Nether fossils are structures made of bone blocks that get generated in soul sand valleys. They're often seen on a small floating island.
Unlike normal fossils, nether fossils are never partially made up from coal ore, only bone blocks. They also always generate the entire structure, rather than cutting out blocks at random.
Nether fossils come in 14 different variants, made up of 5 to 27 bone blocks. Each of these structures represent ribs, except for one, this represents a sternum.