Hi, Betty! I still think your best bet is to check the 1900 census, since your Marion should be listed as a 12-year-old in his parents' household, and that should answer your question for sure. I'm hoping to get over to the National Archives this weekend, so if I have time, I'll try to check on it.
I only have two Thomas Taunton/Tantons in my database whose birthdates are close to the 1855 date you mentioned in your message -- both, though, were born in Alabama. I've entered in my database ALL Taunton/Tantons found in the 1850-1870 censuses for GA, AL, LA, FL, MS, and TX, and I'm currently working through the 1880 census. So, as far as I've found, there weren't any other Thomas Taunton/Tantons born c1855 besides the two Alabama-born ones I've already found.