The New Zealand primitive frogs' defining characteristics are their extra vertebrae (for a total of nine) and the remains of the tail muscles (the tail itself is absent in adults, although it is present in the younger frogs, which need the extra skin surface until their lungs are fully developed). The family Ascaphidae (found only in North America), of the same suborder, shares these primitive characteristics, hence the two have often been described as related, or even part of the same family.