Dimetrodon belongs to a group called the synapsids, which includes all living mammals and a menagerie of extinct animals extending back to about 305 million years ago. Being an ancient animal that looks different from modern-day mammals, non-mammalian synapsids are often mistaken for reptiles, and dinosaurs. Although synapsids and reptiles descend from a common ancestor, neither group is directly ancestral to each other. Plus, Dimetrodon became extinct about 60 million years before the first dinosaurs even evolved. That’s almost the same amount of time that separates humans from Tyrannosaurus.
Dimetrodon was extremely successful. Its fossils have been found in many locations including the United States, Canada, and Germany, and throughout many different layers of rock, suggesting a long history. Since the first description of the genus Dimetrodon in 1878, both researchers and the public have wondered about the purpose of the massive sail.
Because of the sail’s size and imposing appearance, scientists proposed that it could have been used for intimidation or defense. However, these theories don't hold fast when you consider that Dimetrodon species were medium- to large-sized carnivores and the apex predators of their communities. The different species ranged in length from 1.7 to 4.6 meters (5.5 to 15 feet) and weighed between 30 and 250 kg (65 and 550 lb). They were the predators of such large prey as the amphibians Diadectes and Eryops, the latter could reach about 90 kg (200 lb) in adulthood. In addition, the sail was a relatively weak structure; the upper parts of the spine were embedded in a thin, soft tissue and lacked muscles. So, it seems unlikely that the animal would develop such a defense mechanism that couldn’t really defend against predators that, well, didn’t really exist.
When hunting, Dimetrodon may have been an ambush predator—waiting for its food to come along—so maybe the sail helped with camouflage? However, it seems odd that an already large animal trying to hide in the foliage would sport a “billboard,” advertising its presence. Camouflage would have been much better achieved without the sail.
The word sail typically conjures sailing ships, so perhaps the sail’s function was similar to the one used on a boat, to help navigate through the water. However, using the sail as a sail would cause too many disadvantages and not enough to compensate for any usefulness while in the water. Other hypotheses have included the sail as a hearing support, like the tympanic membrane in our ears, or as having no function at all, instead representing “excess vitality.” These hypotheses, especially the latter, seem unlikely. Why expend so much energy to create something useless?
There are two contending hypotheses that hold some promise. They are the regulation of body temperature and display variations between the sexes. All sorts of animals have evolved exuberant appearances to attract potential mates, and the sail is believed to have served a similar purpose. Although both females and males had sails, the sail could have possibly changed color during the mating season. However, testing for display function is difficult, and any remnants of such superficial features haven’t been found in fossils.
Curiously, Dimetrodon was not the only sailed-back animal of the Permian Period. The herbivorous synapsid Edaphosaurus also had a sail but with crossbars. Although Dimetrodon and relatives (sphenacodontids) and Edaphosaurus and relatives (edaphosaurids) are closely related, their sails evolved independently. This coincidence of sail evolution in separate groups suggests that something the two groups shared in common must have triggered this feature. The environment, perhaps?
