[Stark_GOP]

If you have a closed, airtight system, then water does not continue to evaporate until there is no liquid water left. Rather, water evaporates until there is a certain, temperature dependent, partial pressure of water vapor present in the system. Once this partial pressure has been achieved, then evaporation stops. If mineral deposits on the bone surface sealed it off making an airtight system, then it is entirely possible that the equilibrium water vapor would be reached rather quickly thereafter, and further evaporation would be halted.
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Interesting. Fossilization must have taken place at an incredible rate to provide a thick enough barrier to prevent evaporation.

[stremba]

Not necessarily. If the original atmosphere had a pretty constant high humidity, then the evaporation rate would be very slow. In order to maintain the proper vapor pressure in a small, enclosed space, not much water is really needed. Even without an originally humid atmosphere, only some mechanism for sealing is needed, not fossilization to prevent further evaporation. Fossilization can occur after this alternative mechanism occurs. The alternative could be something as simple as immersion underwater or in mud, which would pretty much prevent the water vapor inside the enclosure from escaping.