Samuel did have a daughter Mary who was a few years younger, but her middle initial was M. I don't know what happened to Mary, but I'm fairly sure Timothy Dillard Tittle married Margaret. The marriage record gives his wife as Bettie, and a date of 1881. On the 1880 census for Warren Co, Samuel's widow has daughter Margaret still at home, but daughter Mary is gone.
You have another Rigsby connection. Timothy + Margaret had a daughter Mary who married William Tilden Rigsby. William was a son of Margaret's older brother Leander.