The Prickly lettuce plant looks quite similar to a sow-thistle before the flower heads start to appear, the big difference at that point is the row of spines down the midrib of each leaf. Once the flowers start to emerge it looks quite different, as instead of a few upright dandelion-like flowers they have several nodding pannicles of buds that once they flower stand upright in a wide spray of flowers and seed heads(see picture at bottom). The flowers close-up shop early in the day so when I usually come by in the later afternoon they are all closed up. I had to go out to the test-plot area at midday on purpose to see the actual blooms.
Prickly lettuce leaf with spines down the rib
According to the wikipedia article, this is the closest wild relative of our cultivated lettuce although it doesn't look very similar. It is supposed to be edible, but has a milky white sap that is quite bitter. I wouldn't put it in my salad.