The Middle Ear

Abstract

The middle ear of land vertebrates presents one of the most remarkable and best documented examples of functional transformation in vertebrate evolution (Carroll, 1987; Lombard and Bolt, 1979). The essentially single ear ossicle of the non-mammals (with two sub-components, the columella and extracolumella), is ultimately derived from a series of cartilaginous rods which were part of an extensive system of gill-support arches in the earliest vertebrates. The acquisition of jaws by later fish led to these particular components of the gill-arch system (the hyomandibular arch, which lay immediately behind the arch giving rise to the jaws) being utilized as structures strengthening the jaw articulation. A later change in the articulation of the jaw produced a redundancy of function, so that these rods became free in the region behind the jaw joint. The various components of the hyomandibular arch can be traced today in the ontogeny of various land vertebrates and be seen to build different parts of the ear ossicle of the nonmammals (Fig. 3.1). The two most important parts of this ossicle are the columella (origin in the pharyngobranchial) and extracolumella (origin in the epibranchial of the hyomandibular arch), which together make up the sound-conducting ossicle. Other hyomandibular components make up, for example, the dorsal process of the columella and intercalary cartilage.