you got that backwards, amphibians first appeared during the devonian about 370 million years ago ichthyostega being the first. The first reptiliomorph was westlothiana which was the first precursor to reptiles appearing approximately 350 mill yrs. ago. The first true reptiles known were approx. 300 mill years ago in the Carboniferous period.
Thanks for the corrections: It was counter-intuitive as I typed it and should have looked it up.