You are actually quite familiar with the group of tetrapods known as diapsids, believe it or not. All members of the group called the Reptilia (see below), except for the anapsids (turtles and their ilk), and a few extinct groups, are diapsids. The main diagnostic physical character for a diapsid is the presence of two openings on each side of the skull; openings on each side of the skull; the upper and lower temporal openings. Even the birds are considered diapsids (and hence reptiles), because they are descended from certain dinosaurs (which are also diapsids), and ancestrally have the paired skull openings along with other physical characteristics that unite them with diapsids. Thus, they are considered diapsids by their ancestry, which is illuminated by shared derived traits.
The skull of a champsosaur, an extinct diapsid related to the archosaurs. It is an example of convergent evolution with the crocodilians, as it is not directly related to them.
The evolutionary history of the diapsid lineage is quite complex; diapsids evolved into many shapes, occupying many different ecological niches since they first came onto the scene in the late Carboniferous period (roughly 350 million years ago), when they were represented by the earliest diapsid, the tiny lizardlike Petrolacosaurus. We have found fossils of a multitude of diapsids that swam, slithered, crawled, ran, or even flew since then. They are truly an amazing group -- far more diverse than any other comparable vertebrate group except for Actinopterygii (the ray-finned fishes).
"Reptile" refers to the Reptilia, which includes the ectothermic snakes, lizards, crocodiles, turtles, and the endothermic birds. Or, if you consider yourself in the cladistic school of thought like most paleontologists, then if you say Reptilia, you are referring to all anapsids and diapsids (the usual snakes, lizards, crocodiles, and turtles, and their friends, including dinosaurs (and their descendants the birds)). If this seems overly technical, remember that scientists do need to keep in mind the relationships of organisms, or else evolution makes little sense. Phylogenetic systematics is used to show the relationships of animals (phylogeny).
The term "reptile" may carry a lot of psychological baggage with it, conjuring up outmoded images of slow, stupid, inferior creatures, but it is a valid term applied to the group comprising the first reptile and all of its descendants. By this convention, birds are considered Reptilia, just like bats are mammals and snails are mollusks. Birds are certainly quite different from other living Reptilia, but the traits that modern birds possess were acquired gradually over many million years of evolution. The first birds were quite different than modern birds, and looked much more like good traditional reptiles than hawks, doves, or turkeys do.
Click on any of the pictures above to learn about each group of diapsids!
If you are looking for information on living reptiles, we recommend trying the Herpmed homepage, with links to dozens of relevant Web sites. You may also wish to try our listings of Herpetological Collections on-line.