"From a cladastic point of view, the class [R]eptilia is not valid even today. Reptiles are now considered a polyphyletic classification. They did not branch of at the same time. Reptiles don't have enough in common to be considered a biologically relevant grouping. The crocodile is not a lizard any more than a dolphin is a fish.
Squamata (lizards, snakes), Testudinata (turtles, tortoises), Rhynchocephelia (Sphenodon), and [C]rocodilia (alligators, crocodiles, caimans, gavails) are different orders of extant reptiles. However, the evidence is that these four orders branched off at completely different times.
In terms of a cladogram, crocodilians are closer related to birds then to [S]quamata, testudinata (sic) or [R]hynchocephelia. Some scientists prefer to group crocodilians, birds and dinosaurs into one group called archosaurs.
There are a few groups of mammal-like reptiles that lived before the dinosaurs. Pelycosaurs, therapsids and gorgonoids were first classified as reptiles, just like the dinosaurs were first classified as reptiles. However, research over the last few years shows that these reptiles are closer related to mammals than they were to dinosaurs.
There is a lot of controvery (sic) about grouping testudinata and squamata together. Some evidence indicates testudinata (sic) may be closer related to archosaurs than to [S]quamata. However, the testudinata (sic) split off very early from both archosaurs and [S]quamata.
It appears that in terms of natural history, reptiles are not a well defined group. The four extant orders have a skin deep resemblance to each other. However, they did not split off from each other later than they split off from the mammal line. If you include extinct animals in taxonomy, it gets even more difficult to think of reptiles as a monophyletic clade. Before the Permian Triassic extinction 250 MYA, there was a continuum of creatures that could be classified as archosaurs. There were fewer distinctive features between birds, crocodilians and dinosaurs in the Jurassic. However, these animals differed a great deal from [S]quamata, pterodactyls and mososuars (sic).
Reptiles are a historically useful class derived from Linnean taxonomy. Historical categories are useful for looking up things in the library or even on the internet. If everybody kept reclassifying organisms to keep up with research that is continually being done, no one could keep track of that research. Libraries would go broke reshuffling their books every few months. However, reptiles is not a useful category when correlating facts about nature."
Dinosaurs are still, of course, considered reptiles, leading to a natural follow-up question: if birds are really dinosaurs, and dinosaurs are reptiles, should birds be considered reptiles? The answer again is yes: herpetologists, the scientists who specialize in the study of amphibians and reptiles, take great pleasure in reminding their ornithologist colleagues that birds might correctly be thought of as “avian reptiles.”
"Snakes, lizards, turtles, crocodilians, and birds also all share an evolutionary history. Many years of research has proven that the ancestors of birds were bird-like dinosaurs (visit the Dinobuzz page for more on this). Even though birds look very different from other living reptiles, they’re most closely related to alligators and crocodiles and clearly belong in the reptile group."