content: Metamorphism usually destroys fossils. As far as limestone containing fossils go, it depends on their age.  Before ~500 million years ago any life would have been single celled.  Limestone can be made completely of these organisms shells, or they can be from CO2 precipitating out of ocean water as the acidity rises/falls.  Limestone doesn't necessarily mean fossils.  There are limestones on mars though, and I would give my left nut to get my hands on a sample of martian limestone.