By Heather Heying
Approximately 36 extant species in seven (sometimes presented as eight or nine) genera are currently recognized in this primitive family of salamanders. The hynobiids represent the only family of salamanders with geographic distribution restricted to Asia (the only other Asian salamanders being the large paedomorphic cryptobranchids in the genus Andrias, and some rough-skinned newts (Salamandridae).
Despite their sister relationship to the cryptobranchids, hynobiids bear little external resemblance to those large, non-metamorphosing salamanders. Hynobiids are small -- with no species growing longer than 25 cm, and most less than 10 cm long. All but a few species exhibit complete metamorphosis, although they return to the water to reproduce. Some species (e.g. members of Onychodactylus) remain aquatic as adults. Larvae have external gills, four pairs of gill slits, and a caudal fin, all of which are lost at metamorphosis. Adults have eyelids, four or five toes on each hind limb, and both lacrimal and septomaxillae bones in their skulls, which are absent in the cryptobranchids. The absence of an operculum and opercularis muscle in some species of hynobiids distinguishes these animals from all other salamanders except rhyacotritonids. Well-developed lungs, which are primitive in salamanders, are present in all species, except in the genus Onychodactylus (absent), and Ranodon (reduced). Palatal teeth are present. These are stout-bodied salamanders with smooth skin that are restricted to temperate and cool temperate-sub arctic zones in Asia.
Most species are terrestrial, and return to the water to breed. There is little evidence of courtship. Females lay eggs in two arc-shaped, gelatinous masses containing 35-70 eggs each. The egg sacs are attached to stones or vegetation. Fertilization is external (as in the two other families of primitive salamanders), and parental care is present in the form of egg-guarding. Hynobiid behavior and ecology is poorly understood.
The hynobiids, together with the cryptobranchids, form the suborder Cryptobranchoidea. These two families primarily share plesiomorphic character states, but three synapomorphies link them: 1) fusion of first hypobranchials and first ceratobranchials, 2) fusion of tibialis muscles, and 3) eggs enclosed in paired sacs. The suborder Cryptobranchoidea, often called the primitive salamanders, is sister to the Salamandroidea, the advanced salamanders. Some researchers have proposed a relationship between the hynobiids and the Ambystomatoidea, but similarities between these groups are likely due to shared primitive characters (symplesiomorphies) rather than shared derived ones (synapomorphies), and thus provide little evidence of common ancestry.
No fossil hynobiids are known.
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Larson, A. 1996. Hynobiidae: Tree of Life. (website.) 
http://tolweb.org/tree?group=Hynobiidae&contgroup=Caudata
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Contributors
Heather Heying (author).
