Found two of these in close proximity to each other in a recently (2012 or 2013, not this year) logged and fairly moist area (for W. MT), in the foothills, near Schley Ck. on the north end of Missoula County, MT. At first I figured they were queen carpenter ants, but that doesn't really make sense, and something about them makes me think beetle, maybe a mimic of some kind?
Anyway, would love to know what they are. Both individuals are pictured, perhaps a male/female pair? The one with the "dented" elytra(?) was very active, the other, slightly smaller individual was not very active at all. Did not really interact with each other in the 4-5 minutes I observed them. Did not see any more during the outing. Photographed on a above-average warm, sunny May day.