Trees didn't exist 400 million years ago, which makes the discovery of 20-foot, trunk-like fossils from that era all the more puzzling. Just what is the giant fossil Prototaxites? First, scientists thought it was a conifer; then, they decided it was more likely to be some kind of fungus, algae or lichen. But who ever heard of a 20-foot fungus?

 “No matter what argument you put forth, people say, well, that’s crazy. That doesn’t make any sense,” C. Kevin Boyce, assistant professor in geophysical sciences at the University of Chicago tells The Daily Galaxy. “A 20-foot-tall fungus doesn’t make any sense. Neither does a 20-foot-tall algae make any sense, but here’s the fossil.”

Boyce identified Prototaxites as fungi in 2007, but scientists have remained confused by the fact that the fossils don't display structures usually found in fungi.

After more than a century, the mystery may finally be solved: they're not mushrooms at all. Dr. Linda Graham of the University of Wisconsin, an expert in the evolutionary origin of land plants, points to a seemingly unlikely culprit — rolls of pale-green plants known as liverworts.

According to studies by Graham and her colleagues, Prototaxite fossils and modern-day liverwort plants share similarities in structure, internal anatomy and nutrition — and the shape that mats of the plant create when rolled by wind, water or gravity could explain the concentric, tree trunk-like rings seen in the fossils.

Though technically classified as a plant, liverwort doesn't have leaves and is generally lumped together with lichen, slime molds and algae.

The discovery helps illuminate the Devonian period of the Paleozoic era, during which many dramatic changes took place, like fish evolving legs and the appearance of the world’s first seed-bearing plants.