Dimetrodon

Dimetrodon was an extinct genus of sphenacodontid synapsid. It lived during the Cisuralian to early Guadalupian epoch of the Permian period, around 294.1 to 271 million years ago. Most species were about 1.7 to 4.6 m (5.6 to 15.1 ft) long and weighed 28 to 250 kg (62 to 551 lb).

The most noticeable feature of Dimetrodon was the large neural spine sail on its back. The sail was made of long spines that grew up from the vertebrae. It was an obligate quadruped, which means it could only walk on four legs. It had a tall, curved skull with large teeth of different sizes along its jaws. Most of its fossils have been found in the Southwestern United States, mainly in a rock layer called the Red Beds of Texas and Oklahoma. More recently, fossils were also found in Germany. Over a dozen species have been named since the genus was first named in 1878.

Dimetrodon is often mistaken for a dinosaur or shown living alongside dinosaurs in popular culture. However, it died out by the middle of the Permian, about 40 million years before dinosaurs appeared. Although it looked and worked somewhat like a reptile, Dimetrodon is much more closely related to mammals. It belongs to the closest sister family to therapsids, the group that contains the direct ancestor of mammals.

In the past, Dimetrodon was grouped with the paraphyletic group "pelycosaurs." This name is no longer used and has been replaced by names such as "primitive synapsids" or "basal synapsids." Its skull has a single opening (temporal fenestra) behind each eye. All synapsids share this feature, unlike reptiles and birds, which belong to the clade Sauropsida. These two groups had split apart by the Late Carboniferous.

Dimetrodon was probably one of the apex predators of its Cisuralian ecosystems. It ate fish and tetrapods, including reptiles and amphibians. Smaller species may have had different ecological roles. Scientists have suggested that the sail may have helped stabilize the spine, or helped the body warm up and cool down as a form of thermoregulation. However, some recent studies argue that the sail would have been poor at removing heat, since large species had small sails and small species had large sails. The sail was most likely used in courtship display, such as threatening rivals or showing off to possible mates.

History of discovery

The earliest known Dimetrodon fossil was a maxilla found in 1845 by Donald McLeod, who lived in the British colony of Prince Edward Island, now part of Canada. Canadian geologist John William Johnson bought the specimen. In 1854, Joseph Leidy described it as the mandible of Bathygnathus borealis, a large carnivore that was thought to be related to Thecodontosaurus. In 2015 it was renamed Dimetrodon borealis. Because Bathygnathus had been named first, a request to keep the name Dimetrodon was sent to the International Commission on Zoological Nomenclature in 2015. It was approved in 2019.

In the 1870s, American paleontologist Edward Drinker Cope studied fossils that are now called Dimetrodon. He received them, along with other ancient tetrapods, from collectors working in the Red Beds of Texas. The genus name was introduced in 1878.

Later, American paleontologist E. C. Case wrote many studies on Dimetrodon and described several new species in the early 1900s. Starting in the late 1920s, Alfred Romer studied many specimens again and named more species. In the following decades, Dimetrodon fossils were found outside Texas and Oklahoma, including in Utah, Arizona, and Ohio. In 2001, a new species, D. teutonis, was described from Germany. This was the first time Dimetrodon was found outside North America.

Description

Dimetrodon was a quadrupedal synapsid with a sail on its back. It walked with its legs partly spread out, somewhere between a mammal and a lizard. It could also walk more upright, with its body and most of its tail lifted off the ground. Most species were 1.7 to 5 m (6 to 16 ft) long and weighed between 28 and 250 kg (60 and 550 lb).^{: 470} The smallest known species, D. teutonis, was about 60 cm (24 in) long. The larger species were among the biggest predators of the Early Permian. The closely related Tappenosaurus is known from skeleton pieces in slightly younger rocks. It may have been even larger, at about 5.5 metres (18 ft) long.

Its tall skull was narrow from side to side, with its eye sockets set high and far back. Behind each eye socket was one open hole, called an infratemporal fenestra. A pineal foramen lay between the parietal bones. The back of the skull, the occiput, tilted slightly upward, as in all early synapsids. The front tip of the upper jaw, made by the premaxilla, rose above the rest of the jaw to form a step. There was a diastema gap in the row of teeth. Each side of the back of the skull had a single large opening. This linked it to mammals and set it apart from most of the earliest sauropsids, which either lack openings or have two openings. Its nasal cavity was transitional between those of early four-limbed land-dwelling vertebrates and mammals. Its nasoturbinals were much smaller than those of later synapsids. The jaw joint also showed a change. The articular and quadrate bone were starting to change. Later, in mammal ancestors, they became part of the malleus bone of the middle ear.

The teeth were very different in size along the jaws. This gave Dimetrodon its name, which means "two measures of tooth." Large, pointed, canine-like teeth, called caniniforms, extended from the maxilla. Teardrop-shaped teeth were unique to Dimetrodon and other closely related sphenacodontids. Most species had serrated tooth edges, with tiny notches so fine they looked like small cracks. The dinosaur Albertosaurus had similar notches, but each one had a round void at its base. This spread force over a larger surface area, which Dimetrodon did not have. A 2014 study suggested that Dimetrodon was in an arms race with its prey. Smaller species had no serrations. Larger species developed sharper, serrated teeth. For example, D. limbatus had serrations that looked like those of Secodontosaurus.

The sail was made of long neural spines that stuck up from the backbones. The lower, proximal part of each spine was rough and held the epaxial muscles of the back. The upper, distal part was smoother. The tail was long, with around 50 caudal vertebrae. In 2025, scientists described scaly body impressions from Dimetrodon teutonis found in the Tambach Formation in Germany. These showed that early synapsids likely had scaly skin. Later synapsids, such as Estemmenosuchus, had bare, glandular skin. Even later, mammals evolved hair. Ascendonanus was first classified as a varanopid synapsid. A recent study placed it in a separate group that is closer to diapsid reptiles.

Classification

See also: Synapsid § Linnaean and cladistic classifications

Dimetrodon is an early member of a group called synapsids. This group includes mammals and their extinct relatives. Dimetrodon is not an ancestor of any mammal, because mammals appeared millions of years later. Many people think it was a dinosaur, but it died out about 40 million years before the first dinosaurs appeared in the Triassic period. Because it is a synapsid, it is more closely related to mammals than to dinosaurs or any living reptile. In the early 1900s, most scientists called it a reptile under Linnean taxonomy. This system placed reptiles in a class. Mammals were put in a separate class, so Dimetrodon was called a "mammal-like reptile." Scientists thought mammals had evolved from this group.

Under phylogenetic systematics, if the descendants of the last common ancestor of Dimetrodon and all living reptiles were grouped together, they would include all mammals. To avoid this, scientists split the descendants of the common ancestor of mammals and reptiles. This ancestor lived around 310 million years ago, in the Late Carboniferous. Synapsida includes Dimetrodon and mammals. Sauropsida includes living reptiles and extinct reptiles that are more closely related to them than to mammals. Within Synapsida, Dimetrodon belongs to Sphenacodontia. Paleontologists Alfred Romer and Llewellyn Ivor Price first proposed this group in 1940. They also proposed Ophiacodontia and Edaphosauria. They mainly compared body features such as limbs and backbones, called postcranial features. In 1986, paleontologist Robert R. Reisz suggested a newer view. It still places Dimetrodon among the sphenacodonts within the wider phylogeny. In this view, varanopids are seen as more basal synapsids outside Sphenacodontia. Sphenacodontia contains Sphenacodontoidea. This includes Sphenacodontidae, the group with Dimetrodon and other sail-backed relatives such as Sphenacodon and Secodontosaurus. Sphenacodontia also includes Therapsida, which contains mammals and their relatives. A cladogram of Synapsida from an analysis by Michael J. Benton in 2012 shows how these groups are related.

The cladogram below shows how a few Dimetrodon species are related. It comes from Brink and colleagues (2015).

Species

Main article: Species of Dimetrodon

About twenty species of Dimetrodon have been named. Around 13 are currently recognised:

• †D. angelensis Olson, 1962
• †D. booneorum Romer, 1937
• †D. borealis (Leidy, 1854) Brink, Maddin, Evans, & Reisz, 2015
• †D. dollovianus (Cope, 1888) Case, 1907
• †D. giganhomogenes Case, 1907
• †D. grandis (Case, 1907) Romer and Price, 1940
• †D. limbatus (Cope, 1877) Romer and Price, 1940
• †D. loomisi Romer, 1937
• †D. macrospondylus (Cope, 1884) Case, 1907
• †D. milleri Romer, 1937
• †D. natalis (Cope, 1878) Romer, 1936
• †D. occidentalis Berman, 1977
• †D. teutonis Berman, Reisz, Martens, & Henrici, 2001

In 1940, Alfred Romer and Llewellyn Ivor Price created a classification system for all the Dimetrodon species known at the time. It had two "series." Series A included larger species with short bodies and short lower legs, such as D. grandis. Series B included smaller species with shorter, lower skulls, such as D. loomisi.^{: 80}

Paleobiology

Scientists have suggested many purposes for the large sail on the back of Dimetrodon. Some early ideas were that the sail helped it hide among reeds while waiting for prey, or that it worked like a boat's sail to catch the wind in water. Another idea is that the long spines helped keep the body steady while it walked from side to side. In 1940, Alfred Romer and Llewellyn Ivor Price proposed that the sail helped the animal warm its body in the Sun. Later studies mostly suggest two uses: warming up quickly in the morning and cooling down when the body got hot. Dimetrodon was probably cold-blooded. Because it was large, its body temperature changed slowly, which is called thermal inertia. In 1986, J. Scott Turner and C. Richard Tracy suggested that the sail helped it keep a steady, if low, body temperature, which is called being homeothermic. Mammals also keep a steady temperature, but they do it by making their own heat, which is called being endothermic.

Recent studies suggest that the sail was not very good at controlling body temperature. Small species such as D. milleri and D. teutonis had sails too, and close relatives such as Sphenacodon had only very low crests. Even so, Dimetrodon and other sphenacodontids probably had a fast energy metabolism (tachymetabolism), which may have helped them keep a high, steady body temperature. In larger Dimetrodon individuals, the sails grew faster than their bodies, an example of positive allometry. A 2010 study concluded that this faster growth was too great to be only about heat. It suggested that the way animals choose mates was the main reason the sail evolved. The sail's growth is similar to the way antlers grow in deer. Males and females may have looked different, so Dimetrodon may have been sexually dimorphic. Some skeletons, such as one at the American Museum of Natural History (AMNH 4636), may be males, while others, including ones at the Denver Museum of Nature and Science (MCZ 1347), may be females.

Fossils of Dimetrodon have been found in the United States (Texas, Oklahoma, New Mexico, Arizona, Utah and Ohio), in Prince Edward Island in Canada, and in Germany. These areas were part of the supercontinent Euramerica during the Early Permian. Almost all American finds come from three geological groups in north-central Texas and south-central Oklahoma: the Clear Fork Group, the Wichita Group, and the Pease River Group. Most of these fossils come from lowland areas that were probably large wetlands during the Permian. The Red Beds of Texas are especially rich in fossil tetrapods, which are four-limbed animals with backbones. Along with Dimetrodon, common animals there included the amphibians Archeria, Diplocaulus, Eryops, and Trimerorhachis, the reptiliomorph Seymouria, the reptile Captorhinus, and the synapsids Ophiacodon and Edaphosaurus. Paleontologist Everett C. Olson called this group of animals the "Permo-Carboniferous chronofauna." This fauna dominated the Euramerican ecosystem for several million years. It is thought to have lived in a well-vegetated lowland deltaic ecosystem.

Food web

Olson studied the Texas Red beds and the role Dimetrodon played there. He described several main types of ecosystems. Dimetrodon belonged to the most primitive one, which grew out of water-based food webs. In that ecosystem, water plants were the primary producers. They were mostly eaten by fish and water invertebrates, which in turn were eaten by most land animals. Dimetrodon was probably the top predator of the Red Beds. It ate animals such as the shark Xenacanthus (which may also have preyed on Dimetrodon), the amphibians Trimerorhachis and Diplocaulus, and the land tetrapods Seymouria and Trematops. Insects were probably part of the same food web, feeding small reptiles like Captorhinus. Some of the first large land-living plant-eaters, like Edaphosaurus and Diadectes, also lived there. Olson said the best modern comparison for this ecosystem is the Everglades. Scientists have long debated whether Dimetrodon lived on land or in water. Its bone structure supports a land lifestyle, which suggests it mostly fed on land, on the banks, or in very shallow water. There is also evidence that it hunted aestivating Diplocaulus during droughts. Three young Diplocaulus were found in a burrow of eight, and the burrow had tooth marks from a Dimetrodon that had dug them out. The only Dimetrodon species found outside North America is D. teutonis, from the Tambach Formation in Germany, at a site called the Bromacker locality. It is estimated to have weighed only 14 kilograms (31 lb). It was too small to hunt the large diadectid plant-eaters common there, so it more likely ate small animals and insects. The few large predators found there were probably either large varanopids or small sphenacodonts, which could have hunted it. Unlike the lowland Red Beds, the Bromacker area seems to have been upland with no water animals. Large meat-eaters there may have depended on large water amphibians for food.

Extinction

Dimetrodon, like most "pelycosaurs", had a big drop in its numbers or died out completely during Olson's Extinction. Experts have argued about whether this event was real. Some thought the drop happened because the main fossil sites moved from the palaeo-equatorial to the palaeo-temperate regions. However, newer studies by Brocklehurst and colleagues found signs of changes in animal groups, so a move in fossil sites seems unlikely. The extinction also did not happen the same way everywhere. It was slower in North America than in Russia. Another study found that Dimetrodon and several "pelycosaurs" groups stopped growing or even shrank for 20 million years. Earlier, during the Pennsylvanian, they had grown quickly.

About 278.9 million years ago, during the mid-Kungurian extinction event, six to eight Dimetrodon species died out. Volcanoes were active in several regions at the same time. Scientists also think that melting ice sheets in Gondwana may have released carbon from methane hydrates. The last Dimetrodon species, D. angelensis, lived on into the Middle Permian. Because it had survived Olson's Extinction, it was called a "dead clade walking".