Microstructure and Growth of the Dermal Skeleton in Fossil Actinopterygian Fishes: Birgeria and Scanilepis

The dermal bones of Birgeria and Scanilepis contain numerous odontodes of consecutive generations, each consisting of dentine and a single ganoin layer; superimposition of series of ganoin layers, generally encountered in the scales of the palaeonisciforms. does not occur in any of these bones. In Birgeria, some odontodes near to the dentition resemble jaw‐teeth proper in possessing an apical portion of acrodin; furthermore, nothing indicates the existence here of nerve‐sac groups like those of sturgeons. The histology of the dermal skeleton in Scanilepis does not support the assumption that this form is more closely related to Polypterus than other palaeonisciforms. Remarks are given on the structure of acrodin and ganoin as revealed by SEM study.