It seems that the presence of iron, which was originally locked up in the animals blood, contributed to the preservation of the collagen fibres within the bone through generation of oxygen free radicals. You can read more about it in
this article, also the original journal article can be found
here which should come out from behind the paywall in the next few months. Of course rapid burial and mineralisation, as Dr Adequate highlighted, also contributed to preservation, and remember that it was only after release from it's mineralised state that the collagen tissue became pliable.
Edit: And Coragyps beat me to it
Edited by Malcolm, : No reason given.
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