Dimetrodon. Computer artwork of two Dimetrodon by a prehistoric lake. 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 tree on the left is a Corodite, an ancient ancestor of today's conifers, and on the far right is a Glossopteris, a seed plant that came to dominate this period with over 70 different species.