 The Cryptobranchoidea ( primitive salamanders ) and the Salamandroidea , also known as <unk> , ( advanced salamanders ) are believed to be sister groups . Both seem to have appeared before the end of the Jurassic , the former being exemplified by <unk> <unk> , <unk> sinensis , <unk> paradoxus , <unk> <unk> , <unk> <unk> and <unk> <unk> , and the latter by <unk> <unk> . By the Upper Cretaceous , most or all of the living salamander families had probably appeared . 
