Based on a quick search, Benton County has Ordovician, Devonian, and Mississippian Period marine fossils.  So this covers a lot of time periods, and the fossil's age depends on exactly where you found them.  I would suggest that you google:
"Benton county" Arkansas fossils stratigraphy. 
Put quotes around Benton county.  If you look through the websites that come up from this search, you may be able to narrow the age down based on what part of the county these came from.