Wow! A ten year old thread in need of an update. 1st, not all thistles are in the same family (nor are they in the sunflower sub-genus helianthus). Thistels are in the overall plant family of Asteraceae (which covers about 45% of all plants (or Trachiophytes/Angiosperms if we want to get sciency). Asteraceae and are NOT all edible (Eg. Arnica monatana). The subfamily for sunflowers Hilianthus might have no toxic plants, I'm not sure, but it's not a thistle plants family.
YES - There is some thing like a thistle which is not edible:
Mexican poppy, flowering thistle, cardo, cardosanto, Argemone mexicana in the Poppy family (Papaveraceae). If it's not yet in flower you might think it's a young yummy thistle... think again! It's poisonous and if it wasn't for the poppy-flower-top I might casualty consume it --- loosing the symetry of my shinbones -- along with other damages as a result of its injestion.
There are actualy a few families of thistles; they are usually in the Cardueae tribe: Cardoon, Carduus, Silybum, Cynara, and Cursium. (Notice the mexican poppy above has a similar name "Cardosanto" even though it is in a different -- non-cardueae (thistles) -- family.
Centuria also has some vicious looking thistle-heads (though many of them are more gentile knappweed basketflowers [not listed here amoungst these excellent thistlate-morphology examples]):
Centaurea eriophora; Centaurea benedicta; Centaurea iberica; Centaurea solstitialis – yellow starthistle; Centaurea calcitrapa -- purple starthistle; Centaurea hyalolepis; etc..
Then you have your wild lettuce lookalikes which can be spiney like thistles, but are usually only a novice or child's mistake; as most adults would recognize thistles as being sturdy and hardy, while; wild-lettuces are soft thin limp easy wilters. I personally have heard of only one toxic-lookalike to ☠ wild lettuces: Packera; which luckily, does not have a thistle like appearance. So, if you are foraging wild lettuce, stay away from Packera glabella!
If you are foraging thistles, stay away from Argemone mexicana!
It grows over the entier Eastern-Half of North America from Mexico and Arizona, all the way to the East-Coast and up through Canada's Manitoba and Ontario:
☠ [url=https://plants.sc.egov.usda.gov/plant-profile/ARME4] Argemone mexicana L.
Mexican pricklypoppy (From Canada to Mexico)[/url].
☠ [url=https://plants.sc.egov.usda.gov/plant-profile/AROC2] Argemone ochroleuca Sweet
pale Mexican pricklypoppy (Mx, AZ (US))[/url].
