This group is represented by a single extant species which is thought to be the sister taxon to all other frogs. Not surprisingly, then, this smallish frog (25 - 50 mm in length) has many primitive characteristics.
The most obvious difference between Ascaphus and other frogs is the presence of an intromittent organ (often referred to as a 'tail'). This structure is actually a highly vascularized extension of the cloaca which allows internal fertilization. During amplexus (which is inguinal and underwater) the rectus abdominus muscles contract to bring the 'tail' forward into the cloaca of the female. Many salamanders and all caecilians practice internal fertilization, yet Ascaphus is the only frog to do so. Ascaphus (along with Leiopelma) also has vestigial tail-wagging muscles, likely an evolutionary relict of its tailed ancestors (see Triadobatrachus. Other primitive characteristics of Ascaphus include: nine presacral vertebrae (all other frogs, except Leiopelma, have eight or fewer) with amphicoelous centra, Type 3 aquatic larvae, and an arciferal pectoral girdle.
These highly aquatic frogs are found in cold mountain streams and are rarely found away from water. Larvae hatch from small clutches of large eggs placed under rocks in mountain streams. The larvae have suctorial discs and reduced tail fins, which presumably are adaptations for living in swift flowing streams. The adults have no tympana and have never been observed vocalizing. These characteristics also may be related to the nature of the fast-flowing streams inhabited by Ascaphus.
Among extant frogs, Ascaphus is thought to be most closely related to the Leiopelmatidae of New Zealand and, in fact, Ascaphus is sometimes placed in this family.
Fossils which are sometimes attributed to the Leiopelmatidae rather than Ascaphidae are known from the Jurassic of Patagonian Argentina. Ascaphus truei is found only in the northwestern United States and adjacent southern Canada. This indicates that, while the current distribution of the Ascaphidae is limited, its ancestors were more widespread (North America, Argentina, New Zealand).
Little is known about the conservation status of Ascaphus, but amphibians in general are thought to be sensitive to changes in their environment (see AmphibiaWeb's declining amphibians page).
Duellman, W.M. and L. Trueb. 1984. Biology of the Amphibians.
Pough, F.H., R.M Andrews, J.E. Cadle, M.L. Crump, A.H. Savitzsky, K.D. Wells. 1998. Herpetology. Prentice-Hall, Upper Saddle River, NJ.
Frost, Darrel R. 2002. Amphibian Species of the World: an online reference. V2.21 (15 July 2002). Electronic database available at http://research.amnh.org/herpetology/amphibia/index.html.
Richard M. Lehtenin (author).