Evolutionary trends and possible origin of the Weberian apparatus

A synthesis of anatomical, paleontological and ontogenetic knowledge of the Weberian apparatus is presented. The primitive state of the Weberian apparatus of the extant otophysan orders is deduced and evolutionary trends are discussed in a functional perspective. Main trends are shortening, reduction of weight, loss of intermediate structures, and ankylosis of anterior vertebral centra. Paleontological and developmental data help to develop a tentative reconstruction of a primitive Weberian apparatus, without a manubrium of the intercalarium. Sagemehl's hypothesis on the origin of the Weberian apparatus, which considers the interossicular ligament as the first structure appearing, provides a basis for a more detailed scenario in which each step leads to a more efficient sound transmitter.