I'm not a dentist but I believe that the use of mercury dental fillings stopped long before Suzie & Stacy would have ever been to a dentist in their lifetime. Perhaps Sherrill had some old fillings which had never been replaced. But those would have to have been some pretty old fillings. Most old mercury based fillings have been replaced by now because they break. See how many people you can find today in their 50's or older, who still have these mercury based fillings that have not been replaced.
My vote says that what you are seeing is just some old decaying tree limbs. Nothing more.
Hurricane, please read this article from the American Dental Association. Dental Amalgrams containing mercury are still being used today. They are currently in use because they are safe, durable, easy to use, and relatively inexpensive.
http://www.ada.org/public/topics/fillings.asp
The family of the Oklahoma Three is convinced there is more to it than "decaying tree limbs".