"No, I think that is because of the photographic angle. Also, Brian says he keyed it out, which means (I assume) that since it's a male, the last sternite is notched. That makes it either atrata or proxima. Based on the punctation and shape of the pronotum, I would have thought it was atrata (though it's fairly elongate, which is more proxima-like), but he says it is different from the atrata he's keyed. Pass along to him the following two questions: (1) have you got males of definitive atrata to compare to? The medial depression is more obvious in females, and if all you've keyed out for atrata are females, then that could explain the difference (i.e., this is a male atrata and you just haven't seen them before). (2) Do you have males of definitive proximato compare to? The male sternal notch is NOT as deep in atrata as it is in proxima, and L&C's key does not mention this - you have to compare them side-by-side to see what I mean. Same holds for the punctation difference; L&C don't mention it, but atrata's dorsal punctures are generally stronger and - on the pronotum - more widely spaced.
"I am also very suspicious about the abdomen coloration; I don't think I ever saw a proxima (or atrata, but I saw many fewer specimens of atrata) with an orange abdomen. This is a fairly tough one to call without the specimen in hand, and comparative material. I would not put it past these beetles to have a cryptic species or two, so I would not bat an eye if someone told me this was a fourth, undescribed species."

V, I think you may be right. I have very little experience with Trigonarthris but would say the last sternite is not notched making it T. minnesotana. I was using Lingafelter's key when I initially made the ID and it makes no mention of the notch being a diagnostic character. Since then I have obtained photocopies of the entire L&C pub. I've also posted a comparison shot of what I determined to be T. atrata. vs.

pls edit your last comment and remove spaces after colon in the 'thumb:' links to make them work
[added:] you already did...

Thanks for passing this on V. I posted pics of the specimens of T. atrata that I compared it to. I'll take a better look with Doug's comments in mind when I get a chance.