Fossil Wood, Bone and Shell
How do wood, bone and shells fossilize?
If you’ve ever seen petrified wood or bone, you’ve probably seen that many of the original structures in the wood and bone are still visible in the fossil. However, the fossilized wood or bone is much denser than the original material; a chemical analysis would reveal large quantities of silica (SiO2), calcite (CaCO3) and little or no organic material (compounds consisting largely of carbon, hydrogen, oxygen, and phosphorus). The same is true for shells.
How does this happen?
- First, bone, wood and shell are porous – they are not as solid as, for example, a block of glass. Those tiny pore spaces can be filled with minerals.
- Second, the water that percolates through the sediments where they are buried is not pure water – it contains lots of dissolved chemicals, such as silicon, carbon dioxide and iron hydroxides. This water also tends to be slightly acidic, and organic material will dissolve in acidic conditions.
- Third, bone and wood need to have been buried rather quickly in an anoxic environment –no oxygen (O2) available to speed up decomposition. Shells are harder with less organic material, so they are less likely to decompose.
Permineralization occurs when the pore spaces in substances such as wood or bone are filled with a secondary mineral such as chalcedony or calcite. This process preserves the shape and internal structures of the bone or wood.
Sometimes, after permineralization the original bone and wood material can be dissolved and then replaced with other minerals. This process is called petrifaction. The bone, wood or shell has been turned to stone – petrified. Most often the minerals that replace the original bone, wood or shell material is silica or calcium carbonate and (rarely) pyrite (FeS2).