One group of temnospondyls , the Branchiosauridae , is also known from larval specimens . Branchiosaurids like Branchiosaurus and Apateon 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 terrestrially adapted adult forms suggests that these temnospondyls were neotenic . Unlike other temnospondyls , 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 Apateon gracilis were described with adaptations toward a terrestrial lifestyle , indicating that not all branchiosaurs were neotenic .