The middle ears of some species of anurans display a set of features that are derived. Some of these features have obvious adaptive significance, whereas the adaptive significance of other derived features is rather obscure. The sequence of histological differentiation of the elements of the derived anuran middle ear, as well as the pattern of morphological changes in their shape, were analyzed and compared with the morphogenesis of the generalized middle ear. Most of the derived features are paedomorphic characters associated with truncated development. The accelerated rate of the opercular development, together with the delayed initiation of function in the tympanic system, permit heterochronic development phenomena to affect the formation of anuran sound-conducting systems.