Mechanisms of intracranial kinetics in fossil rhipidistian fishes (Crossopterygii) and their relatives

Abstract

The fishes of the Order Crossopterygii are characterized by a unique articulation within the braincase, by which the anterior division of the endocranium may be moved dorso-ventrally with respect to the posterior division. The structure of the skull in both groups of crossoptery-gian fishes (the fossil Rhipidistia and the fossil and Recent Coelacanthini) is such that ‘normal’ operation of the intracranial mechanism involves lateral movements of the cheek region and palate corresponding to the dorso-ventral movements of the ethmoid portion of the braincase. The hyomandibular has a function of prime importance in integrating the movements of the various skull components relative to each other. There are important differences between the characteristic intracranial mechanisms of Rhipidistia and Coelacanthini which may be interpreted in adaptive as well as morphological terms. Analysis of the intracranial kinetics of the Rhipidistia reveals a trend, in certain lines, for the amount of relative movement between the skull components to be decreased and this may be used to explain the loss of the intracranial joint in the Amphibia during their evolution from the Rhipidistia. The functional significance of the intracranial articulation has both a kinetic and a dynamic aspect and while in the Amphibia the kinetic ability of the skull is almost wholly restricted, the dynamic features of the ancestral condition are modified and developed as the basal articulation between the palate and endocranium is retained.