Editing Dimetrodon (section)

== 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"
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| 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]].