Unusual alteration of fish bones in the Lower Cretaceous lacustrine palaeoenvironment of the Sanfranciscana Basin, Brazil

Taphonomic processes can modify vertebrate hard tissues, producing morphologies and chemical signatures that depart markedly from those of the original living organism. We document an uncommon mode of preservation of bone fragments assigned to the coelacanthiform Mawsonia from the lacustrine deposits of the Quiricó Formation (Lower Cretaceous, Sanfranciscana Basin, southeastern Brazil), in which the internal portions of dermal bones were partially exposed as tubular structures. Combining optical and electronic microscopy, energy-dispersive spectroscopy, X-ray diffraction, X-ray fluorescence, and Fourier-transform infrared spectroscopy, we demonstrate that the tubes are taphonomically transformed cortical bone tissue, being predominantly composed of fluorapatite and infilled with diagenetic calcite. Bone histology of Mawsonia and comparisons with available data for extant coelacanths indicate that the tubes represent remnant vascular cavities bounded by hypermineralized secondary bone, following partial degradation of the less mineralized primary cortical tissue. Although microbial activity likely contributed to bone loss as part of the taphonomic pathway leading to the tubular structure, no definitive evidence supports that the tubes are microbial borings. These findings expand the spectrum of taphonomic modification of vertebrate hard tissues in non-exceptionally preserved freshwater deposits.