Bony-tailed tadpoles: the development of supernumerary caudal vertebrae in larval megophryids (Anura).

The axial skeleton in most anuran families consists of <or=9 presacral vertebrae, a single sacral vertebra, and the urostyle. Tadpoles from one anuran family, the Megophryidae, deviate, however, from this pattern in bearing supernumerary vertebral centra in their tails. At least 5 of 11 genera from this Asian family share this character: Leptobrachella (approximately 30 caudal centra), Leptolalax (5-6), Megophrys (11-15), Ophryophryne (11-14), and Xenophrys (>or=7). Tadpoles from each genus are typically found in streams, where their extended caudal skeleton anchors muscles that facilitate tadpoles wiggling between plant debris and rocks or even burrowing into the stream bed. The extra centra of megophryids ossify differently in each genus. In Leptobrachella and Ophryophryne, the caudal centra ossify around the entire notochord, whereas in Megophrys and Xenophrys each develops from dorsal and ventral pairs of ossifications that expand to meet each other. The evolutionary loss of caudal centra, an apomorphic anuran trait, is reversed in larval megophryids and confirms that the machinery for caudal vertebral development has been retained in some modern anurans. A likely driving force in the reappearance of the trait in megophryids is the selective pressure associated with a riparian lifestyle.