During the 100 million years after the first reptiles appeared, various reptile lines continued to evolve. Today, it is difficult to find agreement about reptile classification. In most cases, they are divided into four living orders (the others have died out over time):

There is also another older method of grouping reptiles: subclasses according to the positioning of the temporal fenestrae, or the openings in the sides of the skull behind the eyes: the anapsids, synapsids, diapsids, and euryapsids. The anapsids had no openings in the skull and eventually evolved into today’s turtles and tortoises. The synapsids, or “same hole,” had a low skull opening, and were once thought to be the ancestors of modern mammals (and are now not considered to be true reptiles). It was the animals of the diapsid line, or “two skull openings,” that eventually gave rise to the dinosaurs. Another debatable line is the euryapsids, characterized by a single opening on the side of the skull, which are now usually included with the diapsids.