THE CHORDA TYMPANI AND MIDDLE EAR AS GUIDES TO ORIGIN AND DIVERGENCE OF REPTILES

Abstract

The tetrapod middle ear originated by modification of parts of the jaw mechanism of fishes, and its subsequent history is closely associated with the history of the jaw apparatus. Its early evolution is a complex of changes induced by two different influences. On one hand there were modifications of j'aw structure that forced changes upon the middle ear, and on the other there was a tendency for modificatiton of middle ear structure toward refinement of hearing, or for maintenance of hearing in the face of alterations induced by structural changes of the jaw. All of these changes were adaptive in nature. The chorda tympani is a small branch of the facial nerve that passes through the tympanic cavity on its way to the lingual side of the lower jaw. Its relationships to the various parts of the middle ear are remarkably consistent among living tetrapods. Position of the nerve has nothing to do with function of either the middle ear or the jaw mechanism and so is unlikely to have been affected by functional adjustments of the middle ear. Therefore the consistency of its relationships is generally thought to be a conservative feature, one that may be of value in determining patterns of phylogeny, and its position has been extensively studied in living forms. Unfortunately the nerve has left little trace of its position in fossil forms, which have therefore been largely ignored in such studies. Also because of its poor record chorda tympani has seldom been considered in studies of relationships among fossil tetrapods, even though the controversy over the origin and radiation of reptiles currently centers on the nature of the middle ear. A reinterpretation of the pelycosaur stapes (Hotton, 1959) has led to recognition of features that mark the route of the chorda tympani in very primitive reptiles. The present paper is an attempt to correlate this information with the abundant material on the chorda tympani in modern forms, in order to demonstrate that the position of the nerve in the pelycosaurs represents a logical intermediate between the condition of fish and amphibians on one hand and all amniotes on the other, and to show that changes of the middle ear from fish to modern amniotes can reasonably be documented by known fossil forms.