By Heather Heying
This speciose, widespread, and diverse family of frogs has undergone such frequent redefinition that it seems likely that the current definition is neither monophyletic, nor permanent. As defined here, there are two subfamilies, containing roughly 35 genera, and more than 600 species. Ranids are cosmopolitan except for temperate South America, most of Australia, the West Indies, and most Oceanic islands. The subfamily Raninae has the same range as the family, while the Petropedetinae is restricted to sub-Saharan Africa. Species of the genus Rana are the only ranids found in the Americas or Australia; most of the family's diversity is in the old world.
No characters unequivocally diagnose Ranidae, lending more strength to the position that this is a paraphyletic group. Most ranids lack ribs, have dentate upper jaws, eight holochordal-procoelous presacral vertebrae, and an astragalus and calcaneum that are fused only at their ends. The omosternum and sternum are usually ossified, except in Petropedetines, in which the sternum tends to be cartilaginous. Palatines are present; a parahyoid is not. The phalangeal formula is normal, and the pupil is horizontal. Diploid number ranges from 16 to 26; some species are polyploid.
Ranids tend to exhibit axillary amplexus and indirect development, laying aquatic eggs that develop into type IV tadpoles. Exceptions to this rule include some African petropedetines, such as Arthroleptella, with nonfeeding tadpoles, and Anhydrophryne, which has direct development from terrestrial eggs. Some Asian ranine genera (e.g. Platymantis) also exhibit direct development. Petropedetines tend to be small (less than 25 mm snout-vent length), terrestrial frogs with slightly warty skin. Ranines vary widely, with sizes ranging from 20 - 300 mm snout-vent length (at 300 mm, Conraua goliath, is the largest known frog). Some ranines are riparian, with long legs and webbed feet (e.g. Occidozyga); others are toadlike in appearance and somewhat fossorial in habit (e.g. Tomopterna); still others are arboreal (e.g. Platymantis). The tadpoles of Amolops species are torrent-adapted, and have suctorial disks on their bellies. The European Rana esculenta is a hybridogenetic form that, in some populations, exhibits a modified form of cyclical parthenogenesis, in which sexual generations alternate with asexual ones. Research on several species of Rana also form the basis for much that is known of amphibian biology, as well as providing edible legs to generations of gourmands.
Ranids are Neobatrachians, but relationships among the families of these "advanced" frogs are almost wholly unresolved. Within the Neobatrachia, ranids are members of the superfamily Ranoidea, a clade of derived forms that likely loses its monophyly if Dendrobatidae is included. Family relationships among the ranoids are in a state of chaos, and should be considered unknown. The number of subfamilies described for Ranidae has fluctuated considerably over the years, coincident with new hypotheses of relationship. The former subfamilies Arthroleptinae and Astylosterninae have here been removed from Ranidae, and placed in their own family (Arthroleptidae). Similarly, the once-ranid Hemisinae has been accorded family status (Hemisotidae). Finally, the Mantellinae, which has been treated as a subfamily of the ranids by many authors, here is diagnosed as a clade within Rhacophoridae. The subfamily Raninae, and its type genus Rana, are likely paraphyletic. Within the Ranoidea, even this reduced Ranidae is analogous to the Leptodactylidae within the Bufonoidea: both families represent a diverse, paraphyletic group of non-treefrogs. The ranids are the primarily old world counterparts to the new world leptodactylids.
Many fossil ranids are known, including both extant and extinct species of Rana from the Tertiary and Quaternary in Europe and North America, and probable Ptychadena fossils from the Miocene of Morocco.
Cannatella, D., L. Ford, and L. Bockstanz. 1996. Neobatrachia: Tree of Life. (Website.) 
http://tolweb.org/tree?group=Neobatrachia
Cogger, H. G., and R. G. Zweifel, editors. 1998. Encyclopedia of Reptiles and Amphibians, 2nd edition. Academic Press, San Diego.
Duellman, W. E., and L. Trueb. 1986. Biology of Amphibians. Johns Hopkins University Press, Baltimore, MD.
Ford, L. S., and D. Cannatella. 1993. The major clades of frogs. Herpetological Monographs 7:94-117.
Pough, F. H., R. M. Andrews, J. E. Cadle, M. L. Crump, A. H. Savitzky, and K. D. Wells. 1998. Herpetology. Prentice-Hall, Inc., Upper Saddle River, NJ.
Stebbins, R. C., and N. W. Cohen. 1995. A natural history of amphibians. Princeton University Press, Princeton.
Zug, G. R. 1993. Herpetology: an introductory biology of amphibians and reptiles. Academic Press, San Diego.
For More Information
Contributors
Heather Heying (author).
