ICHTHYOCLASTS

Many bioclasts yield highly fragmentary fish remains. Isolated scales have previously been undervalued but are particularly suitable for geochemical analyses. These remains are however widely available in certain bioclasts. The hypermineralized ultrastructure in the external layer in Paleozoic and Early Mesozoic fish scales is usually well and quite variably developed. Various methods using a combination of traditional transmittent light microscopy, cathode luminescence, microprobe analysis, and scanning electron microscopy are used and allow interpretation of diagenetic events and systematic re-organization of motley assemblages of such fragmentary fossil fish remains. The continental Permian record of the Parana Basin in Brazil serves as a case scenario, because the bioclastic remains from this former epicontinental sea have been problematic in terms of age determination and characterization of the ichthyofauna. Autochthonous faunal elements may be separated from allochthonous ones using signals from laminated apatites in these fish scales, and also identification of time-sensitive fish remains is possible using thin sections. The great diversity of fossil fish hard tissues in relatively small samples – despite their fragmentary state of preservation – makes ichthyoclasts very useful research objects for a range of comparative studies, including geochemistry, hard tissue ultrastructure and diagenesis. Ichthyoclasts can yield evidence of the composition of the ichthyofauna at a higher taxonomic level and contribute to better resolution in biostratigrahy.