Comparative anatomy of the external and middle ear of palaeognathous birds.

This study provides new anatomical data, morphometric measurements, and functional hypotheses about the middle ear of paleognathous and a basal neognathous bird. The introduction discusses current theoretical concepts and basic functional approaches in vertebrate morphology. A cautious and somewhat skeptical evaluation of the techniques used, errors made, and personal experience gained in this study forms the background of the interpretation of the structural data. The comparative discussion extracts phylogenetic information from the structural data. The external ear opening of paleognathous species is large and unprotected. The auricular feathers show no structural specialization and provide only incomplete opercularization. No muscles insert into the external ear opening. Special attention has been paid to modifications in the cassowary's skin. The skin of its head has developed cavernous blood sinuses that may be flooded and thus inflate the head during booming display. The external auditory meatus of paleognathous birds is large, bends ventrally, and reaches the tympanic membrane from the ventral side. No peculiarities can be described. The osteology of the tympanic region is described in detail. The fusion of all bony elements and the ossification of connective tissue are important features of the avian middle ear region. The fusion of bone is a necessary prerequisite for extended pneumatization. The ossification of connective tissue, however, complicates morphological description since it deviates from morphologically defined bones. Especially in the neognathous button quail, most of the ventral wall of the middle ear cavity consists of ossified connective tissue; it also includes elements of the extracolumella. The fixation of the eardrum to the bony wall of the tympanic cavity is described in detail. The fixation includes the kinetic quadrate and loose connective tissue in the ventro-lateral part of the middle ear cavity. Movements of the bill must change the tension of the eardrum and thus affect the hearing of birds. The recesses of the middle ear cavity have been investigated using X-ray computed tomography. Using this technique, subsequent three-dimensional reconstruction provides unusual and unique insights into the anatomy of such "nonstructures" as air-filled cavities. It has been shown that three tympanic recesses are a character shared by all birds. Of more functional importance is the interaural pathway provided by the anterior tympanic recess and connecting the contralateral middle ear cavities. This structure is present in all recent and mesozoic birds. It has been suggested that the interaural pathway might function as a sound pressure gradient receptor. Physiological data are controversial but in many cases supportive.(ABSTRACT TRUNCATED AT 400 WORDS)