Dimetrodon: The Sail-Backed Synapsid of the Early Permian

Dimetrodon was a large sail-backed predator that lived between about 295 and 272 million years ago, in the early Permian Period, mainly in what is now Texas and Oklahoma. Often found in sets of toy dinosaurs, it was not a dinosaur at all but a synapsid, an early member of the lineage that eventually produced mammals. As one of the first large land predators, it gives a rare view of life on land long before the age of dinosaurs.

Background: the early Permian world

The Permian Period began about 299 million years ago, when the continents had joined into the supercontinent Pangaea. The region that is now Texas lay near the equator, with a warm climate that grew increasingly seasonal and dry.

Rivers, ponds and floodplains supported a rich community of animals. Large amphibians such as Eryops hunted in the water, alongside the strange Diplocaulus, with its boomerang-shaped head, and freshwater sharks. On land lived early reptiles and many kinds of synapsids, both meat-eaters and plant-eaters. At the top of this food web was Dimetrodon.

The Texas Red Beds have become one of the richest sources of early Permian land vertebrates in the world. Their bones were preserved in river channels and floodplain muds, often together with fossil plants and burrows. Because many species are found in the same rock layers, palaeontologists can reconstruct whole communities and follow how they changed as the climate became drier during the early Permian.

Discovery and naming

The first Dimetrodon fossils were collected in the 1870s from the Red Beds of north-central Texas, a series of reddish early Permian rocks. Some of the material was gathered by the naturalist Jacob Boll, working for the American palaeontologist Edward Drinker Cope.

Cope named Dimetrodon in 1878. The name means “two measures of teeth”, referring to its teeth of two different sizes: large, fang-like teeth at the front and smaller ones further back. Over the following decades many more specimens and species were found, especially in Texas and Oklahoma.

In the 20th century the Harvard palaeontologist Alfred Sherwood Romer studied Dimetrodon and its relatives in depth. His review of the group with Llewellyn Price, published in 1940, remained a standard reference for decades.

A surprising find came from Europe. In 2001 a small species, Dimetrodon teutonis, was described from the Bromacker quarry in Thuringia, Germany. It shows that Dimetrodon lived across a wider part of Pangaea than once thought.

What Dimetrodon was like

Dimetrodon walked on four sprawling legs and had a large, deep skull with powerful jaws. Its most striking feature was a tall sail on its back, supported by greatly lengthened spines growing from the vertebrae, which were probably covered in skin.

Species differed greatly in size. The smallest, such as D. teutonis, were only around 60 centimetres long. The largest, such as D. grandis and D. angelensis, are estimated to have reached roughly 3 to 4 metres in length, with some weight estimates up to about 250 kilograms.

Its teeth were advanced for its time. A 2014 study by Kirstin Brink and Robert Reisz found that some Dimetrodon species had serrated, blade-like teeth similar to those of later meat-eating dinosaurs. This makes Dimetrodon the earliest known land animal with such teeth, and shows that its teeth changed over time, probably in response to shifts in its prey and environment.

A relative of mammals

Dimetrodon belongs to the synapsids, a group defined by a single opening in the skull behind each eye. This opening anchored jaw muscles and is still found, much modified, in mammals today. Reptiles, including dinosaurs and birds, belong to a separate branch called sauropsids.

Older books grouped Dimetrodon among the “pelycosaurs”, a name for early synapsids, and often called them “mammal-like reptiles”. Scientists now avoid that term, because synapsids were never reptiles: the two lineages split around 320 million years ago.

Dimetrodon was a member of the Sphenacodontidae, a family of meat-eating synapsids. Close relatives of the sphenacodonts gave rise to the therapsids, the group from which mammals eventually evolved. Dimetrodon was not a direct ancestor of mammals, but it shows features that appeared early on the line leading to them, such as differentiated teeth.

The sail debate

The purpose of the sail has been argued about for more than a century. A long-standing idea was that it helped regulate body temperature. Turned towards the morning sun, it might have warmed the animal quickly; turned away or into the wind, it could have shed heat.

Later studies questioned this. Research published in 2010 suggested that small Dimetrodon species had sails too small to be effective for temperature control, and that the sail’s growth pattern did not match what heat regulation would require. Many researchers now favour display, such as signalling to rivals or attracting mates, possibly together with a minor role in temperature control.

A sail evolved separately in another early synapsid, the plant-eating Edaphosaurus, showing that such structures arose more than once.

Myths and facts

The most common misconception is that Dimetrodon was a dinosaur. It became extinct around 272 million years ago, while the earliest dinosaurs appeared only around 230 million years ago. The gap of roughly 40 million years is about two-thirds of the time that has passed since the dinosaurs died out.

Dimetrodon is also sometimes called a reptile. Although it looked reptilian, it was a synapsid and was more closely related to mammals, including humans, than to lizards, crocodiles or dinosaurs.

Extinction and legacy

Dimetrodon disappears from the fossil record around 272 million years ago, in the middle Permian. Its extinction was part of a broader turnover in which the early synapsids gave way to therapsids, which became the dominant land animals in the later Permian. Many therapsids were themselves wiped out in the Permian–Triassic extinction about 252 million years ago, but one group survived to give rise to mammals.

Today Dimetrodon skeletons are displayed in major museums, including the Smithsonian’s National Museum of Natural History and the American Museum of Natural History. It remains a favourite of museums and toy makers, and a useful teaching example of how life on land looked before the dinosaurs.

By the numbers

Timeline

• c. 299 million years ago: The Permian Period begins.
• c. 295 million years ago: The earliest Dimetrodon species appear.
• c. 272 million years ago: The last Dimetrodon disappear in the middle Permian.
• c. 252 million years ago: The Permian–Triassic extinction ends the Permian Period.
• 1878: Edward Drinker Cope names Dimetrodon from Texas fossils.
• 1940: Romer and Price publish their review of the pelycosaurs.
• 2001: Dimetrodon teutonis is described from Germany.
• 2014: Brink and Reisz describe serrated teeth in Dimetrodon.

Why it matters

Dimetrodon is one of the best-known animals from before the age of dinosaurs. As an early relative of mammals, it helps scientists trace how features such as specialised teeth and powerful jaws evolved on the long road to mammals, including humans. Its familiar sail also makes it an effective way to show the public that the history of life is far older and richer than the dinosaurs alone.