photo of dimetrodon

Restoration of Dimetrodon grandis by Bogdanov. CC BY-SA 3.0

Dimetrodon milleri: An Unexpected Relative

The Dimetrodon milleri specimen at the Harvard Museum of Natural History. Credit: © President and Fellows of Harvard College.

Nearly 300 million years ago, in a vast, sprawling swamp, one of the world’s first great predators exhaled its final breath and perished in the mud. It was the beginning of the Permian Period, and the planet was starkly different from today. All the world’s landmasses were joined together in a massive supercontinent called Pangea. Earth was in the grip of an ice age, its polar regions covered by massive glacial sheets and its equatorial girdle mostly dry due to extreme seasonal fluctuations between wet and dry seasons, that filled and drained massive landscapes like the predator’s swamp.  


The Name

Dimetrodon, which means “two measures of teeth,” was aptly named for its toothy grin. This genus of top predators used their mouth full of both large and small, conical teeth to grab, grip, stab, and tear the flesh of its prey–usually the large amphibians of their day. Dimetrodon species are part of a whole family known for their dentition—the Sphenacodontidae, which means “wedge-point tooth.”

Edward Drinker Cope 1840-1897. Credit: Public Domain

Dimetrodon was first described in the 1870s by legendary paleontologist Edward Drinker Cope. Cope was responsible for naming over 1,000 fossilized vertebrate species, and was one of the greatest driving forces behind the “Great Dinosaur Rush,” a significant period of advancement in the field of paleontology that was scarred by an intense rivalry between Cope and Yale paleontologist Othniel Charles Marsh. In 1875, Cope initially described the first Dimetrodon, a specimen obtained from the Red Beds of Texas, as Clepsydrops limbatus, placing it in a different, but related genus to Dimetrodon. In 1878, Cope obtained three other specimens from the Red Beds, and decided he was dealing with a new genus altogether, which he named Dimetrodon. Two years later, he renamed the original specimen Dimetrodon limbatus.


Dimetrodon and other fossil species tend not to have common names, but some have nicknames. Perhaps the most famous fossil nickname is the Tyranosaurus rex specimen “Sue,” the most complete T. rex ever found and named after its discoverer, paleontologist Sue Hendrickson. Curators from the Houston Museum of Natural Science named a recently discovered Dimetrodon loomisi, "Laslow."

What Did Dimetrodon Look Like?

Much of Dimetrodon’s appearance remains a mystery. The modern world may never know a great deal about the texture or color of the animal’s skin, though some speculate that it was smooth and porous like its close relative, Estemmenosuchus, from which skin imprints have been discovered. There is also some evidence that pelycosaurs, like Dimetrodon, might have had belly scales.

Dimetrodon angelensis. Credit: Illustration by Dmitry Bogdanov. CC BY-SA 3.0

The most distinctive characteristic of Dimetrodon is the large vertical sail that protrudes from its back. The sail is made up of long, pointed spines that extended from the animal’s vertebrae, and were likely connected by skin rich in blood vessels. The most popular explanation for the sail is that Dimetrodon used it to regulate its body temperature, although some scientists speculate that it was used for sexual display. Dimetrodon also had a long tail and sprawling limbs posture characteristic of sphenacodontids.

Over time, Dimetrodon species became larger. The earliest species, Dimetrodon milleri, which lived nearly 300 million years ago, was also the smallest, growing just short of 2 m/6.5 feet from tip-to-tail. During the later part of the early Permian, Dimetrodon species became formidably large, reaching over 4.5 m/147.6 feet. Scientists believe that the size increase might have been due to changes in their environment that allowed prey to grow much larger. The increasing size may have become an evolutionary "arms race" between predator and prey.

Why Isn’t Dimetrodon a Dinosaur?

Despite its almost crocodilian stature, Dimetrodon was neither a reptile nor a dinosaur. Dimetrodon appeared 60 million years before the first dinosaurs and is only distantly related to reptiles. Reptiles, including crocodiles, dinosaurs, and birds belong to a group known as the Sauropsida, or “lizard faces,” while Dimetrodon belongs to a group known as Synapsida, which includes mammals. These two major groups split from a common ancestor about 320 million years ago. Dimetrodon, in fact, is more closely related to humans than it is to dinosaurs.

Derivative work from Peter Bockman. CC BY-SA 3.0

Synapsids can be distinguished from sauropsids by their skulls. In Greek, synapsida means “fused arch,” which refers to a bony arch formed by an opening in their skull behind each eye. Mammals have this arch too. Dimetrodon and its relatives are most often referred to as non-mammalian synapsids—the predecessors to mammals. Fossil evidence indicates that they flourished as the dominant terrestrial vertebrates in Permian ecosystems, but were also quite successful during the Triassic period.

Classification Tree

Classification tree. Credit: Derivative work from Dmitry Bogdanov. CC BY-SA 3.0

Mammals today are quite different from their ancestors. Modifications in posture allowed them to move more upright like a dog and less sprawled out like an alligator. This led to changes in their internal anatomy that enabled them to move for sustained periods as opposed to short bursts. Adaptations of the skull allowed for larger brains and more specialized jaws. Lactation and the ability to suckle allowed mammals to be born at less fully developed stages than their ancestors. These unusual features are distinct from those of the non-mammalian synapsids that predated them, yet these distinctions are critical to guiding our understanding of mammal evolution.

Did you know? Dimetrodon wasn’t the only species of the period that had large sails. The Edaphosaurus was an herbivorous synapsid that lived at the same time and evolved the sail independently. One easy way to tell the two sails apart is that Dimetrodon spines are smooth, while Edaphosaurus spines have humps. 

Type Specimens - Video

The Dimetrodon milleri specimen housed at the Harvard Museum of Natural History is one of the oldest and most complete Dimetrodon specimens known. Scientists come from all over the world to Harvard to study this special “type” specimen, even though it has been a part of the scientific record for more than 80 years. Dr. Stephanie Pierce, Curator of Vertebrate Paleontology at the Harvard Museum of Comparative Zoology, discusses the importance of natural history collections. 

I'm Your Type

Learn More


Encyclopedia of Life Dimetrodon  web page

Biodiversity Heritage Library Dimetrodon Resources


The story of the discovery of “Laslow", Texas Observer: Big Dig 


The first of three videos on Harvard Professor Stephanie Pierce’s work on the origin of mammal movement.

A video from the Field Museum, Chicago, on why Dimetrodon was not a dinosaur.