Pipidae

Introduction

Pipids are highly aquatic frogs that rarely if ever venture out of water. They have several adaptations to aquatic life, including the loss of the tongue (tongues are not generally useful for feeding in water), and the presence of lateral line organs, which are used to detect wave motion in water (these are present in most groups of fishes). The group is sometimes called the Aglossa.

Pipid frogs are found in Africa, South America, and just get into Panama. Some species in South America, such as the Surinam Toad (Pipa pipa) are extremely flattened and look like roadkills. Females of the genus Pipa have an elaborate mating behavior, in which eggs are deposited on the back of the female, and the skin swells up around the eggs to encase them in pockets in which the embryos develop. In some species the eggs hatch out as tadpoles, but in others fully formed froglets emerge from the mother's back.

Tadpoles (when present) lack beaks and denticles, and have paired spiracles (if spiracles are present). This is the Orton type 1 tadpole, also found in Rhinophrynidae. There is much diversity in larval morphology and ecology in pipids. Tadpoles of Xenopus and Silurana are extremely efficient filter feeders. Tadpoles of Hymenochirus are carnivorous, eating larger prey items. In some species of Pipa the eggs (embedded in the mother's back) hatch out as tadpoles, but other species have direct development, in which froglets emerge.

The genus Xenopus (African Clawed frogs) has undergone drastic evolution in chromosome number, producing tetraploid (4n), and octoploid (8n), and even dodecaploid (12n) species. These higher levels of ploidy may have resulted from hybridization of between species. One species, Xenopus laevis, is widely used as a lab animal in molecular and developmental biology. The Dwarf Clawed Frogs (Hymenochirus) are very small, about 20-30 mm, and are widely sold in aquarium stores. The call of many pipid frogs is a clicking sound, which in Xenopus borealis is produced by forcefully pulling apart the large arytenoid cartilages of the larynx (voice-box), thus producing a "pop" by implosion.

Geographic Distribution

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The distribution of living members of the familiy Pipidae is indicated in red.

Discussion of Phylogenetic Relationships

The definition of the name Pipidae is problematic because of the relationships of Mesozoic and Tertiary taxa to living pipids (Báez, 1981). Báez' cladogram placed †Thoraciliacus, †Cordicephalus, †Saltenia, and †Eoxenopoides outside of the living Pipidae. She used two synapomorphies for Pipidae (including the aforementioned taxa): the absence of a quadratojugal and the absence of mentomeckelian bones. The first of these is also present in the closely related †Palaeobatrachidae, and may be diagnostic of a larger clade. Although mentomeckelians are reported in palaeobatrachids, the examination of †Palaeobatrachus fossils and figures in Spinar (1972) has not convinced Cannatella of their presence.

Cannatella and Trueb (1988a) diagnosed the living Pipidae by a large number of synapomorphies including presence of an epipubis cartilage, an unpaired epipubic muscle, absence of a quadratojugal, free ribs in the larvae, a fused articulation between the coccyx and sacrum, a short, stocky scapula, elongate septomaxillary bones, ossified pubis, a single, median palatal opening of the eustachian tube, lateral line organs in the adults, and absence of a tongue. Several of these characters are present in fossil taxa, and thus may be diagnostic of larger clades. Several others cannot be assessed in fossils. By ignoring the fossils, Cannatella and Trueb (1988a) produced a diagnosis for the family that was misleading. Relationships within the living Pipidae were discussed by Báez (1981), Cannatella and de Sá (1993), Cannatella and Trueb (1988a,b), and de Sá and Hillis (1990).

To ensure stability, Ford and Cannatella (1993) defined the node-based name Pipidae to be the most recent common ancestor of living pipids (Xenopus, Silurana, Hymenochirus, Pseudhymenochirus, and Pipa) and all of its descendants. Taxa considered to be fossil "pipids" (†Thoraciliacus, †Cordicephalus, †Saltenia, †Shomronella, and †Eoxenopoides) are assigned only to the level of Pipimorpha.

Other Names for Pipidae

• Aglossa
• Vernacular Names: Tongueless Frogs

References

Báez, A. M. 1981. Redescription and relationships of Saltenia ibanezi, a Late Cretaceous pipid frog from northwestern Argentina. Ameghiniana 18(3-4):127-154.

Cannatella, D. C., and R. O. de Sa. 1993. Xenopus laevis as a model organism. Syst. Biol. 42(4):476-507.

Cannatella, D. C., and L. Trueb. 1988a. Evolution of pipoid frogs: Intergeneric relationships of the aquatic frog family Pipidae (Anura). Zool. J. Linn. Soc. 94:1-38.

Cannatella, D. C., and L. Trueb. 1988b. Evolution of pipoid frogs: morphology and phylogenetic relationships of Pseudhymenochirus. J. Herpetol. 22:439-456.

de Sá, R. O., and D. M. Hillis. 1990. Phylogenetic relationships of the pipid frogs Xenopus and Silurana: an integration of ribosomal DNA and morphology. Mol. Biol. Evol. 7(4):365-376.

Ford, L. S., and D. C. Cannatella. 1993. The major clades of frogs. Herp. Monogr. 7:94-117.

Spinar, Z. V. 1972. Tertiary frogs from central Europe. W. Junk, The Hague.