Dimetrodon, the name of which meaning, 'Two measure of teeth', was the top predator on land, during the Early-Early Mid Permian Period, 285-265 Million Years Ago. They are Pelycosaur Synapsids, of the strange and iconic looking 'sailbacks', that are often incorrectly referred to as Dinosaurs. Fossils of them are known from North America and Europe. They were the largest predatory Pelycosaurs ever, at 2.5-3.2 Metres Long, and probably a weight of a third to one half of a tonne. They were formiddable carnivores in their day, preying on even their 'own kind', as in other Pelycosaurs such as Edaphosaurus. It is most likely they were even cannibals, not being above preying on newly hatched infants of their own species when given the chance to.
They were well adapted to life in the Early-Early Mid Permian, with their 'sails', used for thermoregulation, in an increasingly arid climate. Seasons would bring different conditions, and winter in the time of Dimetrodon, surprisingly may have brought ice and snow. The ability to absorb every bit of energy and heat from the sun upon the large surface area of their 'sails', would have been a great advantage over other reptiles. They preyed on anything they could catch, including smaller amphibians such as Seymouria. These types of animals seem to have given risen to the ancestors of mammals, evolving eventually into the Therapsids (true mammal like reptiles, though Dimetrodon is essentially analogious to a mammal like reptile) which in turn developed into true mammals. The Pelycosaurs as a whole disappeared as they were superceeded by Gorgonopsids and such others.