A microstructural study of the ganoine tissue of selected lower vertebrates

Abstract

The histology and crystallite arrangement of the surface hard tissues in the dermal scales of selected lower actinopterygians, both recent and extinct, as well as several acanthodian genera, were compared, together with the tissue of the collar region in oral teeth of a few actinopterygian genera, to investigate their microstructure and its possible phylogenetic significance. Ganoine exhibits a wide diversity of crystallite arrangements in distinct genera, as does the enamel in sarcopterygian teeth between genera. There are no clear criteria to distinguish between the crystallite arrangement of certain ‘ganoines’ of actinopterygian fish and that of the enamel of some sarcopterygian teeth. Our results suggest that there are basically two morphological groups of ganoine: an enamel-like tissue, the wide variety of crystallite arrangements of which might be of systematic significance, and an enameloid-like type. The first is typical of most actinopterygians, with the exception of at least some Ginglymodi, whose ganoine resembles the acrodin tissue of actinopterygian tooth caps. The term ganoine as traditionally used is more descriptive than systematically relevant, since it can be either single or multilayered. This enamel-like tissue is possibly homologous with other birefringent, single or multi-layered tissues covering the scales of acanthodian, heterostracan and sarcopterygian fish.