Mammalian ancestors took the form of the now-extinct dicynodonts and the cynodonts, the latter of which include modern mammals, as well. In modern mammals, you can see how the skull only has a single hole behind the eye (the space where the coronoid process sneaks in between the main part of the skull and the extruding zygomatic arch), making it a synapsid, or "one-holer." In previous posts, we've talked about primitive, mammal-like animals such as Dimetrodon and Cotylorhynchus. Both of these critters are synapsids. Diapsids, or "two-holers" (remember from our recent Latin/Greek Roots post that the root "di" means "two" [click HERE to read that post]), is another large group, and includes everything from crocodiles to dinosaurs, lizards to snakes, and tuataras to birds. It also includes the archosaurs, a subgrouping of diapsids that are characterized by an additional hole in the skull, bringing the total number of skull holes up to three. So some diapsids are also archosaurs, such as birds, dinosaurs, and crocodiles. There's also the anapsids, which are animals with no holes in the skull, such as amphibians and turtles. Taxonomically, this can get a bit confusing (especially since sometimes an animals classification doesn't correspond to the number of holes that it has at that point in its evolutionary history), and maybe later we can go into greater detail about these different 'apsids, but below we have a nice picture that should help clear things up a little.
|Holes in the skull. On the top left, we have the prehistoric sea turtle Protostega, an anapsid, with no extra holes behind the eye socket. Below Protostega, we have Prestosuchus, a type of archosaur. Not only does Prestosuchus have the two holes in the skull behind the eye socket that characterize older diapsids, but it also has a third hole, in front of the eye socket, but behind the nose openings. On the bottom right, we have Edaphosaurus, a primitive synapsid. The largest hole in the skull, furthest on the right, is what will one day become the hole that the coronoid process sneaks through, between the zygomatic arch and the rest of the skull. In the picture above Edaphosaurus, you can see what I'm talking about, with the extinct mammalian synapsid Hyaenodon. Here, you can see the little nub of the coronoid process between the zygomatic arch and the skull.|
|A trio of cetacean skulls. On the top left, we have Pakicetus, a terrestrial ancestor of the cetaceans, that lived in Pakistan approximately 50 MYA. Below Pakicetus, we have Zygorhiza, a more derived and fully aquatic cetacean. In both Pakicetus and Zygorhiza, you can see how the front teeth and back teeth are different, with the front teeth more for gripping prey, and the back teeth perfect for slicing. On the right, you can see the skull of the modern killer whale, or orca, which has only one type of tooth in their mouth, the conical, gripping teeth.|
|The skull of Heterodontosaurus, on display at the American Museum of Natural History in New York. You can see the two different types of teeth in the skull, especially in the lower jaw.|