Fossil

A fossil is the apparent remains of a giant prehistoric creature.

Generation
Fossils randomly generate 15–24 blocks (i.e., at y-coordinates from 40 to 49) underground in 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.

Structure
Fossils are composed of bone blocks, with random blocks removed and some of them replaced with coal ore. 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

 * Fossils are one of four structures that do not have a loot chest (the others being swamp huts, desert wells, and ocean monuments).