| Dimetrodon | |
|---|---|
 | |
| Skeleton ofD. limbatus,Staatliches Museum für Naturkunde Karlsruhe | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Synapsida |
| Family: | †Sphenacodontidae |
| Subfamily: | †Sphenacodontinae |
| Genus: | †Dimetrodon Cope,1878 (conserved name) |
| Type species | |
| †Dimetrodon limbatus Cope, 1877 | |
| Species | |
Seebelow | |
| Synonyms | |
Genus synonymy
Species synonymy
| |
Dimetrodon (/daɪˈmiːtrəˌdɒn/[1] or/daɪˈmɛtrəˌdɒn/ ⓘ;[2]lit. 'two measures of teeth') is an extinctgenus ofsphenacodontidsynapsid that lived during theCisuralian (Early Permian)epoch of thePermian period, around 295–272 million years ago.[3][4][5] With most species measuring 1.7–4.6 m (5.6–15.1 ft) long and weighing 28–250 kg (62–551 lb), the most prominent feature ofDimetrodon is the largeneural spine sail on its back formed by elongated spines extending from thevertebrae. It was an obligatequadruped (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 theSouthwestern United States, the majority of these coming from a geological deposit called theRed Beds of Texas and Oklahoma. More recently, its fossils have also been found inGermany and over a dozen species have been named since thegenus was first erected in 1878.
Dimetrodon is often mistaken for adinosaur or portrayed as a contemporary of dinosaurs inpopular culture, but it became extinct by the middlePermian, some 40 million years before the appearance of dinosaurs.[6][7] Althoughreptile-like in appearance and physiology,Dimetrodon is much more closely related tomammals, as it belongs to the closestsisterfamily totherapsids, the latter of which contains the direct ancestor of mammals.[4]Dimetrodon is traditionally assigned to theparaphyletic group "pelycosaurs", a term now considered obsolete and replaced by terms such as "primitive synapsids" or "basal synapsids"; another name "mammal-like reptiles" is also used traditionally but incorrectly for non-mammalian synapsids[4] due to some of the features shared with modern mammals such astooth specialization andendothermy, but that term is now also defunct. TheDimetrodonskull 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 belong to the cladeSauropsida, which had diverged from the synapsids by theLate Carboniferous.
Dimetrodon was probably one of theapex predators of the Cisuralian ecosystems, feeding on fish andtetrapods, including reptiles andamphibians. SmallerDimetrodon species may have had differentecological roles. The sail ofDimetrodon may have been used to stabilize its spine or to heat and cool its body as a form ofthermoregulation.[8] 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 incourtship display, including threatening away rivals or showing off to potential mates.[9][10]
Dimetrodon was aquadrupedal, 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.[11] MostDimetrodon species ranged in length from 1.7 to 4.6 m (6 to 15 ft) and are estimated to have weighed between 28 and 250 kg (60 and 550 lb).[12] The smallest known speciesD. teutonis was about 60 cm (24 in) long and weighed 14 kilograms (31 lb).[12][13] The larger species ofDimetrodon were among the largest predators of the Early Permian, although the closely relatedTappenosaurus, known from skeletal fragments in slightly younger rocks, may have been even larger at an estimated 5.5 metres (18 ft) long.[14][15] Although someDimetrodon species could grow very large, many juvenile specimens are known.[16]
 |  |
 |  |
Asingle large opening on either side of the back of the skull linksDimetrodon to mammals and distinguishes it from most of the earliest sauropsids, which eitherlack openings or havetwo openings. Features such as ridges on the inside of thenasal cavity and a ridge at the back of the lower jaw are thought to be part of an evolutionary progression from earlyfour-limbed land-dwelling vertebrates tomammals.
The skull ofDimetrodon is tall and compressedlaterally, or side-to-side. The eye sockets are positioned high and far back in the skull. Behind each eye socket on each side is a single hole called aninfratemporal fenestra. An additional hole in the skull, thepineal foramen (or "third eye") along the midline between theparietal bones, can be seen when viewed from above. The back of the skull (theocciput) is oriented at a slight upward angle, a feature that it shares with all other earlysynapsids.[17] 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 thepremaxilla bone, is raised above the part of the jaw formed by themaxilla bone to form a maxillary "step". Within this step is adiastema, a gap in the tooth row. Itsskull was more heavily built than a dinosaur's skull.
The size of the teeth varies greatly along the length of the jaws, lendingDimetrodon its name, which means "two measures of tooth" in reference to sets of small and large teeth.[18] 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 anddentary bones. Small teeth are present around the maxillary "step" and behind the caniforms, becoming smaller further back in the jaw.[19]
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 toDimetrodon and other closely relatedsphenacodontids, which helps to distinguish them from other early synapsids.[13] As in many other earlysynapsids, the teeth of mostDimetrodon species are serrated at their edges.[13] The serrations ofDimetrodon teeth were so fine that they resembled tiny cracks.[20] The dinosaurAlbertosaurus had similarly crack-like serrations, but, at the base of each serration was a roundvoid, which would have functioned to distribute force over a largersurface area and prevent the stresses of feeding from causing the crack to spread through the tooth. UnlikeAlbertosaurus,Dimetrodon teeth lacked adaptations that would stop cracks from forming at their serrations.[20] The teeth ofD. teutonis lack serrations, but still have sharp edges.[13]
A 2014 study shows thatDimetrodon was in an arms race against its prey.[21] The smaller species,D. milleri, had no tooth serrations because it ate small prey. As prey grew larger, severalDimetrodon 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 onSecodontosaurus). The second-largest species,D. grandis, has denticle serrations similar to those of sharks andtheropod dinosaurs, making its teeth even more specialized for slicing through flesh. AsDimetrodon's prey grew larger, the various species responded by growing to larger sizes and developing ever-sharper teeth.[22] The thickness and mass of the teeth ofDimetrodon may also have been an adaptation for increasing dental longevity.[23]
On the inner surface of the nasal section of the skull are ridges callednasoturbinals, which may have supported cartilage that increased the area of theolfactory 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 ofDimetrodon istransitional between those of early landvertebrates and mammals.[24]
Another transitional feature ofDimetrodon is a ridge in the back of the jaw called the reflected lamina, which is found on thearticular bone, which connects to thequadrate 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 themalleus bone of themiddle ear. The reflected lamina became part of a ring called the tympanic annulus that supports theear drum in all living mammals.[25]
The tail ofDimetrodon makes up a large portion of its total body length and includes around 50caudal vertebrae. Tails were missing or incomplete in the first described skeletons ofDimetrodon. 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 thatDimetrodon had a very short tail. A largely complete tail ofDimetrodon was not described until 1927.[26]
The sail ofDimetrodon is formed by elongatedneural 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.[27] 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.[28] A cross-section of the spine of one specimen ofDimetrodon 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.[29] 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 orproximal portion of the spine has a rough surface that would have served as an anchoring point for theepaxial muscles of the back and also has a network of connective tissues calledSharpey's fibers that indicate it was embedded within the body. Higher up on thedistal (outer) portion of the spine, the bone surface is smoother. Theperiosteum, a layer of tissue surrounding the bone, is covered in small grooves that presumably supported the blood vessels that vascularized the sail.[30]
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 ofDimetrodon preserve deformed areas of the neural spines that appear to be healed-over fractures. Thecortical bone that grew over these breaks is highly vascularized, suggesting that soft tissue must have been present on the sail to supply the site withblood vessels.[28] Layeredlamellar 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.[31] In many specimens ofD. 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.[27]
Scaly body impressions that likely were made byDimetrodon teutonis were described in 2025 from the Early PermianTambach Formation Bromacker site in Germany. Given the ichnogenus nameBromackerichnus, 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.[32][33] Some synapsid groups later developed bare, glandular skin, as indicated by the fossils of the dinocephalian therapsidEstemmenosuchus 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 ofmammals.Ascendonanus from the Early Permian of Germany was found with preserved soft tissues showing squamate-like scales and was initially classified as avaranopid synapsid.[34] However, the taxonomic placement of varanopids has been debated between synapsids or closer todiapsid reptiles.[35][36] A recent study removedAscendonanus from the varanopids (considered synapsids by the researchers) as a member of a separate clade in the Neoreptilia.[37]
_jaw_-_Redpath_Museum_-_McGill_University_-_Montreal%2c_Canada_-_DSC08069.jpg)
The earliest discovery ofDimetrodon fossils were of amaxilla recovered in 1845 by a man named Donald McLeod, living in the British colony ofPrince Edward Island.[38] These fossils were purchased by John William Johnson, a Canadian geologist, and then described byJoseph Leidy in 1854 as themandible ofBathygnathus borealis, a largecarnivore related toThecodontosaurus,[39] although it was later reclassified as a species ofDimetrodon in 2015, asDimetrodon borealis.[40] AlthoughBathygnathus was named first, a petition to conserve the genusDimetrodon and suppress the genusBathygnathus was submitted to theInternational Commission on Zoological Nomenclature (ICZN) in 2015,[41] which was approved in 2019.[42]
Fossils now attributed toDimetrodon were first studied by American paleontologistEdward Drinker Cope in the 1870s. Cope had obtained the fossils along with those of many other Permiantetrapods from several collectors who had been exploring a group of rocks in Texas called theRed Beds. Among these collectors were Swiss naturalistJacob Boll, Texas geologistW. F. Cummins, and amateur paleontologistCharles Hazelius Sternberg.[43] Most of Cope's specimens went to theAmerican Museum of Natural History or to theUniversity of Chicago's Walker Museum (most of the Walker fossil collection is now housed in theField Museum of Natural History).
Sternberg sent some of his own specimens to German paleontologistFerdinand Broili atMunich University, although Broili was not as prolific as Cope in describing specimens. Cope's rivalOthniel Charles Marsh also collected some bones ofDimetrodon, which he sent to the Walker Museum.[44] The first use of the nameDimetrodon came in 1878 when Cope named the speciesDimetrodon incisivus,Dimetrodon rectiformis, andDimetrodon gigas in the scientific journalProceedings of the American Philosophical Society.[45]
The first description of aDimetrodon fossil came a year earlier, though, when Cope named the speciesClepsydrops limbatus from the Texas Red Beds.[46] (The nameClepsydrops was first coined by Cope in 1875 forsphenacodontid remains fromVermilion County, Illinois, and was later employed for many sphenacontid specimens from Texas; many new species of sphenacodontids from Texas were assigned to eitherClepsydrops orDimetrodon in the late 19th and early 20th centuries.)C. limbatus was reclassified as a species ofDimetrodon in 1940, meaning that Cope's 1877 paper was the first record ofDimetrodon.
Cope was the first to describe a sail-backed synapsid with the naming ofC. natalis in his 1878 paper, although he called the sail a fin and compared it to the crests of the modernbasilisk lizard (Basilicus). Sails were not preserved in the specimens ofD. incisive andD. gigas that Cope described in his 1878 paper, but elongated spines were present in theD. rectiformis specimen he described.[45] 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 demonstratearboreal habits, as in the existing genusBasilicus, where a similar crest exists."[19]
In the first few decades of the 20th century, American paleontologistsE. C. Case authored many studies onDimetrodon and described several new species. He received funding from theCarnegie Institution for his study of manyDimetrodon specimens in the collections of theAmerican Museum of Natural History and several other museums.[44] 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, paleontologistAlfred Romer restudied manyDimetrodon specimens and named several new species. In 1940, Romer coauthored a large study withLlewellyn Ivor Price called "Review of the Pelycosauria" in which the species ofDimetrodon named by Cope and Case were reassessed.[47] Most of the species names considered valid by Romer and Price are still used today.[30]
In the decades following Romer and Price's monograph, manyDimetrodon specimens were described from localities outsideTexas andOklahoma. The first was described from theFour Corners region of Utah in 1966[48] and another was described from Arizona in 1969.[49] In 1975, Olson reportedDimetrodon material from theWashington Formation of Ohio, which has been given a tentative assignment ofD. cf.limbatus.[50][51][52] A new species ofDimetrodon calledD. occidentalis (meaning "westernDimetrodon") was named in 1977 from New Mexico.[53] The specimens found in Utah and Arizona probably also belong toD. occidentalis.[54]
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, isolatingDimetrodon to a small region of North America, while a smaller sphenacodontid calledSphenacodon 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 ofDimetrodon.[53][55]
In 2001, a new species ofDimetrodon calledD. teutonis was described from the Lower Permian Bromacker locality at the Thuringian Forest of Germany, extending the geographic range ofDimetrodon outside North America for the first time.[12]
Twentyspecies ofDimetrodon have been named since thegenus was first described in 1878. Many have beensynonymized with older named species, and some now belong to different genera.
| Species | Authority | Location | Status | Synonyms | Images |
|---|---|---|---|---|---|
| Dimetrodon angelensis | Olson, 1962 |
| Valid |  | |
| Dimetrodon borealis | Leidy, 1854 |
| Valid | Previously known as thedinosaurBathygnathus borealis |  |
| Dimetrodon booneorum | Romer, 1937 |
| Valid | ||
| Dimetrodon dollovianus | Case, 1907 |
| Valid | Embolophorus dollovianus Cope, 1888 | |
| Dimetrodon gigahomogenes | Case, 1907 |
| Valid |  | |
| Dimetrodon grandis | Romer and Price, 1940 |
| Valid | Clepsydrops gigas Cope, 1878 Dimetrodon gigas Cope, 1878 Theropleura grandis Case, 1907 Bathyglyptus theodori Case, 1911 Dimetrodon maximus Romer 1936 |  |
| Dimetrodon kempae | Romer, 1937 |
| Possiblenomen dubium | ||
| Dimetrodon limbatus | Romer and Price, 1940 |
| Valid | Clepsydrops limbatus Cope, 1877 Dimetrodon incisivus Cope, 1878 Dimetrodon rectiformis Cope, 1878 Dimetrodon semiradicatus Cope, 1881 | |
| Dimetrodon loomisi | Romer, 1937 |
| Valid |  | |
| Dimetrodon macrospondylus | Case, 1907 |
| Valid | Clepsydrops macrospondylus Cope, 1884 Dimetrodon platycentrus Case, 1907 | |
| Dimetrodon milleri | Romer, 1937 |
| Valid |  | |
| Dimetrodon natalis | Romer, 1936 |
| Valid | Clepsydrops natalis Cope, 1878 |  |
| Dimetrodon occidentalis | Berman, 1977 |
| Valid | ||
| Dimetrodon teutonis | Bermanet al., 2001 |
| Valid |
Dimetrodon limbatus was first described by Edward Drinker Cope in 1877 asClepsydrops limbatus.[46] (The nameClepsydrops was first coined by Cope in 1875 for sphenacodontid remains fromVermilion County, Illinois, and was later employed for many sphenacontid specimens from Texas; many new species of sphenacodontids from Texas were assigned to eitherClepsydrops orDimetrodon in the late nineteenth and early twentieth centuries.) Based on a specimen from theRed Beds of Texas, it was the first known sail-backed synapsid. In 1940, paleontologistsAlfred Romer andLlewellyn Ivor Price reassignedC. limbatus to the genusDimetrodon, makingD. limbatus thetype species ofDimetrodon.[47] Remains tentatively assigned to this species are also known fromWashington County, Ohio, which correspond to a relatively large individual. These remains are slightly older than others assigned toD. limbatus from the west, although potentialD. limbatus remains from New Mexico may be concurrent with it.[52]
The first use of the nameDimetrodon came in 1878 when Cope named the speciesDimetrodon incisivus along withDimetrodon rectiformis andDimetrodon gigas.[45]
Dimetrodon rectiformis was named alongsideDimetrodon incisivus in Cope's 1878 paper, and was the only one of the three named species to preserve elongated neural spines.[45] In 1907, paleontologistE. C. Case movedD. rectiformis into the speciesD. incisivus.[44]D. incisivus was later synonymous with the type speciesDimetrodon limbatus, makingD. rectiformis a synonym ofD. limbatus.[30]
Described in 1881 on the basis of upper jaw bones,Dimetrodon semiradicatus was the last species named by Cope.[56] In 1907, E. C. Case synonymizedD. semiradicatus withD. incisivus based on similarities in the shape of the teeth and skull bones.[44]D. incisivus' andD. semiradicatus are now considered synonyms ofD. limbatus.[30]
Dimetrodon dollovianus was first described by Edward Drinker Cope in 1888 asEmbolophorus dollovianus. In 1903, E. C. Case published a lengthy description ofE. dollovianus, which he later referred toDimetrodon.[57]
Paleontologist E. C. Case named a new species of sail-backed synapsid,Theropleura grandis, in 1907.[44] In 1940, Alfred Romer and Llewellyn Ivor Price reassignedTheropleura grandis toDimetrodon, erecting the speciesD. grandis.[47]
In his 1878 paper on fossils from Texas, Cope namedClepsydrops gigas along with the first named species ofDimetrodon,D. limbatus,D. incisivus, andD. rectiformis.[45] Case reclassifiedC. gigas as a new species ofDimetrodon in 1907.[44] Case also described a very well preserved skull ofDimetrodon in 1904, attributing it to the speciesDimetrodon gigas.[58] In 1919,Charles W. Gilmore attributed a nearly complete specimen ofDimetrodon toD. gigas.[59]Dimetrodon gigas is now recognized as a synonym ofD. grandis.[60]
Dimetrodon giganhomogenes was named by E. C. Case in 1907 and is still considered a valid species ofDimetrodon.[44][30]
Dimetrodon macrospondylus was first described by Cope in 1884 asClepsydrops macrospondylus. In 1907, Case reclassified it asDimetrodon macrospondylus.[44]
Dimetrodon platycentrus was first described by Case in his 1907 monograph. It is now considered a synonym ofDimetrodon macrospondylus.[30]
Paleontologist Alfred Romer erected the speciesDimetrodon natalis in 1936, previously described asClepsydrops natalis.D. natalis was the smallest known species ofDimetrodon at that time, and was found alongside remains of the larger-bodiedD. limbatus.[61]
Dimetrodon booneorum was first described by Alfred Romer in 1937 on the basis of remains from Texas.[61]
Dimetrodon kempae was named by Romer in 1937, in the same paper asD. booneorum,D. loomisi, andD. milleri.[61]Dimetrodon kempae was named on the basis of a single humerus and a few vertebrae, and may therefore be anomen dubium that cannot be distinguished as a unique species ofDimetrodon.[12] In 1940, Romer and Price raised the possibility thatD. kempae may not fall within the genusDimetrodon, preferring to classify it as Sphenacodontidaeincertae sedis.[47]
Dimetrodon loomisi was first described by Alfred Romer in 1937 along withD. booneorum,D. kempae, andD. milleri.[61] Remains have been found in Texas and Oklahoma.
Dimetrodon milleri was described by Romer in 1937.[61] It is one of the smallest species ofDimetrodon in North America and may be closely related toD. occidentalis, another small-bodied species.[54]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 ofDimetrodon.
Besides its small size,D. milleri differs from other species ofDimetrodon 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 otherDimetrodon 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 speciesD. booneorum,D. limbatus andD. grandis, suggesting thatD. milleri may be the first of an evolutionary progression between these species.
Dimetrodon angelensis was named by paleontologistEverett C. Olson in 1962.[62] Specimens of the species were reported from theSan Angelo Formation of Texas.[63] It is also the largest species ofDimetrodon.
Dimetrodon occidentalis was named in 1977 from New Mexico.[53] Its name means "westernDimetrodon" because it is the only North American species ofDimetrodon 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 ofD. occidentalis is similar to that ofD. milleri, suggesting a close relationship.Dimetrodon specimens found in Utah and Arizona probably also belong toD. occidentalis.[54]
Dimetrodon teutonis was named in 2001 from theThuringian Forest of Germany and was the first species ofDimetrodon to be described outside North America. It is also the smallest species ofDimetrodon.[12][64]
In 1878, Cope published a paper called "The Theromorphous Reptilia" in which he describedDimetrodon cruciger.[65]D. cruciger was distinguished by the small projections that extended from either side of each neural spine like the branches of a tree.[66] In 1886, Cope movedD. cruciger to the genusNaosaurus because he considered its spines so different from those of otherDimetrodon species that the species deserved its own genus.[67]Naosaurus would later be synonymized withEdaphosaurus, a genus which Cope named in 1882 on the basis of skulls that evidently belonged to herbivorous animals given their blunt crushing teeth.[68]
E. C. Case named the speciesDimetrodon longiramus in 1907 on the basis of a scapula and elongated mandible from theBelle Plains Formation of Texas.[44] In 1940, Romer and Price recognized that theD. longiramus material belonged to the same taxon as another specimen described by paleontologistSamuel Wendell Williston in 1916, which included a similarly elongated mandible and a long maxilla.[47] Williston did not consider his specimen to belong toDimetrodon but instead classified it as anophiacodontid.[69] Romer and Price assigned Case and Williston's specimens to a newly erected genus and species,Secodontosaurus longiramus, that was closely related toDimetrodon.[47][70]
Dimetrodon is an early member of a group calledsynapsids, which include mammals and many of their extinct relatives, though it is not an ancestor of any mammal (which appeared millions of years later[71]). 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 theTriassic period. As a synapsid,Dimetrodon is more closely related to mammals than to dinosaurs or any living reptile. By the early 1900s most paleontologists calledDimetrodon a reptile in accordance withLinnean taxonomy, which ranked Reptilia as aclass andDimetrodon as a genus within that class. Mammals were assigned to a separate class, andDimetrodon was described as a "mammal-like reptile". Paleontologists theorized that mammals evolved from this group in (what they called) a reptile-to-mammal transition.
Underphylogenetic systematics, the descendants of thelast common ancestor ofDimetrodon and all living reptiles would include all mammals becauseDimetrodon 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, thenDimetrodon 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 theLate Carboniferous) are therefore split into two clades: Synapsida, which includesDimetrodon and mammals, andSauropsida, which includes living reptiles and all extinct reptiles more closely related to them than to mammals.[4]
Within clade Synapsida,Dimetrodon is part of the cladeSphenacodontia, which was first proposed as an early synapsid group in 1940 by paleontologists Alfred Romer and Llewellyn Ivor Price, along with the groupsOphiacodontia andEdaphosauria.[47] All three groups are known from the Late Carboniferous and Early Permian. Romer and Price distinguished them primarily bypostcranial 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 calledTherapsida, which included the closest relatives to mammals. Romer and Price placed another group of early synapsids calledvaranopids within Sphenacodontia, considering them to be more primitive than other sphenacodonts likeDimetrodon.[72] They thought varanopids andDimetrodon-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 paleontologistRobert R. Reisz in 1986, whose study included features mostly found in the skull rather than in the postcranial skeleton.[73]Dimetrodon is still considered a sphenacodont under thisphylogeny, but varanodontids are now considered morebasal synapsids, falling outside clade Sphenacodontia. Within Sphenacodontia is the groupSphenacodontoidea, which in turn containsSphenacodontidae andTherapsida. Sphenacodontidae is the group containingDimetrodon and several other sail-backed synapsids likeSphenacodon andSecodontosaurus, whileTherapsida includes mammals and their mostly Permian andTriassic relatives.
Below is thecladogram Clade Synapsida, which follows this phylogeny ofSynapsida as modified from the analysis of Benson (2012).[72]
| Amniota |
| ||||||
The below cladogram shows the relationships of a fewDimetrodon species, from Brink et al., (2015).[40]
| Sphenacodontidae |
| ||||||||||||||||||||||||||||||
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 whileDimetrodon waited for prey, or as an actual boat-like sail to catch the wind while the animal was in the water.[74] 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.[28]
In 1940,Alfred Romer andLlewellyn 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 inDimetrodon. For example, in a 1973 article in the journalNature, paleontologists C. D. Bramwell and P. B. Fellgett estimated that it took a 200 kilograms (440 lb) individual about one and a half hours for its body temperature to rise from 26 to 32 °C (79 to 90 °F).[75] 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 ofDimetrodon, Haack found that the sail allowedDimetrodon 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 allowDimetrodon to retain a higher body temperature at night.[76] 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.[77]
.jpg)
Most of these studies give two thermoregulatory roles for the sail ofDimetrodon: 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 orpoikilothermic, relying on the sun to maintain a high body temperature. Because of its large size,Dimetrodon had highthermal 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 ofDimetrodon could warm their bodies much faster than could something the size ofDimetrodon. Many paleontologists including Haack have proposed that the sail ofDimetrodon may have allowed it to warm quickly in the morning in order to keep pace with its prey.[76] 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.[74]
In 1986, J. Scott Turner and C. Richard Tracy proposed that the evolution of a sail inDimetrodon was related to the evolution of warm-bloodedness in mammal ancestors. They thought that the sail ofDimetrodon enabled it to behomeothermic, maintaining a constant, albeit low, body temperature. Mammals are also homeothermic, although they differ fromDimetrodon in beingendothermic, 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 ofDimetrodon. The homeothermy that developed in animals likeDimetrodon may have carried over to therapsids through a modification of body shape, which would eventually develop into the warm-bloodedness of mammals.[78]
Recent studies on the sail ofDimetrodon 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 ofDimetrodon such asD. milleri andD. 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 ofDimetrodon such asSphenacodon have very low crests that would have been useless as thermoregulatory devices.[30] The large sail ofDimetrodon is thought to have developed gradually from these smaller crests, meaning that over most of the sail's evolutionary history, thermoregulation may not not have served an important function. It should be noted however global temperatures soared during the Permian, with the region that is now the central United States reaching 165 °F (74 °C).[79]
Although the function of its sail remains uncertain,Dimetrodon and otherSphenacodontids 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 anamniote-wide study that found tachymetabolic endothermy to have been widespread throughout, and likelyplesiomorphic to bothsynapsids andsauropsids. ForDimetrodon 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.[80]
Larger bodied specimens ofDimetrodon have notably larger sails than would be predicted if the sail size was only proportionate scaled to their increase in body size, an example ofpositive 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 inDimetrodon 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 ofDimetrodon grew at a much faster rate than was necessary for thermoregulation, and suggested thatsexual selection was the primary reason for its evolution.[79]
The allometric exponent for sail height is similar in magnitude to the scaling of interspecific antler length to shoulder height incervids. Furthermore, as Bakker (1970) observed in the context ofDimetrodon, 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 ofD. limbatus (Romer and Price 1940).[79]
Dimetrodon may have beensexually dimorphic, meaning that males and females had slightly different body sizes. Some specimens ofDimetrodon 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 theAmerican Museum of Natural History (AMNH 4636) and theField Museum of Natural History may be males and the skeletons in theDenver Museum of Nature and Science (MCZ 1347) and theUniversity of Michigan Museum of Natural History may be females.[47]
Fossils ofDimetrodon 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 supercontinentEuramerica during the Early Permian. Within the United States, almost all material attributed toDimetrodon has come from three geological groups in north-central Texas and south-central Oklahoma: theClear Fork Group, theWichita Group, and thePease River Group.[81][82] 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 fossiltetrapods, or four-limbed vertebrates. In addition toDimetrodon, the most common tetrapods in the Red Beds and throughout Early Permian deposits in the southwestern United States, are the amphibiansArcheria,Diplocaulus,Eryops, andTrimerorhachis, thereptiliomorphSeymouria, the reptileCaptorhinus, and the synapsidsOphiacodon andEdaphosaurus. These tetrapods made up a group of animals that paleontologistEverett C. Olson called the "Permo-Carboniferous chronofauna", afauna that dominated the continental Euramerican ecosystem for several million years.[83] Based on the geology of deposits like the Red Beds, the fauna is thought to have inhabited a well-vegetated lowlanddeltaic ecosystem.[84]
Olson made many inferences on the paleoecology of theTexas Red beds and the role ofDimetrodon 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 theprimary producers and were largely fed upon byfish and aquatic invertebrates. Most land vertebrates fed on these aquatic primary consumers.Dimetrodon was probably thetop predator of the Red Beds ecosystem, feeding on a variety of organisms such as the sharkXenacanthus,[85][86] the aquatic amphibiansTrimerorhachis andDiplocaulus, and the terrestrial tetrapodsSeymouria andTrematops. Insects are known from the Early Permian Red Beds and were probably involved to some degree in the same food web asDimetrodon, feeding small reptiles likeCaptorhinus. The Red Beds assemblage also included some of the first large land-living herbivores likeEdaphosaurus andDiadectes. 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 ecosystemDimetrodon inhabited is theEverglades.[84] The exact lifestyle ofDimetrodon (amphibious to terrestrial) has long been controversial, but bone microanatomy supports a terrestrial lifestyle,[87] which implies that it would have fed mostly on land, on the banks, or in very shallow water. Evidence also exists forDimetrodon preying onaestivatingDiplocaulus during times of drought, with three partially eaten juvenileDiplocaulus in a burrow of eight bearing teeth marks from aDimetrodon that unearthed and killed them.[88]
The only species ofDimetrodon found outside the southwestern United States isD. teutonis from Germany. Its remains were found in theTambach Formation in a fossil site called the Bromacker locality. The Bromacker's assemblage of Early Permiantetrapods is unusual in that there are few large-bodied synapsids serving the role of top predators.D. teutonis is estimated to have been only 14 kilograms (31 lb) in weight, too small to prey on the largediadectid 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 largevaranopids or smallsphenacodonts, both of which could potentially prey onD. teutonis. In contrast to the lowlanddeltaic 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 aquaticamphibians for food.[12]
Definitions from Wiktionary
Media from Commons
Taxa from Wikispecies
Data from Wikidata
.jpg)
