there's actually new research saying prototaxites may not have been fungi, but belonged to an entirely separate kingdom that's now extinct. one of the defining characteristics of fungi are that their cell walls contain chitin, the same stuff that makes arthropod shells. prototaxites was found not to contain any chitin, while other ancient fungi of the same age as prototaxites already did, so it's not a feature of fungi that was adapted later. prototaxites was also found to contain microscopic structures that don't exist in any known fungus species

edit: damn, someone beat me to it while I was trying to find the details