The sacro-urostylic region of anurans, which consists of a single sacral vertebra and a rodlike urostyle, is one of the most unusual specializations of the group and seems to have appeared concomitant with their origin and the move toward saltation. The ontogeny of the sacro-urostylic region follows two different pathways. In some taxa, the anterior end of the hypochord (ventral, unsegmented structure of the urostyle) never exceeds to the level of the anterior end of Postsacral Vertebra 1, whereas in others it extends anteriorly to the level of the sacrum. Thus, as the hypochord migrates dorsally during metamorphosis, it either fuses to the overlying Postsacral 1 or to the sacrum. In the first developmental mode, the adult morphology consists of an articulating sacrum and urostyle, as in the jumping frogs Acris crepitans and Discoglossus sardus. In the second developmental mode, the adult morphology results in a fused sacrum and urostyle, which is characteristic of the hopping-burrowing Spea multiplicata and the aquatic Xenopus laevis. The articulation between the sacrum and urostyle constitutes the best joint for saltatorial locomotion, whereas the loss of mobility between these two elements seems to increase stability for digging and swimming. Further rigidity in the sacro-urostylic complex is achieved by fusion of additional vertebrae to the sacrum and urostyle to form a synsacrum, as in the aquatic Hymenochirus curtipes, and may represent an adaptive trait for maintaining the body straight during swimming.