 The skull of Dimetrodon is tall and compressed laterally , or side @-@ to @-@ side . The eye sockets are positioned high and far back in the skull . Behind each eye socket is a single hole called an infratemporal fenestra . An additional hole in the skull , the supratemporal fenestra , can be seen when viewed from above . The back of the skull ( the occiput ) is oriented at a slight upward angle , a feature that it shares with all other early synapsids . The upper margin of the skull slopes downward in a convex arc to the tip of the snout . The tip of the upper jaw , formed by the premaxilla bone , is raised above the part of the jaw formed by the maxilla bone to form a maxillary " step . " Within this step is a diastema , or gap in the tooth row . 
