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.