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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.