Fossil

Fossils are structures that generate underground in certain biomes.

Generation
Fossils randomly generate 15-24 blocks below the surface in deserts and swamplands, including the "Hills" and "M" variants. Each chunk has a $1/64$ chance to spawn a fossil

Structure
Fossils may generate as one of 4 different variations of skull or spine. Fossils can be made up of bone blocks and coal ore.