How sure are you that Marie Mercier was born in Fall River? I have Mercier's from Fall River, but I cannot find a record of a Marie Mercier (or Messier, or the like) born in Fall River in 1868 plus or minus two years.
There was an Edward Mercier that had a daughter Marie L. Mercier about that time in another city in Massachusetts. His wife was named Lucy.
When did Joseph and Marie die? have you gotten a death certificate for them? How about an obituary for them as well. If after 1930 or so, likely to have some additional info. The cemetery records might reveal the names of the parents.
What other info do you have?