This caught my attention in a big way.  The image below is an artistic rendition of a broken section of a whelk shell engraving found in a mound at Spiro OK.  The date is attributed to 1200 AD.  What does that look like to you?
It was depicted in the book Phillips, Philip and Brown, James A. , Pre-Columbian Shell Engravings from the Craig Mound at Spiro, Oklahoma. Vol. I, Peabody Museum Press. Peabody Museum of Archaeology and Ethnology. Harvard University, Cambridge, MA, 1978.   Plate 9, Figure 12.  I am going to have to save up my "allowance" to afford this book.  I enjoy the shell art stuff anyway; now I have a new reason to get it and learn more.  
If that is a two hole atlatl like I think it is, then that would be evidence that the atlatl was indeed a known object well into the Mississippian period.   Furthermore, if that is a weight on that atlatl (perhaps is and perhaps it isn't), then atlatl weights were a known item into the Mississippian period too.  That looks like a bar weight strapped to a smaller shaft.  I wondered how those bar weights would have looked on an atlatl.  This may be a clue - if it is indeed an atlatl depiction.
Take a look and make up your own mind.  This really makes me think.