While I can't be 100% certain, it looks like it could be Phaeolus Schweinitzii, otherwise known as Schweinitz' butt rot. It is found in Florida, so I don't see why it couldn't be found in Missouri. It is a parasitic fungus that will eventually kill the tree that hosts it. Not much you can do for the tree by the time the fruiting body appears, unfortunately, as one fruiting body can indicate extensive rot in the tree. They usually start off as a firm yellow-green conk that turns reddish-brown and woody over time.
