By Heather Heying
This family of little-known aquatic frogs contains but four species in two genera. Their distribution is disjunct, but limited to South America. They are found in tropical lowlands east of the Andes, and in the Magdalena Valley of Colombia.
Paradox frogs are so dubbed for the extremely large size of their tadpoles (up to 250 mm total length in Pseudis paradoxa) compared to the moderate size of metamorphosed adults (adults of the same species attain 70 mm snout-vent length). Adults are stout-bodied, with strong hindlimbs and webbed feet. They have intercalary elements between the terminal and penultimate phalanx. These are elongate and periochondrially ossified, unlike the intercalary elements of other anuran clades, which has led some researchers to consider them a synapomorphy of Pseudidae. Pseudids have eight holochordal, procoelous presacral vertebrae; cartilaginous sternum and omosternum; palatines but no parahyoid; dentate maxilla and premaxilla; horizontal pupils; and type IV tadpoles with both denticles and beaks. Diploid number is 24.
Pseudids are almost entirely aquatic. The elongate intercalary element may serve to lengthen the digit, and therefore increase the functional area of the webbed foot in these aquatic frogs. They use their long fingers and toes to stir up mud on the bottoms of shallow lakes while foraging. Pseudis paradoxa has been observed floating among vegetation in shallow ponds and roadside ditches. This species may be able to survive dry periods buried in mud. Amplexus is axillary, and eggs are laid in a frothy mass among aquatic vegetation in shallow water. In Pseudis paradoxa, tadpoles grow to their large size in approximately four months, after which time they metamorphose into much smaller juveniles.
Pseudids are members of the Neobatrachia, the relationships of which are highly controversial. Most authors identify a superfamily, alternately called Bufonoidea or Hyloidea, which includes all the neobatrachians that are neither Ranoids nor Microhyloids. The group Bufonoidea is thus sketchy at best. Furthermore, most relationships among the bufonoids are not resolved. However, several researchers have presented evidence that Pseudidae, Hylidae and Centrolenidae are a monophyletic group, based largely on the shared presence of intercalary elements. Other researchers argue that the intercalary elements found in pseudids are not of the same origin as those in hylids and centrolenids; if this is the case, the sister taxon of Pseudidae is unknown. Absent the unique intercalary elements, pseudids would probably be considered part of Leptodactylidae.
No fossil pseudids are known.
Adler, K., and T. R. Halliday, editors. 1986. Reptiles and Amphibians. Torstar Books Inc., New York.
Cannatella, D. 1996. Pseudidae: Tree of Life. (Website.) 
http://tolweb.org/tree?group=Pseudidae
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).
