This family of small frogs, which contains but a single genus, exhibits a highly derived reproductive mode. There are two known species in the genus Rhinoderma, which is synonymous with the family Rhinodermatidae. Rhinodermatids are restricted to the temperate forests of southern Chile and Argentina in South America.
Darwin's frogs are diagnosed as a clade not by morphological or molecular characters, but by a behavior. When the terrestrial eggs hatch, adult males transport the larvae in their enlarged vocal sacs. This reproductive mode is unique among anurans. Other characteristics of rhinodermatids include their small size (30 mm snout-vent length), and the fleshy appendages at the tips of their snouts (from which is derived the name of the genus and family). Their hind feet are webbed. They lack both ribs and teeth, and their skulls are weakly ossified. Furthermore, rhinodermatids have eight holochordal-procoelous presacral vertebrae; broadly expanded sacral diapophyses; horizontal pupils; and their astragalus and calcaneum are fused only at their ends. The type IV tadpoles have beaks and denticles, and the spiracle is sinistral. Some authors report that amplexus is axillary, but others report that it is absent, a rare but not unknown condition in anurans. Diploid number is 26.
Darwin's frogs are primarily terrestrial, living along cold streams in wet temperate beech forests. Small numbers of eggs are laid on moist ground, where they develop quickly, in two to three weeks. Multiple males may attend a clutch, which raises questions about paternity and dispersal patterns. The hatching tadpoles are picked up by one or more of the attending males, each of which may carry from five to fifteen larvae. In Rhinoderma rufum, the tadpoles are carried immediately to small pools of water, where they are released and complete their development. In Rhinoderma darwinii, males retain the tadpoles in their vocal sacs until the young metamorphose. The initiation of muscular activity in the eggs is apparently the cue to males to pick them up, one by one, into their mouths. Generally, anuran parental care is found among territorial species. Although male Rhinoderma clearly exhibit an elaborate form of parental care, it is not known if they are territorial.
Rhinodermatids are placed in the Neobatrachia, but relationships among the families of these "advanced" frogs is controversial at best. Most authors identify a superfamily, alternately called Bufonoidea or Hyloidea, which includes all the neobatrachians that are not Ranoids or Microhyloids. The group Bufonoidea is thus sketchy. Within Bufonoidea, rhinodermatids were originally placed in the Brachycephalidae, though no support exists for that position today. Were it not for the striking reproductive mode found in Darwin's frogs, Rhinoderma would likely be placed in the Leptodactylidae. Currently, there is no consensus regarding which clade(s) might be related to the rhinodermatids.
No fossil rhinodermatids are known.
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Heather Heying (author).