Ever since their discovery more than 165 years ago, massive fossilized structures left by an organism known as Prototaxites approximately 400 years ago have proven impossible to categorize.
Researchers in the UK suggest in a report (yet to be peer reviewed) that Protaxites are a type of organism with no modern equivalent
https://www.sciencealert.com/mysterious-giants-may-be-a-whole-new-kind-of-life-that-no-longer-exists