Dimetrodon

Dimetrodon ("two-measure tooth") is an extinct genus of sphenacodontid pelycosaur synapsids whose species lived in North America and Europe during the Early Permian around 295 to 272 million years.

The most outstanding feature of Dimetrodon is the enormous, fin-like dorsal sail, whose structure is based on elongated vertebral spines. The function it may have played was to stabilize the spine to improve locomotion or to warm and cool the body as a thermoregulation system. However, recent studies maintain that the sail may have been ineffective in regulating body heat and may have instead served a reproductive function during mating displays.

It was certainly one of the super predators of its time and fed on fish and other tetrapods, including reptiles and amphibians. It had a quadrupedal posture and had a large skull with a curved profile with large teeth of different sizes housed in both jaws. Most Dimetrodon fossils were found in the southwestern United States, especially from deposits called the Red Beds of Texas and Oklahoma, and the distribution of the genus was believed to be restricted to North America.. However, a new species found in Europe (Germany) was recently described, which is not surprising given that at the beginning of the Permian North America and Europe were part of the supercontinent Euramerica. The genus name was first used in 1978 and since then around twenty species have been described, of which a dozen are considered valid.

Dimetrodon is frequently confused in popular culture with a dinosaur or one of its contemporaries, but it actually lived at least 40 million years before the first dinosaur appeared during the Triassic. Despite having a reptile appearance, it is more related to mammals than to any of the modern reptiles; However, the family to which it belongs is not part of the lineage that gave rise to living mammals.

History

Description in the 19th century

The first studies carried out on fossils currently classified as Dimetrodon were carried out by the American paleontologist Edward Drinker Cope in the 1870s. Cope obtained the fossils, along with many other remains of Permian tetrapods, from the hands from some fossil hunters who obtained them in the Red Beds of Texas. Among the hunters were the Swiss naturalist Jacob Boll, the American geologist W. F. Cummins, and the amateur paleontologist Charles Hazelius Sternberg.

Sternberg sent some of his fossils to German paleontologist Ferdinand Broili of the University of Munich, however Broili was not as prolific as Cope in describing the specimens. Cope's rival Othniel Charles Marsh also obtained some Dimetrodon fossils which he sent to the Walker Museum. The name Dimetrodon was first used in 1878 when Cope described the species i>Dimetrodon incisivus, Dimetrodon rectiformis and Dimetrodon gigas in the scientific publication Proceedings of the American Philosophical Society.

However, the first description of a Dimetrodon fossil took place a year earlier, when Cope described the species Clepsydrops limbatus from the Red Beds of Texas. (The name Clepsydrops was coined by Cope in 1875 to describe sphenacodontids from Vermilion County, Illinois, and later used to name many sphenacodontid specimens from Texas; late in the century XIX and early XX centuries many sphenacodontids from Texas were assigned to both Clepsydrops and Dimetrodon) C. limbatus was reassessed as a species of Dimetrodon in 1940, so Cope's 1877 publication became the first record of Dimetrodon.

Cope himself was responsible for describing the first species of synapsid with a dorsal sail in 1978, with the name Clepsydrops natalis; He however considered the sail as a fin and compared it to the crest of the current basilisks (Basilicus). Candles were not preserved in the specimens ofD. incisivusand D. gigasalso described in his 1878 publication, but they were in the specimen from D. rectiformisalso described by him.

Descriptions from the beginning of the 20th century

In the first decades of the XX century, the American paleontologist E. C. Case was the author of many studies on Dimetrodonand described some species. Case received funding from the Carnegie Institution to study many specimens in the collections of the American Museum of Natural History and other museums. Many of these specimens were found by Cope, but he did not detail them in great detail, since Cope was known for describing species new ones based only on some bone fragments. In the early 1920s, paleontologist Alfred Romer reviewed many specimens of Dimetrodon and described other species. In 1940, Romer co-authored an extensive study with Llewellyn Ivor Price called Review of the Pelycosauria, in which the species of Dimetrodon described by Cope and Case were revalued. Most of the species considered valid by Romer and Price still maintain that status.

New specimens

In the decades following Romer and Price's monograph, new specimens of Dimetrodon were discovered in localities outside of Texas and Oklahoma in the United States. The first of them was discovered in the Four Corners region of Utah in 1966; another in Arizona in 1969, and in 1975 in Ohio. A new species of the genus called D. occidentalis was described in 1977 from New Mexico. Fossils found in Utah and Arizona probably also belonged to D. occidentalis.

Before these findings, a theory was formulated that proposed the existence of a sea passage in the early Permian that separated what are now the states of Texas and Oklahoma from the western territories, restricting the species of Dimetrodonto a small region of North America, while a smaller sphenacodontid called Sphenacodon dominated the western area. Although this strip of ocean probably existed, the discovery of fossils outside this territory shows that its extension was limited and did not constitute an effective barrier to the distribution of Dimetrodon species.

In 2002, a new species of Dimetrodon that was renamed D. teutoniswas described from a find in the Saar-Nahe Basin in Germany, which was the first find of a species in this genus outside of North America.

Phylogeny

Dimetrodon is a primitive member of the synapsids, a diverse group that includes living mammals and all of their extinct relatives. The genus may have evolved from a primitive form of the Sphenacodontidae family, probably from Sphenacodon ferox during the Upper Carboniferous or Lower Permian (Gzhelian-Asselian); and its disappearance coincides with the rise of the therapsids. eotitanosuchids and dinocephalians during the Ufimian. In popular culture they are often confused with a dinosaur, even though they arose at least 40 million years before the first dinosaur appeared. Dimetrodon is found more related to current mammals than to dinosaurs and any of the modern reptiles. From the end of the 19th century to the beginning of the XX, most paleontologists considered it a reptile according to Linnaeus' taxonomy. In this classification reptiles were considered as a class and Dimetrodon was included as a genus of this class. However, paleontologists recognized that mammals evolved from this group in what they called a transition from reptile to mammal. More recently, phylogenetic taxonomy unified the classification of vertebrates and defined that they all share a common ancestor. Reptiles and mammals are located in different clades, and each group has a common ancestor from which they are all descendants. Under phylogenetic systematics, the descendants of the most recent common ancestor of Dimetrodon and all living reptiles also includes all modern mammals and their ancestors, which are currently considered most closely related to Dimetrodon<. /i> than with any living reptile. The descendants of the most recent common ancestor of mammals and reptiles (which appeared about 310 million years ago in the Late Carboniferous) are divided into two clades: the synapsids, which include Dimetrodon and mammals and sauropsids, which include living reptiles and all extinct reptiles more closely related to each other than to mammals.

Within the synapsids Dimetrodon is part of the Sphenacodontia clade. Sphenacodonts were first classified as primitive synapsids in 1940 by paleontologists Alfred Romer and Llewellyn Ivor Price, along with the groups Ophiacodontia and Edaphosauria. Members of these three groups are known from the Upper Carboniferous and Early Permian. Romer and Price identified them primarily by postcranial characteristics such as the shape of the limbs and vertebrae. They considered Ophiacodontia the most primitive group because of its greater similarity to reptiles; while Sphenacodontia was considered the most advanced due to its greater similarity to therapsids, which contain the synapsids most closely related to mammals. Romer and Price placed a family of primitive synapsids called Varanopidae within Sphenacodontia, considering it more primitive than the other sphenacodontids related to Dimetrodon. They thought that the varanopids and sphenacodontids related to Dimetrodon i> they were closely related since both groups were carnivorous; However, the varanopids were much smaller, lacked a dorsal sail and had a reptilian appearance. The current classification of relationships between synapsids was proposed by paleontologist Robert R. Reisz in 1986, whose study was based mainly on cranial characteristics, leaving the rest of the skeleton in the background. Dimetrodon is considered a sphenacodonts under this classification, but varanodontids are considered basal synapsids and are classified outside the sphenacodonts. Within Sphenacodontia is the clade Sphenacodontoidea, which in turn contains the family Sphenacodontidae and the order Therapsida. The family Sphenacodontidae includes Dimetrodon and other synapsids with dorsal sails such as Sphenacodon and Secodontosaurus, while therapsids include mammals and most from their relatives who lived during the Permian and Triassic. Below is a modified cladogram from Benson's analysis (in press) that follows this phylogeny:

Anatomical description

Dimetrodon was a quadruped synapsid, possessing a large fin-like sail that covered its back. Its size and morphology was similar to a modern alligator, except for the fact that it was an exclusively terrestrial animal. Dimetrodon was the dominant predator in its environment for more than 25 million years, during which time it diversified into about a dozen species, which increased in size over time. Most species ranged in length from 1.7 to 3.2 meters, and an estimated weight from 28 to 250 kg. The largest described species of Dimetrodon is D. grandis 3.2 meters in length and the smallest is D. teutonis only 60 cm long. Large species of Dimetrodon were among the largest predators in the early Permian; However, its relative Tappenosaurus, known from fossil finds in somewhat more recent strata, could have been a little larger with an estimated length of 5.5 m. Among the characteristics that most identify it With mammals than with reptiles, it is worth highlighting the presence of a fused lower jaw, a complex teeth for chewing, a novel mandibular muscular anatomy, a larger brain and three small bones in the middle ear.

Skull

Skull bones Dimetrodon

The skull of Dimetrodon is proportionally tall, narrow and triangular in profile when viewed from above. The eye holes are located at the top and back of the skull. Posterior to each eye socket is a hole called the infratemporal fenestra. Two other pairs of foramina can be seen on the posterior surface of the skull when viewed from above: the supratemporal fenestra and the posttemporal fenestra. The posterior pole of the skull (occiput) is oriented at a slightly higher angle, a characteristic it shares with other primitive synapsids. Viewed laterally, the upper contour of the skull declines downward and forward in a convex arc to the tip of the snout. The tip of the upper jaw, formed by premaxillary bone, extends from above on the maxilla to form a discontinuity, which forms a diastema between the dental arches.

Due to its phylogenetic position, it had a pattern of mandibular musculature from which that of mammals originated. However, the study of the temporal region, the posterior palate and the jaw, indicates that Dimetrodon it had adductor muscles in the jaw with differentiation and distribution similar to modern reptiles. The characteristics of the masticatory muscles and the presence of a powerful prehensile apparatus suggest that Dimetrodon fed on large animals.

Teeth

The length of the teeth differs noticeably according to their location in the jaws, which is why it owes its name Dimetrodon which means "tooth of two sizes", alluding to the two sets of teeth. of different lengths that it had. One or two pairs of canine-like teeth detached from the upper jaw and large incisors projected from the tip of both jaws inserted into the premaxilla and mandible. Smaller teeth are found around the maxillary gap and posterior to the caniniforms, becoming progressively smaller towards the back of the jaws.

Many of the teeth are wider in the middle section and become narrower as they contact the jaw bone, giving them an oval or "water drop" shape. This shape of the teeth is characteristic of Dimetrodon and the more related sphenacodontids, a feature that allows them to be distinguished from other primitive synapsids. As in other basal synapsids, the teeth of most Dimetrodon species have serrated edges. The teeth of the smallest speciesD. teutonislack this type of edge, but retain their sharp edges.

Nasal cavity

In the fossils found, the internal surface of the nasal region has ridges called nasoturbinal, which could have provided the basis for the cartilage that supported the olfactory epithelium, which, as its name indicates, was responsible for detecting odors. These grooves are much smaller than those present in the most recent synapsids from the Permian and Triassic, in which the large nasoturbinals are considered proof that they were warm-blooded animals, because they possibly served as support for the warming mucous membrane. and humidifies the inspired air. Therefore, the nasal cavity of Dimetrodon is transitional between primitive terrestrial vertebrates and mammals.

Mandibular joint and ear

Another transitional feature of Dimetrodon is a ridge on the back of the jaw called the reflex plate. This is located in the articular bone that connects the lower jaw with the square bone of the skull to form the mandibular joint. In the most recent ancestors of mammals, the articular bone and the quadrate separated from the mandibular joint, the articular becoming the malleus of the middle ear. The reflex lamina becomes part of a structure called the tympanic annulus that supports the eardrum in all living mammals.

Queue

The tail of Dimetrodon makes up a large proportion of its body mass and is made up of around 50 caudal vertebrae. In the first described skeletons the tail was absent or incomplete; The only caudal vertebra known in one specimen was a number eleven, which was at the height of the hip. Because the first findings suggested that the caudal vertebrae rapidly decreased in size as they moved toward the tip of the tail, many paleontologists of the 19th century and the early 20th century XX thought that the animal had a very short tail. This concept changed until 1927 when a specimen with an almost complete tail was described.

Sail

The most distinctive feature of Dimetrodon is the large fin-like sail that ran the entire length of its back. This anatomical formation originated from the elongation of the spinous processes of the vertebrae. Each spine was different in its cross section, from the base to the tip. Near the vertebral body the cross section of the spine has a rectangular shape and towards the tip it takes on a figure of eight with a groove in each of its sides. It is believed that this shape gave more resistance to the spines, which prevented bending and fractures. A cross section of a Dimetrodon giganhomogenes spine has a rectangular shape but retains the figure-of-eight configuration toward the center, indicating that the shape of the spines can change with the age of the individuals.

The microscopic anatomy of each spine also varies from the base to the tip and indicates the sites that served as seats for the back muscles and the portion that was part of the fin. The proximal part of the spine had a rough surface that could have served as an anchoring point for the dorsal muscles. It also had a network of connective tissue called Sharpey's fibers, which indicate that it was embedded within the body. In the distal part of the spines the surface of the bone is smoother. The periosteum is covered with small grooves that probably served as a channel for the blood vessels that supplied the sail. It was previously believed that the large lateral grooves of the spines were channels furrowed by large blood vessels, but because the bone lacks vascular channels of significant size, the concept was changed and it is thought that the sail was poorly vascularized. In some specimens of Dimetrodon deformed areas can be observed that may be consolidated fractures. The cortical bone over these breaks is quite vascularized, suggesting that the sail had soft tissues that projected blood vessels over the site. Concentric layers of lamellar bone constitute the majority of the spines when viewed in cross section and contain areas of growth recession that allow us to calculate the age of the specimens at the time of their death. In many finds, the distal portion of the spines rotates abruptly, indicating that in life the sail may have had an irregular profile. This curvature suggests that the soft tissue may not have reached the tip of the bony projections, meaning that the tissue between the spines was not as extensive as previously thought.

Thermoregulatory function

There are many propositions about the function of the dorsal sail in this animal. Among the first, it was suggested that it could have served the animal to camouflage itself among the reeds while it stalked its prey or that it could have been an authentic sail that propelled the animal over the water. Another possible function that its long neural spines could have had was to stabilize the trunk by restricting its vertical movements, which provided better support for the lateral movements required for walking. In 1940, Alfred Romer and Llewellyn Ivor Price proposed that the sail had a thermoregulatory function, which It allowed the animal to heat its body with sunlight. In the following years, many models were created to estimate the thermoregulation capacity of 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-kg animal about an hour and a half to increase its temperature from 26 to 32 °C. In 1986, Steven C. Haack concluded that the warming was slower than estimated and that the process took about four hours. Using a model based on a variety of environmental factors and hypothetical physiological factors of the animal, Haack found that the fin allowed Dimetrodon to warm up quickly in the morning and reach an optimal temperature during the day, but it was ineffective at removing excess heat and keeping warm at night. In 1999, a group of mechanical engineers devised a virtual model to analyze the candle's ability to regulate body temperature during seasonal changes. They concluded that the sail was useful for capturing and releasing heat throughout the year.

Most of these studies attribute two thermoregulatory functions to the candle: one as a means of warming up quickly in the morning or another as a means of lowering body temperature. It is presumed that Dimetrodon, like all terrestrial vertebrates from the beginning of the Permian, was a cold-blooded or ectothermic animal, dependent on the sun to maintain body temperature. Due to their large size, temperature changes were slower than in smaller animals. By increasing the temperature in the morning, the smaller prey of Dimetrodon could warm up and become active faster than its predator. Many paleontologists, including Haack, proposed that Dimetrodon's sail may have allowed it to warm up more quickly at dawn, in time with its prey. The large surface area of the fin may also have allowed it to remove excess moisture. heat, at the expense of metabolism or solar radiation. To cool itself, the animal will orient its fin parallel to sunlight or restrict blood flow to the fin to maintain heat at night.

In 1986, J. Scott Turner and C. Richard Tracy proposed that the development of the sail in Dimetrodon is related to the evolution of warm blood in the ancestors of mammals. They thought that the existence of sail led this animal towards homeothermy, by maintaining constant, although low, body temperature. Mammals are also homeothermic, but they differ from Dimetrodon by being endothermic, since they control their body temperature, without depending on the environment, by increasing their metabolism. The same authors noted that early therapsids, an advanced group of synapsids closely related to mammals, possessed long limbs that could dissipate heat as efficiently as the fin of Dimetrodon. The homeothermy developed in this genus and its related ones may have led therapsids to modify their body shape, which eventually led to warm-blooded mammals.

More recent studies of the sail of Dimetrodon and other sphenacodontids support Haack's 1986 claim that the sail was poorly adapted to release heat or regulate body temperature. The presence of sail in small species such asD. milleriand D. teutonisdo not support the idea that the fin's function was thermoregulatory because small fins are less effective at transferring heat and a small body can easily absorb and release heat without additional attachments. Additionally, close relatives of Dimetrodon such as Sphenacodon had small dorsal ridges that certainly did not serve any thermoregulatory function. The large sails of Dimetrodon are thought to have > evolved from those small crests, which means that throughout the existence of this evolutionary trait, thermoregulation did not play a predominant role.

Sexual selection

Larger specimens of Dimetrodon had larger sails in proportion to their size, an example of positive allometry. This characteristic could be beneficial for thermoregulation, since it implies that the larger the animal, the surface of the sail increases faster than its body mass. Animals with larger bodies generate more heat through metabolism and the amount of heat they must dissipate through the body surface is substantially greater than in smaller animals. Effective heat dissipation can be predicted in different animals in a simple way by the relationship between mass and body surface area. However, a 2010 study of allometry in Dimetrodon found a different relationship between the sail's surface area and its body mass: the actual exponent of the sail's scale was much larger than expected in a animal adapted to dissipate heat. The researchers concluded that the Dimetrodon sail showed a growth rate higher than that required to fulfill a thermoregulatory function and that the main reason for the evolution of this trait was sexual selection.

Sexual dimorphism

Dimetrodon could have sexual dimorphism; the AMNH 4636 skeleton (left) can represent the male type and skeleton of the Museum of Natural History of the University of Michigan (right) can represent the female type

Dimetrodon may have had sexual dimorphism, meaning that there was a size discrepancy between the sexes. Some specimens are classified as males, among others, based on the greater thickness of their bones, larger sails and skulls, and wider maxillary gaps. According to these differences, the skeletons at the American Museum of Natural History (AMNH 4636) and the Field Museum of Natural History may be male, and the skeleton at the Denver Museum of Nature and Science (MCZ 1347) and the Denver Museum of Natural History may be male. the University of Michigan may be female.

Paleoecology

Fossils of Dimetrodon are known from finds in North America (United States) and Europe (Germany), regions that were part of the supercontinent Euramerica at the beginning of the Permian. Most of the material attributed to the genus comes from three geological groups in the states of Texas and Oklahoma: the Clear Fork Group, the Wichita Group and the Pearce River Group. Most of the specimens found were part of lowland ecosystems and flat during the Permian, which could have been large swamps. In particular, the Red Beds of Texas are an area with great diversity of tetrapod fossils. In addition to Dimetrodon, the most common tetrapods in the Red Beds and Lower Permian fossil beds in the southeastern United States, are the amphibians Archeria (animal), < i>Diplocaulus, Eryops and Trimerorhachis, the reptilliomorph Seymouria, the reptile Captorhinus and the synapsids Ophiacodon and Edaphosaurus. These tetrapods are part of a group of animals that paleontologist Everett C. Olson called the "Permo-Carboniferous chronofauna," which dominated the ecosystems of the Euramerican continent for a few million years. Based on the geology of deposits similar to the Red Beds, its environment could have been dominated by lowlands with dense river delta-type vegetation.

Food web

Olson made many inferences about the paleoecology of the Texas Red Beds and the role of Dimetrodon within this ecosystem. He proposed some main types of ecosystems where the first tetrapods lived. Dimetrodon assigned it to the most primitive ecosystem, which depended on the aquatic food web. In this model, aquatic plants functioned as the primary producer, constituting the food source for fish and aquatic invertebrates. Most terrestrial vertebrates fed on these aquatic commensals. Dimetrodon was probably an apex predator in its ecosystem, preying on a diverse number of organisms such as the primitive shark Xenacanthus, the amphibians Trimerorhachis and Diplocaulus , and the terrestrial tetrapods Seymouria and Trematops. There are known insects that inhabited this area that were probably to some degree part of the food web of Dimetrodon, feeding on small reptiles such as Captorhinus. The Red Beds were also the habitat of some of the first large terrestrial herbivores such as Edaphosaurus and Diadectes. These herbivores that fed on terrestrial plants did not depend on the aquatic environment for their survival. According to Olson, the closest modern analog to the Dimetrodon ecosystem is the Everglades.

The only species of Dimetrodon found outside the southeastern United States is Dimetrodon teutonis from Germany. Their remains were found in the Tambach Formation at the Bromacker fossil site. This Early Permian tetrapod site is unusual in its paucity of large predatory synapsids. The speciesD. teutoniswas around 1.7 meters long, very small to catch the large diadectid herbivores that populated the area. It is more likely that it fed on small invertebrates and insects. Only three fossil specimens may correspond to large predators; It is believed that they may belong to large varanopids or small sphenacodonts and both to be potential predators ofD. teutonis. In contrast to the swampy lowlands of the Texas Red Beds, the Bromacker deposits are believed to have had a mountainous environment without aquatic species. It is possible that large carnivores were conspicuous by their absence due to their dependence on aquatic amphibians for survival.

Young specimens

Although some species of Dimetrodon could reach large sizes, many young specimens are known. Paleontologist Robert Bakker suggested in 1982 that adults lived mainly in floodplains, while juveniles preferred swamps. isolated areas and the margins of lakes. The type of environment was deduced from the type of sediment in which the remains were found. Bakker thought that young Dimetrodon specimens behaved similarly to young large modern reptiles, which avoid contact with adults of their species. In modern reptiles, animals of all ages can compete for the same resources and adults can turn the young into their prey. However, a later study by Donald Brinkman casts doubt on Bakker's claim by demonstrating that the specimens found in the two types of habitat represented different species and not adult and juvenile members of the same species.

Species

Species	Authority	Location	State	Synonym	Image
Dimetrodon angelensis	Olson, 1962	Texas	Valid
Dimetrodon booneorum	Romer, 1937	Texas	Valid
Dimetrodon cruciger	Cope, 1878	Texas	Synonym Edaphosaurus cruciger
Dimetrodon dollovianus	Case, 1907	Texas	Valid	Embolophorus dollovianus Cope, 1888
Dimetrodon giganhomogenes	Case, 1907	Texas	Valid
Dimetrodon grandis	Romer and Price, 1940	Oklahoma Texas	Valid	Theropleura grandis Case, 1907 Bathyglyptus theodori Case, 1911 Dimetrodon maximus Romer 1936
Dimetrodon incisivus	Cope, 1878	Texas	Synonym Dimetrodon limbatus
?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
Dimetrodon longiramus	Case, 1907	Texas	Synonym Secodontosaurus obtusidens
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
Western Dimension	Berman, 1977	Arizona New Mexico Utah	Valid
Dimetrodon platycentrus	Case, 1907	Texas	Synonym Dimetrodon macrospondylus
Dimetrodon rectiformis	Cope, 1878	Texas	Synonym Dimetrodon limbatus
Dimetrodon semiradicatus	Cope, 1881	Texas	Synonym Dimetrodon limbatus
Dimetrodon teutonis	Berman et al.2001	Germany	Valid