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

Trivia

 * Fossils are the only generated structure that is originally belong to animals, or living entities. They are one in 3 structure have no loot chest (the others being Witch hut and Desert well), as well as the only structure have different permanent size.