By Heather Heying
These pointy-headed frogs have unique tadpoles and are represented by more genera than any other anuran family. Most authors assign nine subfamilies, containing more than 60 genera, and more than 300 species. Distribution is widespread, and microhylids are found throughout most tropical and temperate regions except for the Palaearctic, most of Australia, the West Indies, and most Oceanic islands. Major radiations, and most of microhylid diversity, are found in Madagascar and New Guinea.
Microhylids often have small, pointy heads, round bodies, and narrow mouths. They range in size from tiny (10 mm) to relatively large (100 mm). The primary synapomorphies of this group are larval characters. Microhylid tadpoles lack cornified denticles, have a medially divided ventral velum, a glottis that is fully exposed on the buccal floor, non-perforated nares, and secretory ridges of branchial food traps with only a single row of secretory cell apices. In addition, adults have two or three palatal folds. Phrynomerines have intercalary elements. There are several additional synapomorphies that diagnose the group Scoptanura, which is defined as Microhylidae except Scaphiophryninae. Scoptanura, and perhaps Microhylidae, has type II tadpoles, without beaks or denticles. In addition, most microhylids lack teeth, have reduced or absent clavicles, have a horizontally elliptical or round pupil, and exhibit axillary amplexus. Diploid number is 22 to 28.
The microhylids represent such diversity of form and habit that it is impossible to efficiently summarize the group. Expanded toe discs are found in arboreal species (e.g. Kaloula); pointy snouts are pronounced in fossorial species; and some terrestrial forms have orb-like bodies. Microhylids are found in arid deserts, extremely wet rainforests, and almost everywhere in between. Most of the subfamilies represent a radiation within a limited geographical area. Madagascan endemics include Cophylinae and Scaphiophryninae; the former are predominantly tree frogs, the latter are terrestrial and semi-fossorial. Some treehole-breeding scaphiophrynines have males that defend their eggs and non-feeding tadpoles until metamorphosis. The Asterophryinae and Genyophryninae are nearly restricted to New Guinea, and all exhibit direct development. Phrynomerines (Africa) and most microhylines (found in the new world and Asia) have small eggs and aquatic type II tadpoles. Breviceps, from Africa, are so round and short-limbed that amplexus is impossible; glands on the male's venter issue secretions that glue him to his partner. Both Breviceps and Hoplophryne construct foam nests, within which nonfeeding tadpoles develop. Two species of new world microhylines form facultative commensal associations with large theraphosid spiders. They share burrows and foraging areas, though other frogs are readily eaten by the spiders.
Microhylids are Neobatrachians, but relationships among the families of these "advanced" frogs are almost wholly unresolved. Within the Neobatrachia, microhylids have sometimes been placed in their own superfamily, Microhyloidea, but are usually assigned to the Ranoidea. Ambiguity at this level of history points to the utter confusion that has supplanted all attempts to make sense of microhylid evolution. The best analysis of microhylid relationships today is almost 70 years old (Parker 1934). Probably, microhylids are part of the ranoid radiation, but diverged early from other ranoids. Within Microhylidae, the inclusion of the subfamily Scaphiophryninae is problematic; it has sometimes been placed in Ranidae, but is usually considered sister to the remaining microhylids. Aside from this subfamily, however, the monophyly of the microhylids is not disputed.
The only fossil microhylids are some Gastrophryne from the Miocene of Florida, U.S.A.
Cannatella, D. 1996. Microhylidae: Tree of Life. (Website.) 
http://tolweb.org/tree?group=Microhylidae&contgroup=Neobatrachia
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.
Parker, H. W. 1934. A monograph of the frogs of the family Microhylidae. British Museum, London.
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.
Contributors
Heather Heying (author).
