The first important thing to understand is the structure of the skull. Amniotes, that is animals that have an amniotic egg (reptiles, mammals, and birds) are divided into three main groups with respect to the 'temporal fenestra', or simply 'holes', in the back of their skull. If you feel above your cheekbone, in your temple, you'll feel that it's not hard and bony, but soft and full of muscle. This is what remains of one of those holes. The first group is the anapsids. These animals have no holes in the back of their skull, and consist mainly of turtles. The second group is the synapsids, which have a single hole in the back of the skull, just like us. This includes mammals, and the extinct reptiles closely related to mammals, like Dimetrodon. That's right, Dimetrodon is not a dinosaur, but more closely related to us and other mammals than a dinosaur! The third group is the diapsids, and like it's name suggests, they have two holes in the back, one on top and one below. All remaining reptiles are diapsids, including dinosaurs, pterosaurs, and birds (which are the direct descendants of dinosaurs, and therefore technically dinosaurs). There is a fourth group you will sometimes hear called the euryapsids, which have only one hole up top (in a different position than the one synapsid hole). These were most likely diapsids that lost the bottom hole over time, and include animals like ichthyosaurs.
|The front hole (on the left) is the nostril, while the large middle hole is for the eye. The holes at the back are the temporal fenestrae. A. Anapsid skull with no holes, B. Synapsid skull with 1 hole, C. Diapsid skull with 2 holes. From Fossil Wiki|
|Reptilian posture: Top shows a typical reptile with the legs splayed to the side, while the bottom shows the legs directly under the body, as seen in dinosaurs. Image from Freethought Forum|
|Cladogram showing the relationships and diversity of dinosaurs. From UCMP Berkeley|
National Museum of Natural History