Fossil

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

Generation
Fossils randomly generate 15–24 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.

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

 * 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).