a pice of fossil能这样用吗?

不可,
如你所说 fossil 是可数名词,
fossil 本身就有 piece 的含义,
因此我们说
a fossil
two fossils
其用法和 cake 不同.
例：
Fossils have been found at a recent excavation site in Egypt.
Two fossils date back to the pre-historic period of human evolution.