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The giant fossil Prototaxites is a big 400-million year old mystery. The fossils resemble tree trunks, and yet they are from a time before trees existed. The stable carbon isotope values are similar to those of fungi, but the fossils do not display structures usually found in fungi. Hence, the enigma.
Prototaxites have sparked controversy for more than a century. Originally classified as a conifer, scientists later argued that it was instead a lichen, various types of algae or a fungus. Whatever it was, it stood in tree-like trunks more than 20 feet tall, making it the largest-known organism on land in its day.
“No matter what argument you put forth, people say, well, that’s crazy. That doesn’t make any sense,” said C. Kevin Boyce, Assistant Professor in Geophysical Sciences at the University of Chicago. “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.”
Plant-like polymers have been found in the fossils, but nutritional evidence supports heterotrophy, which is not commonly found in plants. These are a few of the confounding factors surrounding the identification of Prototaxites fossils.
Prototaxites existed during the Late Silurian to Late Devonian periods-- approximately 420-370 million years ago (ma). Prototaxites fossils have a consistent tubular anatomy, composed of primarily unbranched, non-septate tubes, arranged in concentric or eccentric rings, giving the fossils an appearance similar to that of a cross-section of a tree trunk. The fossil "trunks" vary in size and may be up to 8.8 m long and 1.37 m in diameter, making Prototaxites the largest organism on land during the Late Siluarian and Devonian periods.
Their hypothesis is that Prototaxites fossils may be composed of partially degraded wind-, gravity-, or water-rolled mats of liverworts that are associated with fungi and cyanobacteria
Liverwort is a non vascular plant that does not make seeds.
The name "liverwort" derives from the Anglo-Saxon word "lifer, meaning liver and "wyrt", the Anglo-Saxon word for plant. During the 16th century, it was commonly applied to the genus Marchantia, a flat, branching, ribbon-shaped plant the margins of which were claimed to resemble the lobes of a liver