PelobatidaeSpadefoots, Archaic Frogs, Pelobatids,
Pelobatids are squat and toadlike, with soft skins and fossorial habits. This treatment places Megophryidae in a separate family, leaving but two or three extant genera among the spadefoots, and little more than 10 living species. Spadefoots are found in North America (Scaphiopus and Spea) and Europe, western Asia, and extreme northern Africa (Pelobates).
Spadefoots are so-named for the large, keratinous metatarsal tubercles that are internally supported by a well-ossified prehallux, located on the outside edges of their feet. They use these spades to burrow rear-first into the ground (the common direction for burrowing anurans). Pelobatids have round bodies of moderate size (50 - 85 mm snout-vent length), with short limbs, large eyes, and vertical pupils. Synapomorphies of the group include the spade, fusion of the joint between the sacrum and coccyx, and exostosed frontoparietals. Additional characters include the lack of ribs, broadly expanded sacral diapophyses, astragalus and calcaneum fused only at the ends, two tarsalia, dentate premaxillae and maxillae, inguinal amplexus, and indirect development with type IV tadpoles. Diploid number is 26.
Spadefoots spend much of their time underground, emerging only after heavy rains. They are adapted for dry environments, and can decrease the water potential of their body fluids by accumulating urea in their plasma, such that they can reabsorb water from any soil with a higher water potential. Spadefoots usually dig their own burrows, but are also known to use the burrows of other animals. Multi-species choruses of Scaphiopus and Bufo have been reported from southwestern U.S. Even in situations in which only a single species of Scaphiopus is calling, the chorus can be almost deafening. The North American species (Scaphiopus and Spea) are explosive breeders that live in sandy soils in dry habitats. All spadefoots deposit pigmented eggs in temporary ponds. Some Scaphiopus species have extremely rapid development times, with time elapsed from egg laying to metamorphosis as low as eight days. Some spadefoot species are known to be highly territorial, defending the same territory over the course of several years.
Pelobatids have traditionally been considered archaeobatrachians, members of the paraphyletic group of primitive frogs out of which the neobatrachians evolved. Traditionally, the superfamily Pelobatoidea, to which pelobatids belong, has been considered sister to the neobatrachians. Some authors propose the use of the term Mesobatrachia to refer to the clade containing Pelobatoidea and Pipoidea, suggesting a sister relationship not between pelobatoids and neobatrachians, but between pelobatoids and pipoids. Together, the mesobatrachians and neobatrachians form a large clade referred to as Pipanura. The superfamily Pelobatoidea is widely recognized as a monophyletic group containing Pelobatidae, Pelodytidae, and Megophryidae. Synapomorphies of the Pelobatoidea include the presence of a palatine process of the maxilla, and the ossification of the sternum into a bony style. Many authors have included megophryids within Pelobatidae, as a distinct subfamily. The monophyly of Pelobatidae is in some doubt, although the monophyly of each of its genera is not.
Several fossil spadefoots are known, including Scaphiopus from North America in the Oligocene, and Pelobates from Europe, from the Eocene through the present. There is controversy over the placement of the extinct subfamily Eopelobatinae within the Pelobatoidea.
Cannatella, D. 1996. Pelobatidae: Tree of Life. (Website.) http://tolweb.org/tree?group=Pelobatidae&contgroup=Salientia
Cannatella, D., L. Ford, and L. Bockstanz. 1996. Salientia: Tree of Life. (Website.) http://tolweb.org/tree?group=Salientia&contgroup=Living_amphibians
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.
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.