What state are in? There are a lot of possibilities, from a washed out old grave, a recent burial, a dumped body, a dead homeless person. You need to call the police and report it. It's really a sick thing to display the skull. It sounds terribly heathen and barbaric.
It's terrifically declasse to display a skull that you did not remove from the owner yourself.
I am curious about where your located also, as my father-in-law has been missing since October 2000 with no clues to his whereabouts. We suspect foul play, since he drove off to get an inspection sticker and never returned and his car hasn't been found. He would of been 93 next month, so we don't think he just decided one day to leave his home and never come back. He didn't take anything with him. So it would be good for you to call the police, it may be someone else's loved one that has gone missing and this would be closure for them too.