The Early Ontogeny of the Columella Auris Of Crocodilus Niloticus and its Bearing on Problems Concerning the Upper end of the Reptilian Hyoid Arch

Abstract

The columella amis in Crocodilus niloticus has been found to comprise the two upper segments of the hyoid arch, viz., a forked pharyngohyal and an epihyal. To these two primary arch divisions becomes added a laterohyal component derived from delaminating subepidermal blasteme. The crocodilian stapes represents the infrapharyngohyal; the suprastapedial stem (or basal portion of the lacertilian dorsal process) represents the suprapharyngohyal while the extrastapedial represents essentially the epihyal to which laterohyal material contributes the tympanic process as well as the suprastapedial (or lacertilian intercalary). The lateral limb of the so-called lateral prong of the reptilian and avian columella auris is also a laterohyal derivative. It is concluded that in reptiles and birds: (a) the lateral limb and intercalary represent the dorsal articular head of the rhipidistian hyomandibula, (b) the proximal end of the epihyal represents its ventral articular head and (c) the so-called Huxley's foramen is the original passage between the two. Since the lacertilian internal process arises from the pharyngohyal it cannot represent the original rhipidistian quadrate-hyomandibular articulation.