JOHN SIBBICK / SCIENCE PHOTO LIBRARY JOHN SIBBICK / SCIENCE PHOTO LIBRARY
Dimetrodon and Edaphosaurus synapsids, illustration. Dimetrodon (red and yellow, foreground) existed from 295 to 272 million years ago, and Edaphosaurus (blue and yellow, background) from 300 to 280 million years ago, during the Permian. These animals were the dominant land predators at the time, but are more closely related to mammals than to reptiles. The neural spine sail may have been for thermoregulation, but is thought to have been used for mating displays. Fossils of both animals are found in Texas, USA.
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