The tailed frogs are two species of frog. The "tail" in the name is actually an extension of the male cloaca. Ascaphus is the only genus in the family Ascaphidae. Until 2001, the genus was believed to be monotypic, the single species being the Tailed Frog (Ascaphus truei, Stejneger 1899). However in that year Nielson, Lohman, and Sullivan published evidence in Evolution that promoted the Rocky Mountain Tailed Frog (Ascaphus montanus) from a subspecies to its own species.

The existence of the visible "tail" makes this frog family distinct from all other frogs. Thus its wider classification is difficult. It is usually classified in the Archaeobatrachia suborder of ancient frogs, though some say it should be a sister to all other frogs.

As well as the ancient tail, these frogs have a number of vertebrae higher than that normal to frogs, non-vocalisation and ribs. They are extremely small (2.5 to 5 centimetres) and are found in steeply-flowing streams in Montana, Idaho, Oregon and Washington in the United States and south eastern British Columbia.

References

• Excerpt from Zeiner et al. 1988 describing tailed frogs
• Type information from the American Museum of Natural History