Please id these seedlings
I can't provide a positive identification. I'm guessing that is an ash seedling, although the cotyledons in your photo differ from ash cotyledons which are typically long and narrow, not short and round. "Zillions" of ash seeds are germinating all over Denver right now which fits, as does the opposite leaf structure on your seeding. I can only speculate that the shape of the cotyledons in your photo may have been impacted by weather somehow. Knowing what tree species are in the vicinity might provide a clue.
Try letting one grow on to develop and then we might be able to correctly identify it.