I found several (3-5) of these atypically-shaped rove beetles so far among thousands of other beetles from at least 26 other families (plus many other arthropods) by pawing and sifting through flotsam from a substantially swollen catchment basin after several days of heavy, non-stop rain. Not too long ago this beetle's oddity would have me seriously flumoxed in the ID department. Having seen several similar-looking ones come in to bugguide I recognized what I had as soon as I saw it magnified.