Temnospondyl larvae are often distinguished by poorly developed bones and the presence of a hyobranchial apparatus , a series of bones that gills would attach to in life . However , some fully mature temnospondyls also possess hyobranchial bones but did not have external gills . A dense covering of scales is also seen in larvae and adults . Major body changes occur in metamorphosis , including the reshaping and strengthening of skull bones , the thickening of postcranial bones , and an increase in body size .