Dimetrodon
Dimetrodon | |
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Skeleton of D. limbatus, Staatliches Museum für Naturkunde Karlsruhe
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Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Chordata |
Clade: | Synapsida |
Family: | †Sphenacodontidae |
Subfamily: | † Sphenacodontinae
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Genus: | †Dimetrodon Cope, 1878 |
Type species | |
†Dimetrodon limbatus Cope, 1877
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Species | |
See below
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Synonyms | |
Genus synonymy
Species synonymy
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Dimetrodon (
Dimetrodon is often mistaken for a
Dimetrodon was probably one of the apex predators of the Cisuralian ecosystems, feeding on fish and tetrapods, including reptiles and amphibians. Smaller Dimetrodon species may have had different ecological roles. The sail of Dimetrodon may have been used to stabilize its spine or to heat and cool its body as a form of thermoregulation.[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 in courtship display, including threatening away rivals or showing off to potential mates.[9][10]
Description
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.[11] Most Dimetrodon 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 species D. teutonis was about 60 cm (24 in) long and weighed 14 kilograms (31 lb).[12][13] 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 5.5 metres (18 ft) long.[14][15] Although some Dimetrodon species could grow very large, many juvenile specimens are known.[16]
Skull
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
Teeth
The size of the teeth varies greatly along the length of the jaws, lending Dimetrodon its name, which means "two measures of tooth" in reference to sets of small and large teeth.
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
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.[21] The thickness and mass of the teeth of Dimetrodon may also have been an adaptation for increasing dental longevity.[22]
Nasal cavity
On the inner surface of the nasal section of skull are ridges called
Jaw joint and ear
Another transitional feature of Dimetrodon is a ridge in the back of the jaw called the reflected lamina, which is found on the
Tail
The tail of Dimetrodon makes up a large portion of its total body length and includes around 50
Sail
The sail of Dimetrodon is formed by elongated
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
Skin
No fossil evidence of Dimetrodon's skin has yet been found. Impressions of the skin of a related animal,
Classification history
Earliest discoveries
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.[37] 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,[38] although it was later reclassified as a species of Dimetrodon in 2015, as Dimetrodon borealis.[39]
First descriptions by Cope
Fossils now attributed to Dimetrodon were first studied by American paleontologist Edward Drinker Cope in the 1870s. Cope had obtained the fossils along with those of many other Permian 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.[40] 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
The first description of a Dimetrodon fossil came a year earlier, though, when Cope named the species Clepsydrops limbatus from the Texas Red Beds.[43] (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
Early 20th century descriptions
In the first few decades of the 20th century, American paleontologist
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.[44] Most of the species names considered valid by Romer and Price are still used today.[29]
New specimens
In the decades following Romer and Price's monograph, many Dimetrodon specimens were described from localities outside Texas and Oklahoma. The first was described from the Four Corners region of Utah in 1966[45] and another was described from Arizona in 1969.[46] In 1975, Olson reported Dimetrodon material from Ohio.[47] A new species of Dimetrodon called D. occidentalis (meaning "western Dimetrodon") was named in 1977 from New Mexico.[48] The specimens found in Utah and Arizona probably also belong to D. occidentalis.[49]
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.[48][50]
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.[12]
Species
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.
Summary
Species | Authority | Location | Status | Synonyms | Images |
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Dimetrodon angelensis | Olson, 1962 | Texas | Valid | ||
Dimetrodon borealis | Leidy, 1854 | Prince Edward Island | Valid | Previously known as the dinosaur Bathygnathus borealis | |
Dimetrodon booneorum | Romer, 1937 | Texas | Valid | ||
Dimetrodon dollovianus | Case, 1907 | Texas | Valid | Embolophorus dollovianus Cope, 1888 | |
Dimetrodon gigahomogenes | Case, 1907 | Texas | Valid | ||
Dimetrodon grandis | Romer and Price, 1940 | Oklahoma Texas |
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 | Texas | Possible nomen dubium | ||
Dimetrodon limbatus | Romer and Price, 1940 | Oklahoma Texas |
Valid | Clepsydrops limbatus Cope, 1877 Dimetrodon incisivus Cope, 1878 Dimetrodon rectiformis Cope, 1878 Dimetrodon semiradicatus Cope, 1881 |
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Dimetrodon loomisi | Romer, 1937 | Texas Oklahoma |
Valid | ||
Dimetrodon macrospondylus | Case, 1907 | Texas | Valid | Clepsydrops macrospondylus Cope, 1884 Dimetrodon platycentrus Case, 1907 |
|
Dimetrodon milleri | Romer, 1937 | Texas | Valid | ||
Dimetrodon natalis | Romer, 1936 | Texas | Valid | Clepsydrops natalis Cope, 1878 | |
Dimetrodon occidentalis | Berman, 1977 | Arizona New Mexico Utah |
Valid | ||
Dimetrodon teutonis | Berman et al., 2001 | Germany | Valid |
Dimetrodon limbatus
Dimetrodon limbatus was first described by Edward Drinker Cope in 1877 as Clepsydrops limbatus.[43] (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.[44]
Dimetrodon incisivus
The first use of the name Dimetrodon came in 1878 when Cope named the species Dimetrodon incisivus along with Dimetrodon rectiformis and Dimetrodon gigas.[42]
Dimetrodon rectiformis
Dimetrodon rectiformis was named alongside Dimetrodon incisivus in Cope's 1878 paper, and was the only one of the three named species to preserve elongated neural spines.
Dimetrodon semiradicatus
Described in 1881 on the basis of upper jaw bones, Dimetrodon semiradicatus was the last species named by Cope.[51] In 1907, E. C. Case synonymized D. semiradicatus with D. incisivus based on similarities in the shape of the teeth and skull bones.[41] D. incisivus' and D. semiradicatus are now considered synonyms of D. limbatus.[29]
Dimetrodon dollovianus
Dimetrodon dollovianus was first described by Edward Drinker Cope in 1888 as Embolophorus dollovianus. In 1903, E. C. Case published a lengthy description of E. dollovianus, which he later referred to Dimetrodon.[52]
Dimetrodon grandis
Paleontologist E. C. Case named a new species of sail-backed synapsid, Theropleura grandis, in 1907.[41] In 1940, Alfred Romer and Llewellyn Ivor Price reassigned Theropleura grandis to Dimetrodon, erecting the species D. grandis.[44]
Dimetrodon gigas
In his 1878 paper on fossils from Texas, Cope named Clepsydrops gigas along with the first named species of Dimetrodon, D. limbatus, D. incisivus, and D. rectiformis.[42] Case reclassified C. gigas as a new species of Dimetrodon in 1907.[41] Case also described a very well preserved skull of Dimetrodon in 1904, attributing it to the species Dimetrodon gigas.[53] In 1919, Charles W. Gilmore attributed a nearly complete specimen of Dimetrodon to D. gigas.[54] Dimetrodon gigas is now recognized as a synonym of D. grandis.[55]
Dimetrodon giganhomogenes
Dimetrodon giganhomogenes was named by E. C. Case in 1907 and is still considered a valid species of Dimetrodon.[41][29]
Dimetrodon macrospondylus
Dimetrodon macrospondylus was first described by Cope in 1884 as Clepsydrops macrospondylus. In 1907, Case reclassified it as Dimetrodon macrospondylus.[41]
Dimetrodon platycentrus
Dimetrodon platycentrus was first described by Case in his 1907 monograph. It is now considered a synonym of Dimetrodon macrospondylus.[29]
Dimetrodon natalis
Paleontologist Alfred Romer erected the species Dimetrodon natalis in 1936, previously described as Clepsydrops natalis. D. natalis was the smallest known species of Dimetrodon at that time, and was found alongside remains of the larger-bodied D. limbatus.[56]
Dimetrodon booneorum
Dimetrodon booneorum was first described by Alfred Romer in 1937 on the basis of remains from Texas.[56]
"Dimetrodon" kempae
Dimetrodon kempae was named by Romer in 1937, in the same paper as D. booneorum, D. loomisi, and D. milleri.[56] 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.[12] 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.[44]
Dimetrodon loomisi
Dimetrodon loomisi was first described by Alfred Romer in 1937 along with D. booneorum, D. kempae, and D. milleri.[56] Remains have been found in Texas and Oklahoma.
Dimetrodon milleri
Dimetrodon milleri was described by Romer in 1937.[56] It is one of the smallest species of Dimetrodon in North America and may be closely related to D. occidentalis, another small-bodied species.[49] D. milleri is known from two skeletons, one nearly complete (MCZ 1365) and another less complete but larger (MCZ 1367). D. milleri is the oldest known species of Dimetrodon.
Besides its small size, D. milleri differs from other species of Dimetrodon in that its neural spines are circular rather than figure-eight shaped in cross-section. Its vertebrae are also shorter in height relative to the rest of the skeleton than those of other Dimetrodon species. The skull is tall and the snout is short relative to the temporal region. A short vertebrae and tall skull are also seen in the species D. booneorum, D. limbatus and D. grandis, suggesting that D. milleri may be the first of an evolutionary progression between these species.
Dimetrodon angelensis
Dimetrodon angelensis was named by paleontologist Everett C. Olson in 1962.[57] Specimens of the species were reported from the San Angelo Formation of Texas.[58] It is also the largest species of Dimetrodon.
Dimetrodon occidentalis
Dimetrodon occidentalis was named in 1977 from New Mexico.[48] 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.[49]
Dimetrodon teutonis
Dimetrodon teutonis was named in 2001 from the Thuringian Forest of Germany and was the first species of Dimetrodon to be described outside North America. It is also the smallest species of Dimetrodon.[12]
Species assigned to different genera
Dimetrodon cruciger
In 1878, Cope published a paper called "The Theromorphous Reptilia" in which he described Dimetrodon cruciger.
Dimetrodon longiramus
E. C. Case named the species Dimetrodon longiramus in 1907 on the basis of a scapula and elongated mandible from the
Phylogenetic classification
Dimetrodon is an early member of a group called
Phylogenetic taxonomy of Synapsida
Under
Within clade Synapsida, Dimetrodon is part of the clade
The modern view of synapsid relationships was proposed by paleontologist
Below is the cladogram Clade Synapsida, which follows this phylogeny of Synapsida as modified from the analysis of Benson (2012).[66]
Amniota
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The below cladogram shows the relationships of a few Dimetrodon species, from Brink et al., (2015).[68]
Sphenacodontidae |
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Paleobiology
Function of neural spines
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.[69] 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.[27]
Thermoregulation
In 1940, Alfred Romer and Llewellyn Ivor Price proposed that the sail served a thermoregulatory function, allowing individuals to warm their bodies with the Sun's heat. 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 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).[70] 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.[71] 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.[72]
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
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
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.[29] 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.[74]
Although the function of its sail remains uncertain, Dimetrodon and other
Larger bodied specimens of Dimetrodon have larger sails relative to their size, an example of
Sexual selection
The allometric exponent for sail height is similar in magnitude to the scaling of interspecific antler length to shoulder height in
Sexual dimorphism
Dimetrodon may have been
Paleoecology
Fossils of Dimetrodon are known from the United States (Texas, Oklahoma, New Mexico, Arizona, Utah and Ohio) and Germany, areas that were part of the supercontinent
Food web
Olson made many inferences on the paleoecology of the
The only species of Dimetrodon found outside the southwestern United States is D. teutonis from Germany. Its remains were found in the
See also
- Secodontosaurus – Extinct genus of synapsids
- Dinosaurs in Jurassic Park
- Edaphosaurus – Extinct genus of synapsids
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External links
- Dimetrodon Palaeos page on Dimetrodon
- Introduction to the Pelycosaurs University of California Museum of Paleontology webpage on early synapsids, including Dimetrodon
- "Dimetrodon: Our Most Unlikely Ancestor". PBS Eons. August 21, 2017 – via YouTube.