Their body plan has made them able to fit into certain ecolgical niches very well. The main change was the development increased mobility in their jaws. This may have made them better at catching insects, the meal of choice for most lizards, although some fed on the tough vegetation. It also helps with digging for food or to make burrows. The fossil record of lizards is not very good because they often live in very arid areas, where they have a poor chance of becoming fossilised.
Snakes have even more flexible jaws than lizards.
This is what helps them to swallow their prey whole. Like lizards they have a poor fossil record and are first known from the Upper Cretaceous. These are known from the Late Triassic of Elgin in Scotland. This deposit contains evidence of other diaspids, including some early dinosaurs and how they lived. Rhynchosaurs had beak-like mouths that they would have used to lop the tough plants of the Triassic, probably seed ferns.