From Pixelmon Wiki
Covered Fossils can be obtained from mining Fossil blocks or very rarely when using Forage on sand with a Ground-type Pokémon. There are eight different covered Fossils, each turning into a different type of Fossil when cleaned with a Fossil cleaner. All covered Fossils are labeled as "Covered Fossil", without naming the actual identity of the Fossil.
Covered Helix Fossil
Covered Dome Fossil
Covered Old Amber
Covered Claw Fossil
Covered Skull Fossil
Covered Armor Fossil
Covered Cover Fossil
Covered Plume Fossil
Covered Root Fossil
Covered Jaw Fossil
Covered Sail Fossil