Ever since their discovery more than 165 years ago, massive fossilized structures left by an organism known as Prototaxites have proven impossible to categorize.

Researchers in the UK have suggested in a recently published study that there's a very good reason these oddities don't fit neatly on the tree of life – they belong to a branch all of their own, with no modern equivalent.

Some 400 million years ago, the swamps of the late Silurian period would have sprouted a mix of horsetails, ferns, and other prototype plants that look positively alien today.

Among them stretched 8-meter (26-foot) tall towers that defy easy identification. Wide and branchless, these organisms may have been a form of algae or ancient conifer, researchers suspect, based on what little evidence remains.