Postmaturation Skull Development in Xenopus laevis (Anura, Pipidae): Late-Appearing Bones and Their Bearing on the Pipid Ancestral Morphology

Abstract

Cranial morphology was examined in 2-1/N12-year-old of Xenopus laevis . Cranial development was shown to continue after sexual maturation, and in older animals, elements that are commonly admitted as being absent in X. laevis (and in pipids) make their appearance. Among these are the palatine, the preorbital process of the maxillary, and the lateral alae of the parasphenoid. Vomers, which are usually azygous in Xenopus , become paired in old specimens. Two additional dermal ossifications previously recorded only in some casque-headed hylids (Trueb, 1970), were revealed in the skull of the 9- and 12-year-old X. laevis frogs. These bones, which lie on the premaxillary symphasis and dorsal to the septum nasi, are proposed to be homologous with the postrostrals of crossopterygians. Xenopus is proved to be a neotenic frog in which morphological underdevelopment results from the incomplete realization of its morphogenetic program caused by the retardation of somatic development. Prolongation of somatic development occurring in old animals permits the animal’s morphogenetic program to be realized more completely, and features normally lacking in Xenopus but characteristic of its presumed ancestor may make their appearance.