Hierarchical structure and diagenesis of Sauropod long bones using advanced characterization techniques

Abstract

Sauropod dinosaurs are one of the most fascinating animals, mainly because of the extreme size that they could reach. This gigantism led to the hypothesis that sauropod long bones adopted an optimized hierarchical structure to resist the high loads caused by their heavy weight in excess of 100 tons. The present work aims at evaluating this hypothesis with the following objectives: (1) Investigate modifications linked to diagenesis at different hierarchical levels of the sauropod fossil using new techniques and methods. (2) Analyze the structure of bone samples in terms of crystallographic orientation, crystal size as well as the general arrangement of fibrolamellar (FBL) and secondary bone observed in sauropod cortical bone.<br /> In the first part, diagenetic effects appearing during fossilization process were studied with different techniques. Synchrotron-X-Ray fluorescence was used for the first time on fossil material, allowing investigations of considerably higher resolution compared to conventional techniques. Furthermore, a combination of X-Ray diffraction and energy dispersive X-Ray analysis during scanning electron microscopy (SEM-EDX) as well as transmission electron microscopy (TEM-EDX) was used to evaluate diagenetic changes at different hierarchical levels of fossil bones. Infillings of secondary minerals were mostly detected in natural pores of bones (vascular canals and osteocyte lacunae). The combination of the different analysis techniques applied in this study revealed that although seemingly unaffected at the histological level, the sauropod fossils endured strong diagenetic changes. Additionally, pronounced differences concerning the diagenesis and secondary mineral infillings between bone samples of a same postmortem burial environment could be observed. <br /> In the second part, crystallographic investigations were carried out in an ontogeny series of Apatosaurus sp.. The basic crystallographic orientation of sauropod bones as measured by X-Ray diffraction is shown to be a <001> texture, where the c-axes of the crystals are aligned parallel to the longitudinal axis of the bone. This texture, which was also accounted for in bones of recent animals, indicated that these long bones are mainly loaded in compression. The remodeling and consecutive lamellar bone reconstruction does not seem to be affected by the general crystallographic orientation. A pronounced texture found in sauropod and elephant bone seems to be linked to the comparatively large weight of these animals. Crystal size in long bones of Apatosaurus sp. was additionally investigated, but the observations are made difficult by postmortem alterations that these bones are submitted to, as crystal size generally increases in fossil bones. The results obtained in this study failed to show an increase of the crystal size and aspect ratio along with the growth of the sauropod. Based on these findings, the nanostructure of FBL bone and secondary lamellar bone were further analyzed using TEM methods. The observed crystallographic organization is different between the two bone tissues: Even if the crystals of primary bone seem to be randomly oriented the <001> texture is present in the first stage of development along the sauropod cortical bone, as shown with diffraction experiments. The secondary lamellar bone observed in the sauropod bone, on the other hand, display different crystallographic orientations in a rotate plywood model, similar to observations of recent bones. <br /> The results obtained in the present study do not confirm the initial hypothesis that sauropod bone exhibits a superior high-strength structure compared to bones of recent large mammals.