Fossil

A fossil is a skeletal structure composed of bone blocks, coal ore, or diamond ore. They are one of the rarest Generated structures in Minecraft.

Generation
Fossils randomly generate y-coordinates 40 to 49 underground in the 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.

Fossils first generate the pure-bone block fossil with a structure integrity of 0.9, meaning 10% of blocks are replaced with structure voids. Next, the coal fossil generates with a structure integrity of 0.1.

, fossil generation has changed. There are now two types of fossils, each having a $1/64$ chance to generate per chunk. One fossil generates between levels 0 and 320. Another generates from level -63 to -8, and is composed of diamond ores as opposed to coal ore.

Structure
Note that naturally generated fossils are composed of a mix of bone blocks, coal or diamond ores, and structure voids, as detailed above. They are made with the NBT structure block format and have NBT files for each type of fossil located in the folder minecraft.jar/data/minecraft/structures/fossil.

Trivia

 * Coal ore generating among fossils could be based on the common misconception that coal and other fossil fuels are formed from fossilized animal remains. While coal can indeed originate from and around fossils, it is usually those of plants and plant matter.
 * Diamond ore generating in place of coal ore in fossils below y=0 could also reflect the common misconception that diamond is formed from highly compressed coal; diamond is in reality a pure carbon allotrope dating back millions of years before the first coal appeared, as coal is formed from decayed plant matter.
 * Hidden in the structure NBT is data that credits ProfMobius with the creation of the fossils.