Histology of sculptured cranial dermal bones of the stem salamander Kokartus honorarius (Amphibia: Caudata) from the Middle Jurassic of Kyrgyzstan

Abstract

Kokartus honorarius from the Middle Jurassic (Bathonian) of Kyrgyzstan is one of the oldest and basalmost salamanders in the fossil record, and this taxon is useful for documenting a primitive caudate condition and patterns of character state transformations within Caudata. Here, we provide a histological analysis of its dermal bones based on two thin sections of a fragment of a sculptured dermal skull roof bone (squamosal?). The dermal bone histology of Kokartus is similar to that of basal crown-group tetrapods (e.g. temnospondyls, lepospondyls and seymouriamorphs) in the following features: the presence of basic pattern of organisation of dermal bones (diploë structure); the cancellous middle region occupies larger space than the compact cortices; active remodelling with the formation of numerous erosion bays and secondary osteons; high number of vascular canals with the formation a dense vascular network (= ‘rete vasculosum’); and parallel-fibred bone is a dominant type of primary bone matrix. Crown-group salamanders retained the diploë structure, but demonstrate features of simplification (in comparison with Kokartus): absence of pronounced sculpture, lower degree of vascularisation and lower degree of remodelling. The simplification of the histological structure is a derived feature for the Caudata which appeared during the transition from stem-group to crown-group salamanders.