Dimetrodon

File:Dimetrodon NT2 small.jpg

Dimetrodon was a 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.{{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 }} 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}}.{{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 }} The smallest known species D. teutonis was about {{convert|60|cm|in|abbr=on}} long and weighed {{convert|14|kg|lb}}.{{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 }} 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.{{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 }}{{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}} Although some Dimetrodon species could grow very large, many juvenile specimens are known.{{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}}

A 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 lack openings or have 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 four-limbed land-dwelling vertebrates to mammals.

The skull of Dimetrodon is tall and compressed 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 synapsids.{{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 }} 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, a gap in the tooth row. Its skull was more heavily built than a dinosaur's skull.

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.{{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 }} One or two pairs of caniniforms (large, pointed, 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 bones. Small teeth are present around the maxillary "step" and behind the caniforms, becoming smaller further back in the jaw.{{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 }}

File:Dimetrodon grandis Exhibit Museum of Natural History.JPG

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 sphenacodontids, which helps to distinguish them from other early synapsids. As in many other early synapsids, the teeth of most Dimetrodon species are serrated at their edges. The serrations of Dimetrodon teeth were so fine that they resembled tiny cracks.Abler, W.L. 2001. A kerf-and-drill model of tyrannosaur tooth serrations. p. 84-89. In: Mesozoic Vertebrate Life. Ed.s Tanke, D. H., Carpenter, K., Skrepnick, M. W. Indiana University Press. The dinosaur Albertosaurus had similarly crack-like serrations, but, at the base of each serration was a round 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. The teeth of D. teutonis lack serrations, but still have sharp edges.

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 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.{{Cite web|url=https://www.nationalgeographic.com/environment/topic/great-energy-challenge|title=Great Energy Challenge|website=Environment}} The thickness and mass of the teeth of Dimetrodon may also have been an adaptation for increasing dental longevity.{{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 }}

On the inner surface of the nasal section of the skull are ridges called 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 between those of early land vertebrates and mammals.{{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 }}

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.{{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}}

File:PSM V73 D566 Restoration of dimetrodon.png

The tail of Dimetrodon makes up a large portion of its total body length and includes around 50 caudal vertebrae. 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.{{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 }}

File:Dimetrodon Royal Tyrrell Museum.jpg]]

The sail of Dimetrodon is formed by elongated neural spines projecting from the vertebrae. Each spine varies in cross-sectional shape from its base to its tip in what is known as "dimetrodont" differentiation.{{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 }} 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.{{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 }} 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.{{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 }} 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 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.{{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 }}

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 vessels. 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.{{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}} 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.

File:Dimetrodon grandis 3D Model Reconstruction.png

Scaly body impressions that likely were made by Dimetrodon teutonis were described in 2025 from the Early Permian Tambach Formation 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.{{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 }}[https://www.museumfuernaturkunde.berlin/en/museum/media/press/reptile-skin-and-sociability-among-early-mammalian-ancestors Reptile skin and sociability among early mammalian ancestors] Press release 5-23-2025. Museum für Naturkunde Berlin The fossilized bodies of the varanopid Ascendonanus with preserved soft tissues from the Early Permian of Germany also indicate some early synapsids may have had squamate-like scales.{{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 }} However, the taxonomic placement of varanopids has been debated between synapsids or closer to diapsid reptiles.{{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}}{{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}} 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.

File: Dimetrodon borealis (Bathygnathus borealis) jaw - Redpath Museum - McGill University - Montreal, Canada - DSC08069.jpg, 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.{{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"}} These fossils were purchased by John William Johnson, a Canadian geologist, and then described by Joseph Leidy in 1854 as the mandible of Bathygnathus borealis, a large carnivore related to Thecodontosaurus,{{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"}} although it was later reclassified as a species of Dimetrodon in 2015, as Dimetrodon borealis. 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,{{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}} which was approved in 2019.{{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}}

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 tetrapods from several collectors who had been exploring a group of rocks in Texas called the Red Beds. Among these collectors were Swiss naturalist Jacob Boll, Texas geologist W. F. Cummins, and amateur paleontologist Charles Hazelius Sternberg.{{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 }} 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.{{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}} 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.{{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}}

The first description of a Dimetrodon fossil came a year earlier, though, when Cope named the species Clepsydrops limbatus from the Texas Red Beds.{{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}} (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 19th and early 20th centuries.) C. 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 basilisk lizard (Basilicus). Sails were not preserved in the specimens of D.  incisive and D. gigas that Cope described in his 1878 paper, but elongated spines were present in the D. rectiformis specimen he described. 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 habits, as in the existing genus Basilicus, where a similar crest exists."

File:DimetrodonKnight.jpg (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. 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. Most of the species names considered valid by Romer and Price are still used today.

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{{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 |journal=Contributions in Science (Los Angeles County Museum of Natural History) |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 }} and another was described from Arizona in 1969.{{cite journal |last=Vaughn |first=P.P. |year=1969 |title=Early Permian vertebrates from southern New Mexico and their paleozoogeographic significance |journal=Contributions in Science (Los Angeles County Museum of Natural History) |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 }} In 1975, Olson reported Dimetrodon material from the Washington Formation of Ohio, which has been given a tentative assignment of D. cf. limbatus.{{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}}{{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 }}{{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 }} A new species of Dimetrodon called D. occidentalis (meaning "western Dimetrodon") was named in 1977 from New Mexico.{{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}} The specimens found in Utah and Arizona probably also belong to D. occidentalis.{{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}}

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.{{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 }}

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.

File:Dimetrodon species2DB15.jpg

Twenty species of Dimetrodon have been named since the genus was first described in 1878. Many have been synonymized with older named species, and some now belong to different genera.

File:Dimetr incis22DB.jpg]]

Dimetrodon limbatus was first described by Edward Drinker Cope in 1877 as Clepsydrops limbatus. (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, 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.{{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 }} 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.

The first use of the name Dimetrodon came in 1878 when Cope named the species Dimetrodon incisivus along with Dimetrodon rectiformis and Dimetrodon gigas.

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. In 1907, paleontologist E. C. Case moved D. rectiformis into the species D. incisivus. D. incisivus was later synonymous with the type species Dimetrodon limbatus, making D. rectiformis a synonym of D. limbatus.

Described in 1881 on the basis of upper jaw bones, Dimetrodon semiradicatus was the last species named by Cope.{{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}} In 1907, E. C. Case synonymized D. semiradicatus with D. incisivus based on similarities in the shape of the teeth and skull bones. D. incisivus' and D. semiradicatus are now considered synonyms of D. limbatus.

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.{{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 }}

File:Dimetrodon grandis in "high walk" pose.png

Paleontologist E. C. Case named a new species of sail-backed synapsid, Theropleura grandis, in 1907. In 1940, Alfred Romer and Llewellyn Ivor Price reassigned Theropleura grandis to Dimetrodon, erecting the species D. grandis.

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. Case reclassified C. gigas as a new species of Dimetrodon in 1907. Case also described a very well preserved skull of Dimetrodon in 1904, attributing it to the species Dimetrodon gigas.{{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 }} In 1919, Charles W. Gilmore attributed a nearly complete specimen of Dimetrodon to D. gigas.{{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 }} Dimetrodon gigas is now recognized as a synonym of D. grandis.{{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}}

File:Dimetrodon gigashomog DB.jpg

Dimetrodon giganhomogenes was named by E. C. Case in 1907 and is still considered a valid species of Dimetrodon.

Dimetrodon macrospondylus was first described by Cope in 1884 as Clepsydrops macrospondylus. In 1907, Case reclassified it as Dimetrodon macrospondylus.

Dimetrodon platycentrus was first described by Case in his 1907 monograph. It is now considered a synonym of Dimetrodon macrospondylus.

File:D natalisDB.jpg

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.{{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}}

Dimetrodon booneorum was first described by Alfred Romer in 1937 on the basis of remains from Texas.

Dimetrodon kempae was named by Romer in 1937, in the same paper as D. booneorum, D. loomisi, and D. milleri. 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. 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.

File:Dimetrodon loomisi.jpg

Dimetrodon loomisi was first described by Alfred Romer in 1937 along with D. booneorum, D. kempae, and D. milleri. Remains have been found in Texas and Oklahoma.

File:Dimetrodon milleri.jpg

Dimetrodon milleri was described by Romer in 1937. It is one of the smallest species of Dimetrodon in North America and may be closely related to D. occidentalis, another small-bodied species. 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.

File:Dimetrodon angelensisDB.jpg

Dimetrodon angelensis was named by paleontologist Everett C. Olson in 1962.{{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 }} Specimens of the species were reported from the San Angelo Formation of Texas.{{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 }} It is also the largest species of Dimetrodon.

Dimetrodon occidentalis was named in 1977 from New Mexico. 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.

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.{{Cite web |last=Henrici |first=Amy |title=The Bromacker Fossil Project Part XI: Dimetrodon teutonis, an apex predator |url=https://carnegiemnh.org/the-bromacker-fossil-project-part-xi-dimetrodon-teutonis-an-apex-predator/|access-date=2 June 2025 |website=carnegiemnh.org|publisher=Carnegie Museum of Natural History}}

In 1878, Cope published a paper called "The Theromorphous Reptilia" in which he described Dimetrodon cruciger.{{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 }} D. cruciger was distinguished by the small projections that extended from either side of each neural spine like the branches of a tree.{{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}} 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.{{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 }} 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.{{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}}

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. 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. Williston did not consider his specimen to belong to Dimetrodon but instead classified it as an ophiacodontid.{{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}} Romer and Price assigned Case and Williston's specimens to a newly erected genus and species, Secodontosaurus longiramus, that was closely related to Dimetrodon.{{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 }}

{{See also|Synapsid#Linnaean and cladistic classifications}}

Dimetrodon is an early member of a group called synapsids, which include mammals and many of their extinct relatives, though it is not an ancestor of any mammal (which appeared millions of years laterThe mammals appear). It is often mistaken for a dinosaur in popular culture, despite having become extinct some 40 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 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.

File:Dimetrodon skeleton.JPG]]

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 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.

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. 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 varanopids within Sphenacodontia, considering them to be more primitive than other sphenacodonts like Dimetrodon.{{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 }} 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.{{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}} Dimetrodon is still considered a sphenacodont under this phylogeny, but varanodontids are now considered more 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 Therapsida 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 (2012).

{{clade| style=font-size:85%;line-height:85%

|label1=Amniota

|1={{clade

|1=Sauropsida (including dinosaurs, living reptiles and birds) 50px

|label2=Synapsida

|2={{clade

|1={{clade

|1={{extinct}}Ophiacodontidae 50px

|2={{extinct}}Varanopidae 50px}}

|2={{clade

|1={{extinct}}Caseasauria 50px

|2={{clade

|1={{extinct}}Ianthodon schultzei

|2={{clade

|1={{extinct}}Edaphosauridae 50px

|label2=Sphenacodontia

|2={{clade

|1={{extinct}}Haptodus garnettensis 50px

|2={{clade

|1={{extinct}}Pantelosaurus saxonicus

|2={{clade

|label1={{extinct}}Sphenacodontidae

|1={{clade

|1={{extinct}}Cutleria wilmarthi 50px

|2={{clade

|1={{extinct}}Secodontosaurus obtusidens 50px

|2={{clade

|1={{extinct}}Cryptovenator hirschbergeri

|2={{clade

|1={{extinct}}Dimetrodon spp. 50px

|2={{extinct}}Sphenacodon spp. 50px}} }} }} }}

|2=Therapsida (including mammals)50px

}} }} }} }} }} }} }} }} }}

The below cladogram shows the relationships of a few Dimetrodon species, from Brink et al., (2015).{{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}}

{{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 }} }} }} }} }} }}

File:Dimetrodon8DB.jpg

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.{{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 |hdl=20.500.14279/1658 |hdl-access=free }} 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.

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, paleontologists C. D. Bramwell and P. B. 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}}.{{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 }} 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.{{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 }} 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.{{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 }}

File:Dimetrodon milleri (1).jpg

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 poikilothermic, 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. 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.

In 1986, J. Scott Turner and C. 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.{{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 }}

File:Sphenacodon ferox 2.jpg 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. milleri and D. 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. 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.{{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 }}

Although the function of its sail remains uncertain, Dimetrodon and other Sphenacodontids were likely to have been whole-body endotherms, characterised by a high energy metabolism (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 plesiomorphic to both synapsids and 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.{{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}}

Larger bodied specimens of Dimetrodon have larger sails relative to their size, an example of 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.

The allometric exponent for sail height is similar in magnitude to the scaling of interspecific antler length to shoulder height in 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).

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 (MCZ 1347) and the University of Michigan Museum of Natural History may be females.

{{multiple image

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| image1 = Dimetrodon limbatus AMNH 4636.JPG

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| caption1 = The mounted D. limbatus skeleton AMNH 4636 may represent the male type.

| image2 = Dimetrodon incisivus Exhibit Museum of Natural History.JPG

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| caption2 = The D. incisivus skeleton in the University of Michigan Museum of Natural History may represent the female type.

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File:Permian Reptiles and Amphibians by Knight.jpg (center), Casea (left) and the amphibian Diplocaulus (right) in a Permian wetland scene from Texas]]

File:Dimetrodon track.jpg]]

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.{{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 }}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. 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 tetrapods, 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, 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.{{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 }} Based on the geology of deposits like the Red Beds, the fauna is thought to have inhabited a well-vegetated lowland deltaic ecosystem.{{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 }}

File:Dimetr eryopsDB.jpg Eryops, both found in the Red Beds of Texas]]

Olson made many inferences on the paleoecology of the 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 producers 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,{{Cite web |last=Anonymous |orig-date=26 October 2007 |title=Xenacanthus versus Dimetrodon: a battle of epic proportions (with Robert Bakker)|url=https://hmnspaleo.blogspot.com/2007/10/xenacanthus-vs-dimetrodon-battle-of.html |access-date=2 June 2025 |website=Prehistoric CSI |publisher=Houston Museum of Natural Science}}{{Cite web |last1=Zoehfeld |first1=K. Weidner |last2=Mossbrucker |first2=Matthew T. |last3=Bakker |first3=Robert T. |last4=Flis |first4=Chris J.|year=2014 |title=Dimetrodon feeding on sharks and sharks feeding on Dimetrodon: Texas Early Permian food webs based on aquatic predation|url=https://gsa.confex.com/gsa/2014AM/webprogram/Paper248167.html|access-date=2 June 2025 |work=2014 GSA Annual Meeting Program Abstracts |pages=Paper 133-7 }} 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. The exact lifestyle of Dimetrodon (amphibious to terrestrial) has long been controversial, but bone microanatomy supports a terrestrial lifestyle,{{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 }} 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 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.{{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}}

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 tetrapods is unusual in that there are few large-bodied synapsids serving the role of top predators. D. 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 varanopids or small sphenacodonts, both of which could potentially prey on D. teutonis. In contrast to the lowland 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 amphibians for food.

{{Portal|Paleontology}}

• {{annotated link|Secodontosaurus}}
• {{annotated link|Edaphosaurus}}
• Olson's Extinction — an extinction event that wiped out most of the pelycosaurian synapsids, including Dimetrodon

{{Reflist|30em}}

{{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 }}

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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