Trophy Skull at the Hopewell Mound Group in Ross County. What Does it Signify?

Trophy Skull at the Hopewell Mound Group in Ross County. What Does it Signify?

The skeleton on the left is flanked by a ocean shell and then a trophy skull that appears to have a copper helmet. What does this signify? 

The burial was mound # 5 within the enclosure marked "effigy."