Nether Fossil

Nether fossils are fossils that generate in soul sand valleys.

Generation
Nether fossils generate randomly throughout the Nether in soul sand valleys.

Structure
Nether fossils are composed of 5 to 27 bone blocks. Unlike regular fossils, they do not have a lowered structure integrity and no blocks are replaced with coal ore. They are made with the NBT structure block format and the NBT files for each fossil are stored in the folder minecraft.jar/data/minecraft/structures/nether_fossils.

There are 14 different variants of nether fossils:

Trivia

 * Nether Fossils are one of the five generated structures that never generate with loot chests (the others being swamp huts, overworld fossils, desert wells, and ocean monuments).
 * Fossils probably represent the remains of huge dead animals in the soul sand valley, which in fact seems similar to a pyroclastic deposit, or what remains of an imposing volcanic eruption, probably from the basalt deltas, which probably killed these animals.
 * They are fossils that do not generate with Coal Ore despite there being no nether coal ore existing in the game yet.