Fossil

A fossil is a rarely-occurring skeletal structure composed of bone blocks, coal ore, or diamond ore.

Generation
Fossils randomly generate y-coordinates 40 to 49 underground in the Overworld (deserts, swamps, and their variants). Each chunk has a $1/64$ chance to generate a fossil. They may generate as one of the four variants of skull or one of the four variants of spine.

Fossils first generate the pure-bone block fossil with a structure integrity of 0.9, meaning 10% of blocks are replaced with structure voids. Next, the coal fossil generates with a structure integrity of 0.1.

, fossil generation has changed. There are two types of fossils, each having a $1/64$ chance to generate per chunk. One fossil generates between levels 0 and 320. Another generates from level -63 to -8, and includes diamond ore rather than to coal ore.

Structure
Note that naturally generated fossils are composed of a mix of bone blocks, coal or diamond ores, and structure voids, as detailed above. They are made with the NBT structure block format and have NBT files for each type of fossil located in the folder minecraft.jar/data/minecraft/structures/fossil.

Trivia

 * Hidden in the structure NBT is data that credits ProfMobius with the creation of the fossils.
 * The appearence of the fossil "skull_4" got related to the Crewmate-characters of 2018 video game Among Us.