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. In Upcoming Java Edition 1.16 (Nether Update), uncovered fossils can be found in the Soulsand Valley biome in the Nether

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 one of three structures that do not have a loot chest (the others being swamp huts and desert wells), as well as the only structure that has a different permanent size.