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.

Skeletal morphology and development of the olfactory region of Spea (Anura: Scaphiopodidae).

The nasal capsules of anurans are formed by an intricate set of sac-like cavities that house the olfactory organ and constitute the beginning of the respiratory system. In tadpoles, nasal capsules do not have a respiratory function, but each is composed of a single soft tissue cavity lined with olfactory epithelium. Our study has revealed that in Spea the nasal cartilages and septomaxillae are de novo adult structures that form dorsal to the larval skeleton of the ethmoid region. The only element of the adult nasal capsule that is partially derived from the larval skeleton is the solum nasi. Development of the nasal skeleton begins at about Gosner Stage 31, with chondrification of the septum nasi and lamina orbitonasalis. The alary cartilage and superior prenasal cartilage are the first of the anterior nasal cartilages to chondrify at Gosner Stage 37. By Gosner Stages 40/41, the ethmoid region is composed of the larval structures ventrally and the adult structures dorsally. By Stage 44, the larval structures have eroded. The adult nasal capsule is characterized by: (1) a septum nasi that projects ventrally beyond the plane of the nasal floor; (2) a paranasal commissure that forms the ventral margin of the fenestra nasolateralis; and (3) a large skeletal support for the eminentia olfactoria formed by the nasal floor and vomer. The timing of chondrification of the anterior nasal cartilages and the development of the postnasal wall, inferior prenasal cartilage, fenestra nasolateralis, and paranasal commissure are discussed and compared with those of other anuran species. This study also includes a discussion of the morphology of the skeletal support for the eminentia olfactoria, a structure best developed in distinctly ground-dwelling frogs such as spadefoot toads. Finally, we propose a more precise restriction of the terminology that is used to designate the posterior structures of the olfactory region of anurans.