A mystery indeed, especially with the lack of names.
I am not convinced of pirate graves however and the skull and bones found there way onto tombstones well into the 18th century. Here are 3 examples dated 1720, 1774 and 1783. I found these rather quickly in my search and could probably turn up quite a few more if i put the effort in.
http://www.rodcollins.com/wordpress/...ry-explanation