I have no real qualification for this, but I am thinking an oxygen free environment, suh as bog water.
One paragraph inculde a comment that all ofthe fossils from that location have a decay type smell. I'd suggest inspecting new digs ASAP for further evidence.
I read somewhere they are doing just that.
peat mummies came to my mind, as well.
also, this tissue is all bone-marrow, isn't it? I'd be interested to see a study of the porosity of the bone/fossilized-bone envelope: I'm willing to bet that its pores were too small to admit decomp organisms.