Dimetrodonlimbatus

Overview: Despite historically featuring in many dinosaur books and toy sets, Dimetrodon is not a dinosaur. It is actually a synapsid, part of the mammal lineage of land animals which split from the reptile lineage about 318 million years ago, thus it is not a close relative of dinosaurs. Nor did it live alongside them, ruling over a strange ecosystem that existed over 30 million years before the first dinosaurs evolved. It was an apex predator feeding on other synapsids, reptiles, amphibians, and possibly fish, aided by its unique dentition which was already becoming more mammal-like, including early precursors to incisors and canines, despite the fact that it would be some 100 million years before distant relatives of Dimetrodon would evolve into first true mammals. The most distinctive feature of Dimetrodon is its large sail, which was probably used for display, and which may be the primary reason for its mistaking as a dinosaur, as it superficially resembles sail backed dinosaurs like Spinosaurus.

Discovery: The first Dimetrodon fossil ever found was a maxilla found in Prince Edward Island by Donald McLeod in 1845 and described by American paleontologist Joseph Leidy in 1854. However, this fossil was not identified as Dimetrodon until 2015. Dimetrodon limbatus, the best-known species of Dimetrodon was described by American paleontologist of Bone Wars fame Edward Drinker Cope in 1877 as a species of Clepsydrops, an ophiacodont synapsid from the Carboniferous of Illinois, also named by Cope. The next year, Cope named Dimetrodon based on three species, Dimetrodon incisivus, Dimetrodon rectiformis, and Dimetrodon gigas. Dimetrodon means “two measure tooth”, named for its different and distinct tooth types. In 1940, Alfred Romer reclassified “Clepsydrops” limbatus as Dimetrodon limbatus and made it the type species of the genus. Most Dimetrodon limbatus remains come from Texas and Oklahoma, with additional remains which may belong to D. limbatus also known from the Washington Formation of Ohio. Several other species of Dimetrodon are also known from Texas and Oklahoma. One species, D. occidentalis, is known from New Mexico, Arizona, and Utah. Another species, D. borealis, is known from Prince Edward Island. And one more species, D. teutonis, is known from Germany. Dimetrodon is known from numerous good remains including juveniles through adults.

Evolution: Dimetrodon was a synapsid, the group which includes all land animals closer to mammals than to reptiles. Synapsids split from the reptilian lineage in the Carboniferous Period, around 318 million years ago. By 295 million years ago, synapsids had diversified into several groups and had become the dominant animals on land. Dimetrodon is a member of a broad group called Eupelycosauria, which includes most synapsids, including the herbivorous sail-back Edaphosaurus. Within Eupelycosauria, Dimetrodon belongs to Sphenacodonta, a large clade of mostly carnivorous forms, including the mammal-like therapsids, from which mammals would eventually evolve, and the sail-backed sphenacodontids, which included Dimetrodon. While therapsids would come to dominate the Middle and Late Permian, sphenacodontids like Dimetrodon were the top predators in the Early Permian and early Middle Permian. Though they are relatives of the therapsids, from which mammals are the living descendants, the sphenacodonts themselves left no descendants.

Description: Dimetrodon outwardly resembled a large reptilian: quadrupedal, with a long tail, short neck, and short sprawling legs and five-fingered hands and feet. However, it had a distinctively shaped deep, boxy skull with unique teeth. Unlike most reptiles, Dimetrodon had different shapes and sizes of teeth for different tasks, precursors to the extreme heterodonty seen in mammals. Despite its name meaning “two measure tooth”, Dimetrodon actually had three distinct tooth types, thus “Trimetrodon” may have been a more accurate name. In the front of its robust jaws, it had a type of early incisiform teeth. Behind these incisiforms on the top jaw was a distinctive notch between the maxilla and premaxilla bones. Near the front of the maxillae were one or two pairs of large, robust caniform teeth, the precursors to the true canines of therapsids and mammals. The back of the jaws featured smaller more blade-like teeth for slicing flesh. Uniquely, the teeth of Dimetrodon and its close relatives are widest at their midline and narrow toward the root, making them “teardrop” shaped, which is a powerful identifying aid.

Dimetrodon’s most distinctive feature was its large sail, formed by hyper-elongated vertebral spines along its back and a skin membrane stretched between them. Many scientists have put forward hypotheses about what Dimetrodon’s sail was used for. One theory that used to be popular was that the sail was used for temperature regulation. However, more recent studies have cast doubts on this, pointing out that if it was for temperature control, then the sail size should correlate differently with body size than it does. Additionally, non-sail-backed synapsid predators in the same ecosystem also seem to have been successful in their environment, suggesting the sail did not grant a disproportionate survival advantage. What’s more, Dimetrodon and other early synapsids may have had a mild degree of endothermy, and thus would have had less need for external sources of temperature control. The current hypothesis is that, like most weird animal structures today, the sail was probably used for display, either to intimidate rivals or to attract mates. Thus, the sails were likely colorful and may have born distinctive patterns. There is even evidence that male and female Dimetrodons had different sail shapes, providing further evidence that the sails were used as mating displays.

Little is known about the skin of Dimetrodon as no skin impressions from it exist. More derived therapsids had smooth glandular skin, like a hairless mammal. But early synapsids may have had scales like a reptile, with some even having large belly scales like a crocodile. As an earlier synapsid, Dimetrodon probably have more reptilian skin, but until skin impressions are found, we can’t know for sure.

Dimetrodon as a genus had a large range of sizes. The smallest species, the German D. teutonis was only 60 cm (24 in) in length and weighed 14 kg (31 lb). The largest were 4.6 m long and may have weighed over 250 kg (550 lb). Dimetrodon limbatus grew to about 9 m (9.8 ft) in length, similar to a modern Komodo Dragon, though Dimetrodon was a bit heavier built, and probably weighed somewhere in the ballpark of 200 kg (440 lb).

Ecology: Dimetrodon lived in lowland deltaic environment, with Early Permian Texas and Oklahoma somewhat resembling today’s Everglades in Florida, save the plants were completely different (for one, grass wouldn’t evolve until the Cretaceous Period). For some time it was debated as to whether Dimetrodon was semiaquatic like an alligator or a fully terrestrial predator. However study of bone microanatomy, plus the shape of its skull and teeth, suggest a fully terrestrial lifestyle. However it likely preyed upon a wide variety of animals, including the large sail-backed synapsid herbivore Edaphosaurus, the small reptile Captorhinus, and amphibians such as Diplocaulus. Direct fossil evidence exists for Dimetrodon’s predation upon Diplocaulus in the form of a burrow containing the remains of eight juvenile Diplocaulus, three of which were partially eaten and had tooth marks matching the teeth of Dimetrodon. It is likely that the Diplocaulus were aestivating underground during the dry season and were dug up and partially eaten by a Dimetrodon. These amphibians may have been a highly convenient prey item during lean times. Dimetrodon probably competed with other large predators for prey. These other predators included the giant amphibian Eryops and the large synapsid Ophiacodon, both of which could reach similar sizes to Dimetrodon.

Extinction and Legacy: At the end of the Early Permian, about 273 million years ago, a minor, but notable extinction event is thought to have taken place, called Olson’s Extinction, named after Everett C. Olson, who first identified the gap in fossil record suggesting a sudden shift between the Early and Middle Permian faunas. The causes of Olson’s extinction are not fully known, nor is it definitively known to what extent it was a distinct event, but it may have been caused by climate change, with some evidence that the climate became very hot and water became more acidic. These factors certainly would have affected freshwater wetlands like those in which Dimetrodon lived. The aftermath of this extinction saw the decline and extinction of non-therapsid synapsids, and the rise of the therapsids. While Dimetrodon seems to have survived the initial decline of its kind, it ultimately went extinct about 270 million years ago. The role of dominant predator would after be filled by therapsid predators such as the dinocephalians, therocephalians, and gorgonopsids. Today, Dimetrodon can be seen in many museums. A few of them include the American Museum of Natural History in New York City, New York, USA, the Field Museum of Natural History in Chicago, Illinois, USA, the Museum of Ancient Life at Thanksgiving Point in Lehi, Utah, USA, the Royal Tyrrell Museum in Drumheller, Alberta, Canada, and the BYU Museum of Paleontology in Provo, Utah, USA. Dimetrodon was featured in the 2022 film Jurassic World: Dominion where it was incorrectly shown/implied to be semi-aquatic and portrayed with an incorrect head design which resembles something like a Tyrannosaur-Dimetrodon hybrid. A more accurate Dimetrodon was featured in the 2005 BBC documentary Walking With Monsters: Life Before Dinosaurs.