Fossilization of Haversian bone in aquatic environments

The process of fossilization is still in many respects poorly understood. Changing chemical conditions in the fossilizing bone allows us to distinguish successive stages of bone diagenesis. As long as organic compounds are present in the bone, the first stage, the early diagenesis, is not completed. However, the early diagenesis itself can be subdivided into three stages. Initially the organic compounds, mainly collagen, allow intensive microbial activity on the bone and this microbial decay characterizes the first stage by high decay rates and strongly reducing conditions. After about one year, the microbial activity ceases and the second stage begins, which lasts much longer and is characterized by chemical gelatinisation of collagen in the bone. Finally imbibition of the gelatinised collagen produces cracks across cement lines of secondary osteons and opens additional pathways for diffusion. In this last stage of the early diagenesis, collagen is replaced by apatite and other minerals. After this replacement, late diagenesis begins. The redox milieu in the fossilizing bone is now controlled by the environment, but the high phosphate content of the apatite still buffers the pH to high values. During late diagenesis, pH-dependent precipitation is the most important mineral formation process.