You may want to contact the Office of the State Archaeologist (OSA) at the University of Iowa. You can also ask /r/Iowa. The OSA would be the best place; they can tell you how the artifacts are best handled, and they may be interested in the site.

If this is part of work on federally-funded or permitted construction, you may have legal obligations in which case you should let your supervisor know.

A curator at a public museum (natural history/paleontology or archaeology) is another good place to get them looked at. At a glace I am pretty sure that they are not from the modern bison species, but they need to be measured to be sure.