Microstructure and Growth of the Dermal Skeleton in Fossil Actinopterygian Fishes: Boreosomus, Plegmolepis and Gyrolepis

The odontodes of some of the palatal dermal bones in Boreosomus piveteaui Nielsen tend to form odontocomplexes, e.g. symmetrical areal ones where some degree of superimposition may occur between ganoin layers belonging to the component odontodes, and asymmetrical areal ones where this is quite insignificant. In the areal odontocomplexes of the dermal bones in Plegmolepis sp., the degree of overlap between the corresponding layers is somewhat more pronounced. Finally, in the areal odontocomplexes of the dermal bones in Gyrolepis cf. albertii Agassiz, we have a more advanced stage of phyletic specialization where each of the ganoin layers of the component odontodes lies directly superimposed on the preceding one throughout the extent of the latter. At the same time, the effect of phyletic dentine reduction is clearly noticeable here by the development of “extra” ganoin layers no more retaining their original connections with that hard tissue (also developed in the scales of Plegmolepis sp.). Remarks are i.a. given on the characters by which Acropholis and Plegmolepis are said to be distinguishable from each other.