Andreolepis

The scales of Andreolepis have a rhombic shape and contain a thin monolayer of ganoine.[1] The squamation pattern has been divided into ten morphotypes, each presumably covering a distinct section of the body.[5] However, some variation in scale morphology might be due to different developmental stages and different species, as no distinction was made between A. hedei and A. petri when reconstructing the squamation. Peg-and-socket articulation of the scales is already present in this basal fish genus and its scales are reminiscent of those of actinopterygian scales in early developmental stages, which indicates that developmental heterochrony might have been a mechanism by which differences in scale morphology evolved.[5]

Previously it was thought that the dentary of Andreolepis did not contain true teeth, but instead harbored denticles. The lack of teeth and the recognition of initial denticle organisation suggested a basal phylogenetic position within the osteichthyes.[4] It was even argued that the presumed dentary fossil of A. hedei is uninformative of dental evolution, as the fossil did not represent dental development, but rather development of the dermal skeleton.[12] This would mean the tooth-like structures of Andreolepis neither match with teeth of chondrichthyans nor with those of osteichthyans and are more similar to the development of structures in dermal scales.[12] Recently it was shown that A. hedei did have functional teeth that were shed by basal-resorption, something that was overlooked during previous research efforts due to methodological limitations.[6]

The location of the resorption cups, places where resorption of the tooth base took place, and newly formed teeth are not perfectly aligned, which suggests a flexible form of tooth replacement.[6] Nonetheless, some form of patterning can be recognised in the teeth. Multiple layers of resorption cups have been found, which means shedding by resorption could take place multiple times. The presence of a primitive form of tooth development in the most basal osteichthyans sheds light on the manner by which this has evolved.[6]

Fossils including those of Andreolepis together with genetic inferences also helped to elucidate the evolution of enamel.[13] The scales of Andreolepis contain the enamel homologue ganoine, but the dermal bones and teeth don't. Moving up in the phylogenetic tree, more derived extinct and extant species show a shift of enamel-containing structures from the scales, to the dermal plate and eventually the teeth, with enamel lost in dermal teeth-like structures and in some cases even in the teeth of the most derived groups of tetrapods and teleosts. This might indicate that enamel first evolved on dermal tissues like scales and only later in teeth.[13]