Prototaxites, once thought to be tree trunks, were revealed as giant fungi from the Late Silurian to Late Devonian (~420–370 Mya). Fossil analysis indicated non-photosynthetic fungi based on carbon isotopes, concentric growth rings, and internal tubular structures. Alternative theories, such as rolled liverwort mats, were debated but lacked support. Recent laser-scanning microscopy confirmed Prototaxites as a basal ascomycete fungus with polysporous asci. Its massive size and nutrient acquisition remain unresolved, though symbiosis with algae or cyanobacteria is speculated. These towering fungi dominated early terrestrial landscapes, shaping prehistoric ecosystems.