This sounds bogus but check the source - Smithsonian Mag. Weird, but apparently according to the SM article I was linked to, true.

your friendly neighborhood autistic with paleontology as a special interest, here to expand on this a bit!

the image is fake. but:

• currently, the world’s largest single organism IS a mushroom. it’s a massive growth of honey mushrooms, growing under a forest in oregon. it takes up about 4 square miles, stretched out over 60 miles.
• we don’t know exactly how old it is, but based on it’s growth rate, it’s anywhere between 2000-8650 years old. (!!! *excitement*)
  
• the caption is also true, although also completely unrelated to the (fake) picture. here are real pictures of them:

• (lol)
• the fossils found are called prototaxites, and there’s actually still some debate about what exactly they are- mostly because a lot of scientists simply can’t wrap their heads around the idea of giant mushrooms covering the earth. the alternate theories of what they were are lichen or rolled-up formations of moss, but giant mushrooms seems to be the leading theory.
• whatever they were, they were everywhere, and at aprox. 20 feet tall, they were the tallest organisms on land from approximately 420 million to 350 million years ago.
• fan art!!!

conclusion: ANCIENT EARTH HAD GIANT DONGS GROWING OUT OF IT. ALL OVER THE PLACE.

thank you for your time.

@tkingfisher! Your “Valley of Wang” was prehistorically accurate!