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 measured 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, made of long spines that stretched up from the vertebrae. It was an obligate quadruped, which means it could walk only on four legs. It had a tall, curved skull with large teeth of different sizes along its jaws. Most of its fossils were 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 became extinct by the middle Permian, about 40 million years before dinosaurs appeared. Although it looked and functioned 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 This term is now considered obsolete and has been replaced by names such as "primitive synapsids" or "basal synapsids." Its skull has a single opening (temporal fenestra) behind each eye, a feature shared by all synapsids, unlike the skulls of 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 fed on 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 heat 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 were found with small sails and small species with large sails. The sail was most likely used in courtship display, such as threatening rivals or showing off to potential 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 thought to be related to Thecodontosaurus. In 2015 it was reclassified as Dimetrodon borealis. Because Bathygnathus had been named first, a petition to keep the name Dimetrodon was sent to the International Commission on Zoological Nomenclature in 2015 and approved in 2019. In the 1870s, American paleontologist Edward Drinker Cope studied fossils now assigned to Dimetrodon. He got 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. Beginning in the late 1920s, Alfred Romer restudied many specimens and named more species. In the decades after that, 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, which was the first time Dimetrodon was found outside North America.

Description

Dimetrodon was a quadrupedal synapsid with a sail on its back. It most likely had a semi-sprawling posture, somewhere between that of a mammal and a lizard, and it could also walk more upright with its body and most or all of its tail off the ground. Most species ranged from 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 largest predators of the Early Permian. 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. Its tall skull was compressed from side to side, with eye sockets set high and far back. Behind each eye socket was a single hole called an infratemporal fenestra, and a pineal foramen lay between the parietal bones. The back of the skull, the occiput, angled slightly upward, as in all early synapsids. The tip of the upper jaw, formed by the premaxilla, rose above the rest of the jaw to make a step, with a diastema gap in the tooth row. A single large opening on each side of the back of the skull 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, and the jaw joint showed a change from the articular and quadrate bone, which later became part of the malleus bone of the middle ear in mammal ancestors.

The teeth varied greatly in size along the jaws, which gave Dimetrodon its name, meaning "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 serrations so fine they looked like tiny cracks. The dinosaur Albertosaurus had similar serrations, but it had a round void at the base of each one that spread force over a larger surface area, which Dimetrodon lacked. A 2014 study suggested that Dimetrodon was in an arms race with its prey. Smaller species had no serrations, while larger species developed sharper, serrated teeth, such as D. limbatus, whose serrations resembled those of Secodontosaurus. The sail was formed by elongated neural spines projecting from the vertebrae. The lower, proximal part of each spine was rough and anchored the epaxial muscles of the back, while the upper, distal part was smoother. The sail's tail was long, with around 50 caudal vertebrae. Scaly body impressions from Dimetrodon teutonis were described in 2025 from the Tambach Formation in Germany, showing that early synapsids likely had scaly skin. Later synapsids, such as Estemmenosuchus, developed bare, glandular skin, and later mammals evolved hair. Ascendonanus was first classified as a varanopid synapsid, but a recent study placed it in a separate group closer to diapsid reptiles.

Classification

See also: Synapsid § Linnaean and cladistic classifications

Dimetrodon is an early member of a group called synapsids, which includes mammals and their extinct relatives. It is not an ancestor of any mammal, because mammals appeared millions of years later. Many people mistake it for 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, which ranked Reptilia as a class. Mammals were placed in a separate class, and Dimetrodon was called a "mammal-like reptile." Scientists thought mammals had evolved from this group during a reptile-to-mammal transition.

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, the descendants of the common ancestor of mammals and reptiles, which appeared around 310 Ma in the Late Carboniferous, are split into two groups. Synapsida includes Dimetrodon and mammals, while 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, along with Ophiacodontia and Edaphosauria, mainly by comparing postcranial features such as limbs and vertebrae. In 1986, paleontologist Robert R. Reisz proposed a newer view, which still places Dimetrodon among the sphenacodonts within the wider phylogeny. In this view, varanopids are now seen as more basal synapsids outside Sphenacodontia. Sphenacodontia contains Sphenacodontoidea, which holds Sphenacodontidae, the group with Dimetrodon and other sail-backed relatives such as Sphenacodon and Secodontosaurus, along with Therapsida, which includes mammals and their relatives. A cladogram of Synapsida from an analysis by Michael J. Benton in 2012 shows how these groups are related.

The below cladogram shows the relationships of a few Dimetrodon species, from Brink and colleagues (2015).

Species

Main article: Species of Dimetrodon

Around twenty species of Dimetrodon have been named, and 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 torsos and short lower limbs, such as D. grandis. Series B included smaller species with shorter and 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 while the animal was in water. Another idea is that the long spines held the body steady, which may have made walking side to side more efficient. 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 became hot. Dimetrodon was probably cold-blooded, and its large size meant that 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 stay at a steady, if low, body temperature, which is called being homeothermic. Mammals also keep a steady temperature, but they do it by creating 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 high energy metabolism (tachymetabolism), which may have helped them keep a high, stable body temperature. Larger Dimetrodon individuals had sails that 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, and suggested that the way animals choose mates was the main reason for the sail's evolution. 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 vast 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 include 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 and 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, and Dimetrodon belonged to the most primitive one, which grew out of aquatic food webs. In that ecosystem, aquatic plants were the primary producers, and they were mostly eaten by fish and aquatic invertebrates, which in turn were eaten by most land animals. Dimetrodon was probably the top predator of the Red Beds, eating 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, but 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, with three partly eaten young Diplocaulus found in a burrow of eight that bore 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), 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, so large meat-eaters may have depended on large water amphibians for food.

Extinction

Dimetrodon, as well as most "pelycosaurs", saw a major drop in diversity or total extinction as a result of Olson's Extinction. Experts have debated whether this event was real. Some suggested the drop was due to the main fossil sample locations shifting from the palaeo-equatorial to the palaeo-temperate regions. However, more recent reviews by Brocklehurst and colleagues found evidence of changes in animal groups, which made a shift in sample locations unlikely. The extinction was also not the same everywhere. It was more gradual in North America than in Russia. Another analysis found that Dimetrodon and several "pelycosaurs" groups stopped growing or even declined for 20 million years, unlike their rapid growth during the Pennsylvanian.

Six to eight Dimetrodon species went extinct during the mid-Kungurian extinction event, about 278.9 million years ago. This event happened at the same time as volcanic activity in several regions. The melting of the Gondwanan ice sheet was also thought to have released light carbon from destabilized methane hydrates. The last Dimetrodon species, D. angelensis, lived on into the Middle Permian. As a survivor of Olson's Extinction, however, it was described as a "dead clade walking".