I did not get pictures of the underside. Had there been something in the mines, I would have collected them. I may still be able to go back and take a look (or find others that are closer).

I should probably note that my sense of 'similar' might not be sufficiently refined, but I've noticed leaf mines that seemed vaguely like these in Heracleum maximum (Apiaceae), Atriplex gmelinii (Chenepodiaceae), and maybe Polygonum fowleri (Polygonaceae). That said, I think maybe the mines in those cases tended to be a bit more blotchy in total. I'll search through my pictures and see what I can find.

I'm not sure if you're interested in taking a look at the variety I've found, but if so, would it be easiest for me to post a bunch to bugguide separately, or create a page on my website where they can all be seen together?

Thanks!

If you post all the photos directly to Unidentified Leaf Mines at the same time, they will be more or less grouped together. Seeing variations on the same mine type can definitely be helpful. In addition to posting shots of the leaf underside when possible, higher resolution shots that show the frass pattern would help a lot.

Pegomya depressiventris belongs to a group of anthomyiid flies that all mine in Asteraceae, so if my initial thought was right, there wouldn't be anything similar on those other hosts you mentioned. My second thought is that this reminds me of certain Calycomyza (Agromyzidae) mines on Lamiales. There are a number of Calycomyza species known from Asteraceae, but none on Arnica. All of the agromyzids known from Arnica are species of Phytomyza.

I am reasonably certain these mines were made by fly larvae, rather than moth, beetle, or sawfly. I've written a key to all known North American leaf mines on Asteraceae, and Pegomya depressiventris is the only one on Arnica that seems close to this, but I'm not sure if that species would ever make such long projections; I'd expect it to look more like this:

The little frass grains scattered throughout don't seem right either. Yours could easily be something no one has documented before.

I'll be interested in checking out any leaf mine photos you have. I've already written a key to Apiaceae miners; Caryophyllales (including Atriplex and Polygonum) is the next group I'll be working on when I get back to my manuscript this fall.