Want to know something else awesome?
Dimetrodons are actually more closely related to mammals than they are to reptiles. They are synapsids, tetrapods with one hole in the skull located closely behind each eye. Mammals are also synapsids, while animals like birds and reptiles are saurapsids. Synapsids were dominant creatures during the Permain period (299-251 million years ago), and the Dimetrodons in particular were apex predators. The name actually refers to the fact that they have two types of teeth, canines as well as sheering teeth.
Unfortunately, the Permian period met a disastrous end. Around 251 million years ago the largest mass extinction ever took place, wiping out 90% of all marine life and 70% of all land animals. Dimetrodons did not make it (in fact, they were already gone 30 million years before that-- around 280 million years ago), but some other synapsids did, including our very own distant relatives!