Ontogeny of the bizarre: an osteological description of Pipa pipa (Anura: pipidae), with an account of skeletal development in the species.

The adult osteology of the direct-developing pipid frog, Pipa pipa, is described based on cleared-and-stained and dry skeletal specimens. Observations on skeletal development are based on cleared-and-stained embryos and young removed from the backs of preserved females. Osteologically, P.pipa is distinguished from its congeners and other pipid anurans by its large size and peculiar skull, which is extremely depressed and hyperossified. Skulls of the smallest individuals are not significantly different from those of other basal anurans at a similar stage of development; comparisons are made with Bombina orientalis, Discoglossus sardus, Spea bombifrons, Rhinophrynus dorsalis, and Xenopus laevis. The general sequence of chondrification and ossification resembles that of X.laevis; however, there is evidence that the mandible forms earlier in Pipa than in Xenopus. The major allometric transformations that result in the morphologically bizarre skull of adult P.pipa commence after the embryo has resorbed its tail, an event interpreted as marking the end of metamorphic climax in this taxon. In addition, ontogenetic comparisons reveal that the sacrum forms differently in Discoglossus sardus,Silurana tropicalis, and P.pipa. The development of the sphenethmoid region of the skull is the same in P.pipa and X.laevis, and distinctly different from the development of this region of the skull in other non-pipid basal anurans and neobatrachians for which ontogenetic descriptions exist.