Prototaxites formed tall structures like tree trunks, shown in this illustration of a landscape from the Silurian PeriodRICHARD JONES/SCIENCE PHOTO LIBRARYA bizarre ancient organism previously thought to be a giant fungus may actually belong to an undiscovered branch of the tree of life that mysteriously went extinct.Prototaxites, which lived between 420 million and 375 million years ago, was the first giant terrestrial life form to inhabit Earth. It grew trunk-like structures up to 8 metres tall and 1 metre wide.Its fossils were first