Hi Daph,
I did the POL test (I use two pairs of polarized sun glasses) and it was negative. The spores of L. flowersiana are 10-14(18) X (4)4.5-6(7). L. zosterae are 9-13.5 X 5-6, according to Biotic Soil Crust Lichens of the Columbia Basin. My spores are 10.24-17.35 X 7.68-15.36. It's on dead bunchgrass over desert soil (mostly detritus).
I also thought about Caloplaca but that doesn't key out either.