Arnica angustifolia Vahl.
Arnica angustifolia usually blooms in late June in interior Alaska. I see it along roadsides and highways and trails in spruce/hardwood forests. Its bright yellow color is eye-catching and its form is daisy-like. It grows on a long (about 12-15 inches) slender stem with alternate, lanceolate leaves with parallel venation. Some main stems support several flowers.
11-12 sepals (the one I dissected had 11 with 1 v. small sepal)
12 ray flowers (petals)
44 disc flowers of 5 connate petals
44 carpels
# stamens? I think there are 4 in each disc flower, so 176?
I wasn’t able to determine placentation.
Great dissection Lucia! Placentation is general apical or basal, the fruits are one-seeded achenes, but it is hard to do with the tiny ovary and early stage of development. The green parts that look like sepals, are instead the involucral bracts (or phyllaries), they are found below where the ray and disk flowers and are inserted on the enlarged receptacle that supports the flowers. The sepals in the sunflower family are highly reduced to the pappus, the white fluffy hairs that you see in the close-up of the ray flowers (your second to last image). In dandelion it it is the “umbrella-like” structure that propels the seeds. They are sometimes highly modified for wind or animal dispersal.