User blog comment:AmazingPythor/dinos here 1000 years ago/@comment-25461348-20160131053331/@comment-26067066-20160201015733

First of all that very rarely happens, though from what I've read there's a small chance that the iron in a corpse's body can form free radicals, which preserve tissue by acting similarly to formaldehyde, connecting the amino acids found in proteins, thus making them much more resistant to decay. I'm not a paleontologist, nor do I have especially advanced knowledge in any scientific field really, but this article explains it well:

http://www.livescience.com/41537-t-rex-soft-tissue.html