Longitudinal sliding articulations in pipid frogs

Abstract

The fused sacrum and urostyle of the pipid frog Xenopus laevisl/Ig is a familiar object. It has, for instance, been found at archaeological sites and used to identify the animal in the gut contents or excreta of predators. Yet it exhibits a striking feature which appears to have been mentioned or illustrated very rarely, namely, fluted articulating facets. The fluting, in the longitudinal direction, limits lateral turning and vertical bending between vertebrae. Longitudinal movement also occurs between the ilia and the sacral vertebra, but investigations in the latter half of the twentieth century have not included documentation of earlier work, nor reference to longitudinal fluting of vertebral articulations. In this account, previously unconnected data on the functional anatomy of this familiar animal and its fossil relatives on both sides of the South Atlantic are brought together, and taxonomic implications are indicated.