Why is it assumed that these mushrooms would grow straight and tall? Trees grow tall to maximize their exposure to their primary energy source, which is of course light. But why would a mushroom need to grow really tall. Don't they obtain all their energy from the land?
It's assumed they grew that way because that's what the fossils look like.
If a fungus had a photosynthetic symbiont, it would have a good reason to compete for height. In fact, an algal symbiont is a leading theory, based on chemical traces that point to algae in the fossils.
Interestingly, lichens do this today: they are composite organisms of fungus and photosyth (algae or cyanobacteria). They don't currently get very high, but perhaps that's just because plants are so much better at height that all tall forms have gone extinct.
If there's no photosynthesis going on, it's harder to explain. Perhaps it's handy in a very low-growing world to lift most of your energy-storage mass or hard-won water above the ground, away from predators?
Part of the reason a mushroom would be so tall is that when the cap opens, the stem is connected to the middle. So a mushroom top that is say, 10 feet across, would be at the very least 5 feet tall. As the top gets heavier the stem would have to be larger in diameter to hold up the increased weight.
It's also not uncommon to find fungi that are several feet long growing on trees and such. Not sure if the article specifically said mushroom or if it was just a fungi they found a fossil of.
Plausible, but not apparently a problem from most fungi currently alive. I would guess that the trade-off in terms of marshaling resources vs. increase in spore distribution would not be favorable enough. I expect that's why the lichen argument appeared - we can easily see why being tall is a plus for photosynthesis.
