BrachycephalidaeShield Toads
Two genera are recognized in this small family of tiny frogs. Both genera are likely monotypic (such that the family comprises but two species), though some scholars have argued that Brachycephalus includes two previously synonymized species. Geographic distribution is limited to the humid Atlantic forests of southeastern Brazil.
Gold frogs are easily recognized by their tiny body size and reduced digits on both hind- and forelimbs. Psyllophryne grows to less than 10 mm body length, making it both one of the smallest frogs and indeed one of the smallest tetrapods. Brachycephalus attains an adult size that is only slightly larger. Both genera have but three toes on each foot, and two on each hand, and even these are reduced. In Brachycephalus, there is a dermal bony shield that ossifies dorsal to the vertebrae. The complete ossification of epicoracoid cartilages with coracoids and clavicles comprises one synapomorphy of the family. In all members of the family a sternum is absent. The pectoral girdles of gold frogs also lack omosternums; and palatines and prevomers are absent from the head skeleton. The maxillae and premaxillae lack teeth. The pupil is horizontal. Diploid number is 22.
Gold frogs are diurnal and terrestrial, living on the leaf litter of wet forests. Brachycephalus ephippium is bright orange, and some of its skin glands secrete tetrodotoxin. Psyllophryne (and the possible second species of Brachycephalus) are cryptically colored. Unlike most neobatrachians, amplexus (mating posture) appears to be initiated in an inguinal position, which shifts to an axillary position as eggs are being laid. Direct development of terrestrial eggs (in which eggs hatch as tiny frogs, without going through a tadpole stage) is known from one (Brachycephalus), and probably in the other, genus of brachycephalid. Females coat eggs with soil particles after they are laid, which may serve both to camouflage them and protect them from desiccation.
Gold frogs were originally mistaken for toads (bufonids), but the absence of a Bidder's organ in Brachycephalus prompted the move to the family Brachycephalidae. Brachycephalids are unambiguously 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 at best. Furthermore, relationships among the bufonoids are not resolved.
No fossil brachycephalids are known.
Cannatella, D. 1996. Brachycephalidae: Tree of Life. (Website.) http://tolweb.org/tree?group=Brachycephalidae&contgroup=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.
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.
Zug, G. R. 1993. Herpetology: an introductory biology of amphibians and reptiles. Academic Press, San Diego.
Contributors
Heather Heying (author).
Glossary
- bilateral symmetry
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having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.
- ectothermic
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animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature
- metamorphosis
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A large change in the shape or structure of an animal that happens as the animal grows. In insects, "incomplete metamorphosis" is when young animals are similar to adults and change gradually into the adult form, and "complete metamorphosis" is when there is a profound change between larval and adult forms. Butterflies have complete metamorphosis, grasshoppers have incomplete metamorphosis.
- motile
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having the capacity to move from one place to another.