Prototaxites

The genus Prototaxites (pro-toe-tax-eye-tees) describes terrestrial organisms known only from fossils dating from the Devonian period, approximately 420 to 370 million years ago. Prototaxites formed large trunk-like structures up to 1 m (3 ft.) wide, reaching 8 m (26 ft.) in height, made up of interwoven tubes just 50 &mu;m in diameter. Whilst traditionally very difficult to assign to an group of organisms, current opinion is converging to a fungal home for the genus.

Morphology
With a diameter of up to a meter, and a height reaching 9 m, Prototaxites fossils are by far the largest from its period of existence. Viewed from afar, the fossils take the form of tree-trunks, spreading slightly near their base in a fashion that suggests a connection to unpreserved root-like structures. Concentric growth rings, sometimes containing embedded plant material, suggest that the organism grew sporadically by the addition of external layers. It is probable that the preserved "trunks" represent the fruiting body, or "sporophore", of a fungus, which would have been fuelled by a net ("mycelium") of dispersed filaments ("hyphæ"). On a microscopic scale, the fossils consist of narrow tube-like structures, which weave around one another. These come in two 'flavours': skeletal "tubes", 20-50 &mu;m across, have thick (2-6 &mu;m) walls and are undivided for their length, and  "filaments", which are septate - that is to say, they bear internal walls. In Prototaxites, they are also thinner (5-10 &mu;m diameter) and branch frequently; these mesh together to form the organism's matrix. The similarity of these tubes to structures in early plant Nematothallus has led to suggestions that the latter may represent leaves of Nematothallus. Unfortunately for this hypothesis, the two have never been found in connection — although this may be a consequence of their detachment after the organisms' death.

History of research
First collected in 1843, it was not until 14 years later that the ey of John William Dawson, a Canadian scientist, was caught by Prototaxites fossils, which he described as partially rotten giant conifers, containing the remains of the fungi which had been decomposing them. This concept was not disputed until 1872, when a rival scientist named Carruthers poured ridicule on the idea. Such was his fervour that he rebuked the name Prototaxites — loosely translated as "first conifer" — and insisted that the name Nematophycus ("stringy plant") be adopted — a move strongly against scientific convention. Dawson fought adamantly to defend his original interpretation until studies of the microstructure made it clear that his position was untenable - whence he promptly attempted to rename the genus himself (to Nematophyton'), denying with great clout that he'd ever considered it to be a tree. Despite these political attempts to re-name the genus, the rules of convention mean that the name "Prototaxites", however inappropriate in meaning, remains in use today.

Despite the overwhelming evidence that the organism grew on land, Carruthers' interpretation - that it was a giant marine alga - was challenged just the once — in 1919, when Church suggested that Carruthers had been too quick to rule out the possibility of the fungi. Nobody paid him the blindest bit of attention. The lack of any characters diagnostic of any extant group made the presentation of a firm hypothesis difficult, and so the fossil remained an enigmatic mystery and subject of debate: it was not until 2001, after 20 years of research, that Francis Hueber, of Washington's National Museum of Natural History, published a long-awaited paper which attempted to throw out the bathwater and put Prototaxitesin its place. The paper deduced, based on its morphology, that Prototaxites was a fungus.

This idea was faced with disbelief, denial and strong scepticism, but further evidence is emerging to support it. In 2007, isotopic analyses by a team including Hueber and Kevin Boyce of the University of Chicago concluded that Prototaxites was a giant fungus. They detected a highly variable range of values of carbon isotope ratios in a range of Prototaxites specimens: autotrophs — that is, organisms such as plants and algae, that make a living via photosynthesis — living at the same time draw on the same (atmospheric) source of carbon; as organisms of the same type share the same chemical machinery, they reflect this atmospheric composition with a constant carbon isotope trace. The inconsistent ratio observed in Prototaxites appears to show that the organism did not survive by photosynthesis, and Boyce's team deduce that the organism fed on a range of substrates, such as the remains of whichever other organisms were nearby.

Ecological context
This organism would have been the tallest living thing in its day by far; wispy Cooksonia (pictured in navigational box below) only reached 1 m, and itself towered over the "moss forests"; invertebrates such as the euglenids were the only other land-dwelling life. Prototaxites became extinct as shrubs and trees rose to prominence. The organism could have used its raised platform for spore dispersal, or - if Nematophytes really did form its leaves - in competition for light. The presence of bio-molecules often associated with the algae may suggest that the organism was covered by symbiotic (or parasitic) algae - or an alga itself! Prototaxites mycelia (strands) have been fossilised invading the tissue of vascular plants; in turn, there is evidence of animals inhabiting Prototaxites: mazes of tubes have been found within some specimens, with the fungus re-growing into the voids, leading to speculation that the organisms' extinction may have been caused by such activity.