Fossil

A fossil is a skeletal structure composed of bone blocks or coal ore.

Generation
Fossils randomly generate 1–1 blocks (i.e., at y-coordinates from 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 will be replaced with structure voids. Next, the coal ore fossil generates with a structure integrity of 0.1.

Structure
Note that naturally generated fossils are composed of a mix of bone blocks, coal ore, 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

 * Fossil are one of the five generated structures that do not have a loot chest (the others being swamp huts, desert wells, nether fossil and ocean monuments).
 * The one-eyed skull variant is based on an incident where the Ancient Greeks mistook mammoth fossils for the bones of a cyclops.