The main factor for preservation is a lack of biological activity to break down the creature's remains. If it's incased in anything hot that cools quickly, has any antibacterial qualities, or even just the right amount of soil alkalinity then it can be preserved. For hides, though, it's normally more of an imprint left behind than any recoverable bodymass.

In fact, some fossils found in swamps have been almost perfectly preserved due to the Saponification of the oils and lipids in the carcass.

In general, being covered by a wave of hot dirt like in my previous example would seal them up like a can of soup. All of the liquids and chunks would keep moving around until they settled, but any thick hides or bones might still leave recoverable fossils.