How a bone becomes a fossil
The rift valley in East Africa provides the perfect environment to
preserve animal bones. Almost 100
percent of the time, if an animal dies, its
bones will disappear completely. First
other animals scavenge the body. Then,
the sun's hot rays help the remaining
flesh rot away, and disintegrate the
bones to powder until there's no record
of the animal. As Bob Walter, a
geologist at the University of Toronto,
explains, very specific conditions have to be in place before a bone
can become a fossil.
1. Die near a riverbed.
The animal must die near an ancient river or lake bed that's
undergoing active sedimentation. That means the mud and sand in
the river or lake are accumulating rapidly. If an animal dies in or
around the environment, its bones get covered up by the sediment
very quickly. This prevents two things: the bones getting chewed up
by other animals, and the bones eroding over time.
2. Be exposed to the right kind of water.
Not every bone that gets buried becomes a fossil. Sometimes they
simply rot away. But for those that don't, it's all due to specific
conditions in the sediment. Scientists don't know what those
conditions are exactly, but it has to do with "the right kind of water
percolating through the sediment, and probably a series of wetting
and drying cycles," explains Walter.
3. Change organic molecules into inorganic ones.
Animal bone is mostly made up of inorganic material, mainly calcium
phosphate. But about 10 percent of it is organic - carbon molecules
with life-performing functions. These organic molecules easily break
apart after death and are replaced with inorganic elements such as
calcium, silica and magnesium. This process actually changes the
bone into a stone without changing its shape. And voila! A fossil.