Historical constraints and morphological novelties in the evolution of the skeletal system of pipid frogs (Anura: Pipidae)

Abstract

Abstract IPipid frogs of the genera Xenopus, Silurana, Hymenochirus, Pseudhymenochirus and Pipa are dissimilar to all other living anurans and highly derived rather than primitive. Among living pipids, Xenopus is the most primitive-i.e., the most similar to the hypothesized ancestor of the Pipidae. Nonetheless, the adults of this taxon possess many morphological features that distinguish them from their closest living non-pipid relatives, Rhinophrynus dorsalis (Rhinophrynidae) and the pelobatoid frogs. Many of the morphological novelties seem to be associated with the evolution of an aquatic anuran from a terrestrial saltatorial ancestor. The sprawled, laterally positioned limbs and dorsoventral compression of the body in pipids that facilitate their swimming inhibit their hopping or jumping and are correlated with modifications of the thigh musculature and structures of the pectoral and pelvic girdles. Movement of the trunk is limited to anterior-posterior shifts of the pelvic girdle on the sacrum; dorsal-ventral rotation of the posterior trunk, which is critical to saltatorial locomotion, is not possible. Many cranial modifications in Xenopus seem to be associated with its mode of feeding without a tongue, but little is known about the feeding mechanism. The mandible is robust, whereas the upper jaw is weak and suspended from the skull by a modified suspensory apparatus. The hyolaryngeal apparatus is highly modified. The plectra! apparatus is hypertrophied and the tympanic annulus is ossified and in synostosis with the dermal squamosal medially. The orbital region of the braincase is depressed and formed by membrane, rather than endochondral, bone.