Secondary Articulation of the Avian Mandible

THE avian skull, and especially its jaw apparatus, has always been a favorite subject for students of evolutionary and functional anatomy.Any analysis of this ddicate and complicated "machine" requires an extensive knowledge of mechanics as well as of morphology; indeed, the resources of the most competent anatomist are taxed whenever he undertakes a functional study of the avian skull.Omission of even a single feature from analysis usually means that the entire study must be redone.Such is the problem with functional investigations of the avian mandible.Anatomists have always assumed that the quadratearticular hinge is the only suspension of the mandible, and the working hypothesis used in previous studies on the mechanics of the jaw apparatus has been that the forces which counteract the disarticulating forces on the mandible must be supplied by the quadrate hinge.The jaw muscles and ligaments attaching to the mandible provide, of course, some support for it, but for the sake of simplicity these nonbony elements will be omitted from discussion.However, the quadrate is not the only support of the mandible.In many groups of birds, a secondary bony support of the mandible--the roedial brace of the mandible lies medial to the qnadrate-articular hinge.This brace is formed by the medial process of the mandible abutting against the base of the skull, the hinge between the two bones being the badtemporal articulation of the mandible.The functional consequences of this secondary brace are obvious.For many anatomists, information about this brace will be comparable to telling an engineer that there is a third support in the middle of a bridge after he has determined the bridge stresses by assuming that the only supports were at the ends.The discovery of the roedial brace was quite by accident during a taxonomic review of the plovers.Among other features of the skull, I had to examine a process on the lateral margin of the basitemporal plate (Bock, 1958: 46).Much to my surprise, this process was capped by a pad of tissue which was fibrous in nature.At the time, I was preoccupied with the comparisons and did not realize the significance of this capping pad.Several days later during a lull in the taxonomic investigations, I returned to the problem of why this process of the basitemporal plate should have a fibrous capping pad.Only then did I realize that fibrous pads on bony processes mean, in general, only one thing--another bone articulates on this process.