I didn't realize beetles could be quite this tiny--I thought they were springtails before I saw them through my lens. There were a dozen or so on the underside of a fleshy, white polypore, evidently feeding on the spores as they entered one pore after another. I guess they must be ptiliids, based on this info from the guide page: "the smallest known beetles, most species being under 1 mm, and the smallest known being 0.35 mm".
Edit: The fungus is Tyromyces cf. chioneus
. More photos here