Well, reptiles seem to have split pretty early on between the ancestors of lizards and snakes (and the lonely tuatara), and the ancestors of turtles, crocodiles, and dinosaurs, the main differences seemingly being in the bones of the skull, and specifically for the group with the snakes and lizards in, the ability to self-amputate the tail (though that's lost in many of their descendants), and the keratinized scales; you won't see a turtle, crocodile, or dinosaur molting its whole skin in one go like lizards and snakes do, they'll molt their scales (or feathers, or scutes) one at a time.