Dimetrodon

Dimetrodon (/daɪˈmiːtrəˌdɒn/ or /daɪˈmɛtrəˌdɒn/ ^ⓘ) was an extinct genus of sphenacodontid synapsid that lived during the Cisuralian to early Guadalupian epoch of the Permian period, around 294.1–271 million years ago. It had a tall, curved skull with large teeth of different sizes and a large neural spine sail on its back formed by elongated spines extending from the vertebrae. Most fossils of Dimetrodon have been found in the Southwestern United States, especially in a place called the Red Beds of Texas and Oklahoma, but fossils have also been found in Germany.

Although Dimetrodon looks similar to reptiles and is often mistaken for a dinosaur, it lived long before dinosaurs existed—it went extinct about 40 million years before dinosaurs first appeared. Instead, Dimetrodon is more closely related to mammals. It was likely one of the top hunters of its time, eating fish and other animals like reptiles and amphibians. The sail on its back might have helped it control its body temperature or been used to show off to other Dimetrodon during mating or to threaten rivals.

History of discovery

The first fossils of Dimetrodon were found in 1845 on Prince Edward Island, which is now part of Canada. These fossils were studied by scientist Joseph Leidy in 1854 and were originally thought to belong to a different animal called Bathygnathus. Later, in 2015, they were recognized as Dimetrodon fossils.

American scientist Edward Drinker Cope began studying Dimetrodon fossils in the 1870s. He found them in rocks in Texas called the Red Beds. Cope named several species of Dimetrodon, including Dimetrodon incisivus. Another scientist, Alfred Romer, reviewed many Dimetrodon fossils in the 1940s and helped us understand them better today.

In the years that followed, fossils of Dimetrodon were discovered in many places beyond Texas and Oklahoma, such as Utah, Arizona, and even Germany. These discoveries showed that Dimetrodon lived in many parts of North America and also in Europe.

Description

Dimetrodon was a quadrupedal, sail-backed synapsid that most likely had a semi-sprawling posture between that of a mammal and a lizard and also could walk in a more upright stance with its body and the majority or all of its tail off the ground. Most Dimetrodon species ranged in length from 1.7 to 4.6 m (6 to 15 ft), and are estimated to have weighed between 28 and 250 kg (60 and 550 lb). The smallest known species, D. teutonis, was about 60 cm (24 in) long and weighed 14 kg (31 lb). The larger species of Dimetrodon were among the largest predators of the Early Permian.

The skull of Dimetrodon is tall and compressed side-to-side. The eye sockets are positioned high and far back in the skull. Behind each eye socket on each side is a single hole called an infratemporal fenestra. An additional hole in the skull, the pineal foramen (or "third eye") along the midline between the parietal bones, can be seen when viewed from above. The back of the skull (the occiput) is oriented at a slight upward angle. The upper margin of the skull slopes downward in a convex arc to the tip of the snout.

The size of the teeth varies greatly along the length of the jaws, lending Dimetrodon its name, which means "two measures of tooth" in reference to sets of small and large teeth. One or two pairs of caniniforms (large, pointed, canine-like teeth) extend from the maxilla. Large incisor teeth are also present at the tips of the upper and lower jaws. Small teeth are present around the maxillary "step" and behind the caniforms, becoming smaller further back in the jaw.

The sail of Dimetrodon is formed by elongated neural spines projecting from the vertebrae. Each spine varies in cross-sectional shape from its base to its tip. The large groove that runs the length of the spine was once thought to be a channel for blood vessels, but the sail is not thought to have been as highly vascularized as once thought.

The tail of Dimetrodon makes up a large portion of its total body length and includes around 50 caudal vertebrae. Tails were missing or incomplete in the first described skeletons of Dimetrodon. A largely complete tail of Dimetrodon was not described until 1927.

Scaly body impressions that likely were made by Dimetrodon teutonis were described in 2025 from the Early Permian Tambach Formation Bromacker site in Germany. The impressions left by animals resting on mud show a scaly epidermis pattern on the belly, and on the underside of the forelimbs and the tail, supporting the idea that early synapsids in general had a scaly body covering similar to reptiles.

Classification

See also: Synapsid § Linnaean and cladistic classifications

Dimetrodon is an early member of a group called synapsids, which include mammals and many of their extinct relatives. It is often mistaken for a dinosaur, but it lived long before dinosaurs existed. As a synapsid, Dimetrodon is more closely related to mammals than to any living reptile.

There are about thirteen recognized species of Dimetrodon. Scientists have studied their family tree and found that Dimetrodon belongs to a group called Sphenacodontia, which includes several sail-backed synapsids. This group is part of the larger Synapsida clade, which also includes mammals.

Paleobiology

Scientists have thought about many reasons why Dimetrodon might have had its big sail on its back. Some ideas include using it for hiding in tall plants, helping it move efficiently, or even catching the wind like a boat.

One popular idea is that the sail helped Dimetrodon control its body temperature. The sail could have let the animal warm up quickly in the morning sun or cool down when it got too hot. This would have been important because Dimetrodon lived in a world where temperatures could change a lot, and bigger animals like Dimetrodon took longer to warm up or cool down than smaller animals. Some scientists think the sail might have also helped Dimetrodon keep a steady body temperature, a trait that later evolved in mammals.

Recent studies suggest that the sail might not have been very good at controlling temperature, especially in smaller Dimetrodon species. Some scientists now think the sail might have been used to attract mates or show off, similar to how some animals today use big antlers or colorful feathers. Evidence from the size and shape of the sails supports this idea.

Paleoecology

Fossils of Dimetrodon have been found in the United States, Canada (Prince Edward Island), and Germany, which were all part of the supercontinent Euramerica during the Early Permian period. In the United States, most fossils come from Texas and Oklahoma, especially from places called the Clear Fork Group, the Wichita Group, and the Pease River Group. These areas were wet, low-lying lands with many different kinds of four-legged animals, including amphibians and early reptiles.

Dimetrodon likely was a top predator in its environment. It probably ate many different animals, such as small fish, amphibians like Trimerorhachis and Diplocaulus, and early reptiles like Seymouria. There were also large plant-eaters like Edaphosaurus living there. In Germany, a different species of Dimetrodon lived in a drier area without large aquatic predators.