What is this black shelled insect found in Western Arkansas?
Looks like the larvae of a carrion or burrowing beetle (i.e. some beetle species of the family Silphidae). Not sure on the species yet. 

Wikipedia provides a good introduction to this family of insects. 

Here's a picture:


And another:
Silphidae larvae 

Did you see (or more likely smell) any dead carcasses around your property?