Fossil

Fossils represent the remains of giant extinct creatures.

Generation
Fossils randomly generate 15–24 blocks (i.e. at y coordinates ranging from 40 to 49) below the surface in deserts and swamps, including the "Hills" and "Desert Lakes" variants. Each chunk has a $1/64$ chance to generate a fossil. They may generate as one of four different variants of skull or one of four different 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.