Fossilization is a process that requires certain conditions so that it can take place. The climate is very important, a hot and wet place is not a place where there's a chance of finding fossils, while the places with permafrost are a much better candidate because the bacteria can not dissolve the organisms. The pressure also can play a vital role. A place that manages to keep the organisms safe from outside influence, like the tar pits, are excellent preservers. Also the structure of the organisms play a key role, a boneless organism is very hard to preserve and be fossilized, while an organism with bone structure has a bigger chance of being preserved.