Prototaxites

Prototaxites ("first yew") was a mysterious organism that lived during the Silurian and Devonian periods. Although it is not known for sure what type of organism it is, current opinion favors a fungal placement for the genus.

Description
Fossils of Prototaxites are remarkably treelike, and only distinguishable under a microscope. They could grow up to a meter wide and 8 meters tall, by far the largest organism on land during this time.

When examined through a microscope, Prototaxites is made of tiny tube-like structures. Some traits of these structures are only found today in fungi and red algae. These tubes are similar to structures found inside the fossil plant Nematothallus, and it has been suggested that the latter is in fact leaves of the former. However, the two have never been discovered together.

Classification
Two species of Prototaxites are known: P. loganii and P. southworthii.

History
Prototaxites was discovered in 1843, but it was not until 1857 that Canadian geologist John William Dawson described it as fossilized rotting wood from conifers. Its translated name, "first yew", references the conifer family Taxaceae. This classification was accepted until 1872 when a rival scientist named Carruthers ridiculed it and asserted that Prototaxites was a form of marine alga. He even proposed a new genus name, Nematophycus, that better suited the identification. Dawson adamantly fought his original interpretation until enough evidence was gathered that proved it was not rotting wood after all. He promptly attempted to rename the genus Nematophyton and denied that he ever identified it as a tree. Despite the attempts to rename the genus, the rules of botanical nomenclature state that the oldest genus name takes priority, and as such Prototaxites remains in use today despite its inappropriate meaning.

Carruthers' interpretation was tentatively accepted, although nobody was still quite clear on what the fossil actually was. In 2001, Francis Hueber published a paper that deduced Prototaxites was in fact a giant type of fungus based on its morphology. Although met with skepticism, this interpretation was supported by isotope analysis in 2007.

It has also been suggested that Prototaxites was a form of liverworts, but this interpretation is not very well supported.

Paleoecology
Prototaxites would have been the most prominent organism on land in the Devonian. Even so, nobody is sure whether they appeared as large stumps or had leaves to photosynthesize.

Prototaxites fossils have been found that were bored into by ancient insects. Intriguingly, this is behavior recorded before the evolution of the woody stems of plants.