Possible Clues:

• Solidify; turn into bone
• If so, Y's able to get hard like this
• Loss if your houses become inflexible
• Turn to fossil (if fifty were unknown)
• Turn bone-hard
• Convert into bone
• Turn to bone with age
• Lose flexibility, thus getting up stiffly on odd occasions
• Change into bone, or become unprogressive
• Loss, if York houses become fixed