Skeleton Spawner in Soul Sand Valley

Soul Sand Valleys have large rib like structures made of bone blocks. I believe there could be a structure of a complete fossil with a head all the way to the tail. It would house a skeleton spawner in it. This skeleton spawner (unlike the skeleton spawners in the overworld) could have a very low (maybe 5%) chance of spawning a wither skeleton. Although this would be a very rare structure, it could give players a better way of getting wither skulls.