 <unk> first appeared in the Early Carboniferous around 330 million years ago ( Mya ) . During the Carboniferous , <unk> included basal medium @-@ sized forms such as <unk> or large semiaquatic forms such as <unk> . Other , more derived <unk> , such as the <unk> , were smaller and more terrestrial . They resembled salamanders , and some taxa , such as the genus <unk> , even retained external gills like the modern @-@ day axolotl . During the latest Carboniferous and Early Permian around 300 Mya , several groups , such as the <unk> and <unk> evolved strong , robust limbs and vertebrae and became adapted to life on land while others such as the <unk> , developed into large semiaquatic predators . The <unk> , a group of small aquatic <unk> , evolved from terrestrial ancestors in the Late Carboniferous . 
