 <unk> first appeared in the Early <unk> around 330 million years ago ( <unk> ) . During the <unk> , <unk> included basal medium @-@ sized forms such as <unk> or large <unk> forms such as <unk> . Other , more derived <unk> , such as the <unk> , were smaller and more terrestrial . They resembled <unk> , and some taxa , such as the genus <unk> , even retained external gills like the modern @-@ day <unk> . During the latest <unk> and Early <unk> around 300 <unk> , 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 <unk> predators . The <unk> , a group of small aquatic <unk> , evolved from terrestrial ancestors in the Late <unk> . 
