Mineralogy and compositional changes in bone fragments attributed to an ornithopod dinosaur from the Barremian locality of Buenache de la Sierra (La Huerguina limestones Formation, Cuenca, Spain)

Abstract

X-ray diffraction, optical polarizing microscopy, scanning electron microscopy and electron microprobeto, have been used to analyse the diagenetic processes of isolated dinosaur bones and ribs attributed to an ornithopod dinosaur from Buenache de la Sierra (Upper Barremian, La Huérguina Formation). We also examine the geochemical and mineralogical bone mineralization of extant archosaurs (caiman and bird) for comparison. Diagenetic processes have modified the original chemical composition of the studied fragments of bones, as well as its crystallinity, which increases in the fossil samples. In recent bones, hydroxyapatite results the main mineral constituent, while in fossils is fluorapatite. We found two types of mineralogical arrangements: one group exhibiting the spongy structure completely collapsed forming breccias of bone fragments, and a second group with well preserved spongy structure but substituted by calcium carbonate. Calcite is the most abundant authigenic mineral infilling pores and fissures of the fossilized remains. The presence of pyrite in bone cavities suggests an early burial stage under euxinic conditions of a reductor environment. The biostratinomic and fossildiagenetic alterations allow us to conclude that the remains have two distinct taphonomic histories, and that the association undergone taphonomical resedimentation.