Bone is composed of an organic part and an inorganic part.
So a fossil is a bone whose organic part has been replaced to some degree by inorganic elements.
So it has slowly become more like stone.
Burial is the most important step in making a fossil.
It has to be buried and it has to be quickly buried before . . . after death.
So if you leave it lying around for a while after it’s dead,
chances are something is going to get to it and eat it
or a herd of wildebeests is going to pass over it and crush it into millions of pieces
or it’s going to just deteriorate from the weather.
Even after it’s buried, it takes a long time to fossilize.
It doesn’t always get preserved.
And also even when it is preserved in these ideal conditions, we don’t always find it.