From Pixelmon Wiki

All nine types of Fossils, exhibited using Fossil displays
Fossils are rocks that contain the remains and DNA of Pokémon from millions of years ago. They can be resurrected into certain Pokémon.
Use
Turning a Fossil into a Pokémon requires multiple steps.
- A Fossil can be obtained by finding a Fossil block, found within gravel piles. Mining this block will yield a covered Fossil.
- A covered Fossil can be cleaned using a Fossil cleaner to obtained a cleaned Fossil.
- A cleaned Fossil can be resurrected into a Pokémon using a Fossil machine. This process takes a few minutes.
- Once the Fossil machine has finished resurrecting a Pokémon, an empty Poké Ball can be placed in the Fossil machine to claim the Pokémon. The Pokémon will start at level 1.
Aside from being resurrected into Pokémon, Fossils can also be displayed using a Fossil display.
Fossil types
There are eleven different types of Fossils, and each can be resurrected into a different Pokémon.