg|thumb|Possible ''Dimetrodon'' footprint, [[Prehistoric Trackways National Monument]]]]
Fossils of ''Dimetrodon'' are known from the United States (Texas, Oklahoma, New Mexico, Arizona, Utah and Ohio), Canada ([[Prince Edward Island]]) and Germany, areas that were part of the supercontinent [[Euramerica]] during the Early Permian. Within the United States, almost all material attributed to ''Dimetrodon'' has come from three geological groups in north-central Texas and south-central Oklahoma: the [[Clear Fork Group]], the [[Wichita Group]], and the [[Pease River Group]].<ref name=LSRM09>{{cite book |last=Lucas |first=S.G. |author2=Spielmann, J.A. |author3=Rinehart, L.F. |author4=Martens, T. |year=2009 |title=''Dimetrodon'' (Amniota: Synapsida: Sphenacodontidae) from the Lower Permian Abo Formation, Socorro County, New Mexico |series=New Mexico Geological Society Guidebook |volume=60 |publisher=New Mexico Geological Society |pages=281–284 |url=http://nmgs.nmt.edu/publications/guidebooks/downloads/60/60_p0281_p0284.pdf |archive-date=2021-10-05 |access-date=2012-08-24 |archive-url=https://web.archive.org/web/20211005011423/https://nmgs.nmt.edu/publications/guidebooks/downloads/60/60_p0281_p0284.pdf |url-status=dead }}</ref><ref>Nelson, John W., Robert W. Hook, and Dan S. Chaney (2013). [https://books.google.com/books?id=5f4oCgAAQBAJ&q=pease&pg=PA286 Lithostratigraphy of the Lower Permian (Leonardian) Clear Fork Formation of North-Central Texas] from The Carboniferous-Permian Transition: Bulletin 60, ed. Spencer G. Lucas et al. New Mexico Museum of Natural History and Science, pg. 286-311. Retrieved December 28, 2017.</ref> Most fossil finds are part of lowland ecosystems which, during the Permian, would have been vast wetlands. In particular, the Red Beds of Texas is an area of great diversity of fossil [[tetrapod]]s, or four-limbed vertebrates. In addition to ''Dimetrodon'', the most common tetrapods in the Red Beds and throughout Early Permian deposits in the southwestern United States, are the amphibians ''[[Archeria (animal genus)|Archeria]]'', ''[[Diplocaulus]]'', ''[[Eryops]]'', and ''[[Trimerorhachis]]'', the [[reptiliomorph]] ''[[Seymouria]]'', the reptile ''[[Captorhinus]]'', and the synapsids ''[[Ophiacodon]]'' and ''[[Edaphosaurus]]''. These tetrapods made up a group of animals that paleontologist [[Everett C. Olson]] called the "Permo-Carboniferous chronofauna", a [[fauna]] that dominated the continental Euramerican ecosystem for several million years.<ref name=SRM00>{{cite journal |last=Sullivan |first=C. |author2=Reisz, R.R. |author3= May, W.J.  |year=2000 |title=Large dissorophoid skeletal elements from the Lower Permian Richards Spur fissures, Oklahoma, and their paleoecological implications |journal=Journal of Vertebrate Paleontology |volume=20 |issue=3|jstor=4524117 |pages=456–461 |doi=10.1671/0272-4634(2000)020[0456:LDSEFT]2.0.CO;2|s2cid=140709673 }}</ref> Based on the geology of deposits like the Red Beds, the fauna is thought to have inhabited a well-vegetated lowland [[River delta|deltaic ecosystem]].<ref name=OEC66>{{cite journal |last=Olson |first=E.C. |year=1966 |title=Community evolution and the origin of mammals |journal=Ecology |volume=47 |issue=2 |pages=291–302 |doi=10.2307/1933776|jstor=1933776 |bibcode=1966Ecol...47..291O }}</ref>

=== Food web ===
[[File:Dimetr eryopsDB.jpg|thumb|left|Restoration of ''D. grandis'' and the [[temnospondyl]] ''[[Eryops]]'', both found in the Red Beds of Texas]]
Olson made many inferences on the paleoecology of the [[Red Beds of Texas and Oklahoma|Texas Red beds]] and the role of ''Dimetrodon'' within its ecosystem. He proposed several main types of ecosystems in which the earliest tetrapods lived. ''Dimetrodon'' belonged to the most primitive ecosystem, which developed from aquatic food webs. In it, aquatic plants were the [[primary producer]]s and were largely fed upon by [[fish]] and aquatic invertebrates. Most land vertebrates fed on these aquatic primary consumers. ''Dimetrodon'' was probably the [[top predator]] of the Red Beds ecosystem, feeding on a variety of organisms such as the shark ''[[Xenacanthus]]''{{Citation needed|date=February 2018}}, the aquatic amphibians ''[[Trimerorhachis]]'' and ''[[Diplocaulus]]'', and the terrestrial tetrapods ''[[Seymouria]]'' and ''[[Trematops]]''. Insects are known from the Early Permian Red Beds and were probably involved to some degree in the same food web as ''Dimetrodon'', feeding small reptiles like ''[[Captorhinus]]''. The Red Beds assemblage also included some of the first large land-living herbivores like ''[[Edaphosaurus]]'' and ''[[Diadectes]]''. Feeding primarily on terrestrial plants, these herbivores did not derive their energy from aquatic food webs. According to Olson, the best modern analogue for the ecosystem ''Dimetrodon'' inhabited is the [[Everglades]].<ref name=OEC66 /> The exact lifestyle of ''Dimetrodon'' (amphibious to terrestrial) has long been controversial, but bone microanatomy supports a terrestrial lifestyle,<ref name=K08>{{cite journal |last=Kriloff |first=A. |author2=Germain, D. |author3= Canoville, A. |author4= Vincent, P. |author5= Sache, M. |author6= Laurin, M.  |year=2008 |title=Evolution of bone microanatomy of the tetrapod tibia and its use in palaeobiological inference |journal=Journal of Evolutionary Biology |volume=21 |issue=3 |pages=807–826 |doi=10.1111/j.1420-9101.2008.01512.x|pmid=18312321 |s2cid=6102313 |doi-access=free }}</ref> which implies that it would have fed mostly on land, on the banks, or in very shallow water. Evidence also exists for ''Dimetrodon'' preying on [[Aestivation|aestivating]] ''[[Diplocaulus]]'' during times of drought, with three partially eaten juvenile ''Diplocaulus'' in a burrow of eight bearing teeth marks from a ''Dimetrodon'' that unearthed and killed them.<ref>{{cite web | url=http://www.livescience.com/40842-finned-monster-chomped-heads-off-ancient-amphibians.html | title=Finned Monster Chomped Heads off Ancient Amphibians| website=[[Live Science]]| date=31 October 2013}}</ref>

The only species of ''Dimetrodon'' found outside the southwestern United States is ''D. teutonis'' from Germany. Its remains were found in the [[Tambach Formation]] in a fossil site called the Bromacker locality. The Bromacker's assemblage of Early Permian [[tetrapod]]s is unusual in that there are few large-bodied synapsids serving the role of top predators. ''D.&nbsp;teutonis'' is estimated to have been only {{convert|1.7|m|ft}} in length, too small to prey on the large [[diadectid]] herbivores that are abundant in the Bromacker assemblage. It more likely ate small vertebrates and insects. Only three fossils can be attributed to large predators, and they are thought to have been either large [[Varanopidae|varanopids]] or small [[Sphenacodontidae|sphenacodonts]], both of which could potentially prey on ''D.&nbsp;teutonis''. In contrast to the lowland [[River delta|deltaic]] Red Beds of Texas, the Bromacker deposits are thought to have represented an upland environment with no aquatic species. It is possible that large-bodied carnivores were not part of the Bromacker assemblage because they were dependent on large aquatic [[amphibian]]s for food.<ref name=BRMH01 />

==See also==
{{Portal|Paleontology}}
* {{annotated link|Secodontosaurus}}
* {{annotated link|Edaphosaurus}}
* [[Olson's Extinction]] — an [[extinction event]] that wiped out most of the [[pelycosaurian]] synapsids, including ''Dimetrodon''

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

== External links ==
{{Sister project links|display=''Dimetrodon''|d=Q130869|species=Dimetrodon|n=no|b=no|v=no|voy=no|m=no|mw=no|s=no|q=no|wikt=dimetrodon|c=Category:Dimetrodon}}
* [http://palaeos.com/vertebrates/synapsida/sphenacodontidae.html#Dimetrodon ''Dimetrodon''] [[Palaeos]] page on ''Dimetrodon''
* [https://ucmp.berkeley.edu/synapsids/pelycosaurs.html Introduction to the Pelycosaurs] [[University of California Museum of Paleontology]] webpage on early synapsids, including ''Dimetrodon''
* {{cite web |title=Dimetrodon: Our Most Unlikely Ancestor |date=August 21, 2017 |work=[[PBS Eons]] |url=https://www.youtube.com/watch?v=SR3OOP9mImI |via=[[YouTube]] }}

{{Basal synapsids|H.}}
{{Taxonbar|from=Q130869}}
{{Authority control}}
[[Category:Permian Germany]]
[[Category:Sphenacodontidae]]
[[Category:Cisuralian synapsids of Europe]]
[[Category:Cisuralian synapsids of North America]]
[[Category:Prehistoric synapsid genera]]
[[Category:Transitional fossils]]
[[Category:Taxa named by Edward Drinker Cope]]
[[Category:Fossil taxa described in 1878]]
[[Category:Cisuralian genus first appearances]]
[[Category:Cisuralian genus extinctions]]
[[Category:Apex predators]]