From Ganoid To Elasmoid Scales in the Actinopterygian Fishes*)

Abstract

The present paper summarizes recent results dealing with the development and structure of the scales in the actinopterygian fishes and presents a new phylogenetical interpretation of the routes leading from the ancestral thick ganoid scales to the modern thin elasmoid ones. The different regions constituting the ganoid (ganoine, dentine and osseous basal plate) and elasmoid (Limiting layer, External layer and isopedine) scales are compared in view of possible homologies. The outer Limiting layer of the elasmoid scale is not ganoine, but some common features lead us to suggest that ganoine and the outer Limiting layer might be closer than generally admitted. On the other hand, the study of the development of the ganoid scales in polypterids has undoubtedly shown that the very young ganoid scales arc in fact elasmoid ones. It is concluded that the elasmoid scales of the actinopterygian fishes are derived from the ancestral ganoid scales by a process of paedomorphosis and that the osseous basal plate, and probably the dentine have not been retained in the elasmoid scales.