The first amniotes , such as Casineria , looked like small lizards . They evolved about 340 million years ago , in the Mississippian or Lower Carboniferous . 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 .