Fossil

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

Nether fossils are fossils that generate in soul sand valleys in the Nether.

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.

Nether fossils generate randomly throughout the Nether in soul sand valleys. Unlike normal fossils, they do not have random blocks removed or replaced with coal ore. There are 14 different variants of nether fossils.

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