HANDRIGAN, G.R.; HAAS, A.; WASSERSUG, R.J.*; Dalhousie University, Halifax, Canada; University of Hamburg, Germany; Dalhousie University, Halifax, Canada: Bony-tailed tadpoles: The form, function, and fate of supernumerary caudal vertebrae in anurans

The anuran axial skeleton consists of no more than 9 presacral vertebrae, a single sacral vertebra, and a coccyx fused with the urostyle. The coccyx is formed from 2-4 neural arch pairs, whereas the urostyle arises from the hypochord, an endoderm-derived tissue. Tadpoles from one anuran family, the Megophryidae, deviate from this archetype in bearing supernumerary vertebrae in their tails. At least 4 of 11 genera from this Asian family share the character: Leptobrachella (>25 caudals), Leptolalax (2-3), Megophrys (12-14), and Ophryophryne (12-14). Tadpoles from each genus are typically found in streams, where their additional vertebrae may facilitate escape from strong currents by burrowing into the stream bed. The caudal vertebrae of megophryids are rudimentary�they have centra without neural arches�and ossify differently in each genus. In Leptobrachella, for instance, the centra are initially cartilaginous; however, in Megophrys, each centra develops from dorsal and ventral pairs of ossified anlage with no apparent cartilaginous precursors. At metamorphosis, the most proximal caudal vertebrae are incorporated into the coccyx. The remaining vertebrae, however, are resorbed along with other tail structures, a process assisted by multinucleate, osteoclast-like cells. The presence of caudal vertebrae, a plesiomorphic trait, in megophryids confirms that the underlying machinery for caudal vertebral development has been retained in some modern anurans, and suggests that the anuran Bauplan, tightly conserved in most families, can still change in response to extreme environments. It remains to be seen which genes involved in axial skeletal development (e.g., Pax1, Pax9, Uncx-4.1) have been suppressed in non-megophryid anurans.