Original: The first amniotes (referred to as "basal amniotes" or "stem amniotes"), such as Casineria, resembled small lizards and had evolved within reptiliomorphs which were amphibian about 340 million years ago, in the Carboniferous geologic period.
Simplified: The first amniotes, such as Casineria, looked like small lizards. They evolved about 340 million years ago, in the Mississippian or Lower Carboniferous.