Dimetrodon was the apex predator of its time. They came as large as 11 feet long, 4 feet tall, and weighing as much as 300 pounds. Their reptilian hide, a relative novelty for that period, ensured their survival out of water. The large sail may have helped them to regulate body temperature, a survival advantage during the extremes of hot and cold during the Early Permian. . . Alethopteris was a seed fern that populated much of the world during the Carboniferous and Early Permian periods. Long extinct, fossilized leaves from the Alethopteris are commonly found today.