Reptiles evolved from labyrinthodont amphibians 300 million years ago.    From Wordnik.com.  [Reptile] Reference

The exposures in the park are Late Triassic in age and contain early dinosaurs, labyrinthodont amphibians, and many crocodile-line archosaurs and well as numerous plants and freshwater invertebrates.    From Wordnik.com.  [Archive 2009-03-01] Reference

It shows several things: all vertebrates evolved from an amphibian ancestor thought to have been a labyrinthodont. amphibians diverged from the other vertebrates very early on. turtles, crocodiles, and dinosaurs appeared before the other vertebrate taxa. dinosaurs, birds, crocodiles, and snakes are more closely related to each other than to turtles or mammals.    From Wordnik.com.  [Reptile] Reference