Fossil

Fossils are the apparent remains of unknown giant prehistoric creatures in Minecraft.

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 were made with NBT structure block format and they have NBT files of different types of fossils in minecraft.jar/data/minecraft/structures/fossils.