The type of fossil that forms when pore spaces in the original hard parts of an animal are filled in by minerals is called a permineralized fossil.

Here's how it happens:

1. Original Hard Parts: The animal's hard parts, such as bones, teeth, or shells, are buried in sediment.

2. Groundwater Flow: Groundwater carrying dissolved minerals flows through the sediment.

3. Mineral Deposition: The dissolved minerals precipitate out of the groundwater and fill the tiny pores and spaces within the hard parts.

4. Petrified Fossil: Over time, the minerals replace the original organic material, creating a petrified fossil.

Permineralization is a common process in fossilization, and it can produce incredibly detailed and preserved fossils.