 One group of <unk> , the <unk> , is also known from larval specimens . <unk> like <unk> and <unk> are represented by many fossils preserving skin and external gills . An entire growth series is exhibited in the wide range of sizes among specimens , but the lack of <unk> adapted adult forms suggests that these <unk> were neotenic . Unlike other <unk> , their postcranial skeletons developed quickly but were still partly cartilaginous when fully mature . Adults likely had an aquatic lifestyle similar to juveniles . Recently , large specimens of <unk> gracilis were described with adaptations toward a terrestrial lifestyle , indicating that not all <unk> were neotenic . 
