Believe it or not, this stuff was assembled as late as the 1870s - it is not a relic from the barbarian dark ages! Someone had far, far too much time on their hands I think, to come up with this idea. The bones were removed from the cemetary around 1500 to make room for new customers and placed in the ossuary; a perfectly sensible thing to do and a pretty regular practice. Actually _decorating_ the crypt with them is rather more irregular!