Dimetrodons, computer artwork. Dimetrodon were the dominant land predators that lived around 270 million years ago, during the Early Permian Period, and long before the dinosaurs. They were more closely related to mammals than reptiles and could grow up to 11 feet (3.4 metres) long and 4 feet (1.2 metres) tall. The large sail on their back may have helped to regulate body temperature, an advantage during the extremes of hot and cold during the Early Permian.