Illustration of two Dimetrodon fighting. This extinct prehistoric animal is a synapsid, an early form of the evolution from reptiles to mammals. It lived 295 to 272 million years ago, during the Early Permian. Most of the known fossils have been found in Texas, USA. Around 3 metres in length, it was one of the dominant land predators at the time. The neural spine sail may have been for thermoregulation, but is also thought to have been used for mating displays.