Dimetrodon grandis is an iconic Permian vertebrate that existed between 295-275 ma. With its distinctive neural spine sail and powerful build it is believed to have been an apex predator that grew up to 4.5 metres in length.
Still often mistaken as an early form of dinosaur or reptile (sauropsida) dimetrodons actually belong within the synapsid clade -with a single synaptic arch behind the eye socket - and are therefore more closely related to mammals than dinosaurs.
Dimetrodon means “two measures of teeth” which refers to the large incisor type teeth and smaller cutting teeth that its powerful jaws contained - another similarity with mammals. Dimetrodons actually had a third type of teeth, known as pharyngeal teeth that sat high up in the palate region of the skull. All of Dimetrodon's teeth had serrated edges indicative of a carnivore. Dimetrodon's head was narrow - being laterally compressed - with a distinctive step in its upper jaw called the Maxillary Step. It would have had a very powerful bite.
Being a tetrapod with a low-slung pectoral girdle and limbs that extended horizontally from the main body, Dimetrodon has long been portrayed as a semi-recumbent, belly dragging, creature. Recent examination of fossil tracks indicates however that it could move at some speed with its trunk clear of the ground - in a similar fashion to modern caymans (Abbott, Sues, 2017). Dimetrodon is now thought to have had quite powerful limbs and a more elevated posture.
The most distinctive Dimetrodon feature is its spinal sail and there is still debate about how this evolved and what role it played. Hypotheses include it being used as a mating signal, a warning signal or even as an actual sail to cross water (not as improbable as it may first sound - if you did have such a sail why wouldn't you use it to tack across bodies of water?) however it is unlikely that any one of these hypotheses alone could fully account for its emergence. Another point of note is that such a distinctive feature, whilst being useful at this stage in history, almost completely disappeared from the future anatomical landscape - the only modern creatures to possess such a feature are a small group of reptiles such as the Obelisk Lizard.
Another hypothesis is that dimetrodons were cold blooded (ectothermic) and the sail was a form of thermal regulation, warming blood in the early part of the day so that it could move upon prey that was still relatively sluggish. Modelling suggests that the sail may not have been a very efficient heat regulator but even a small difference in this regard could amount to a decisive advantage. The fact that future evolutionary periods become dominated by warm blooded (endothermic) creatures could account for its disappearance as an anatomical trait.