IV. Was Prototaxites a lichen?
II. A fungus? (Hueber)
III. An alga? (Schweitzer)
VI. Literature and credits
In 2002 Marc-André Selosse from Paris published a reaction on Hueber's paper on Prototaxites. He did value Hueber's research very much, but he had also serious doubts about his conclusions. Particularly on the following three points.
1. The remains of the reproductive structures, described by Hueber, are incomplete and not convincing. Furthermore it is peculiar that no spores have been found, not even in the direct vicinity of embedded specimens of Prototaxites.
2. The huge size of the fruiting body of the fungus cannot be clearly explained. Organisms become big in the competition with other organisms, but plants in the Early Devonian didn't grow much higher than 50 cm. Besides it is doubtful how the giant fungus could gain enough food. A rule of thumb is that the biomass in forests at a certain level of the food chain is at most about 10% of the next lower level. The fungi have a mass of about 10 % of the layer of humus in which they live. Prototaxites however seems to have had about the same mass as the layer on which it lived.
3. Prototaxites became extinct in the Late Devonian. Hueber thinks that this was possibly caused by predation by animals and by competition of trees and bushes. The latter however doesn't make sense, for fungi and plants are not food competitors and a fungus is not dependent on light. Plants only bring more food for the fungi.
Selosse thinks it possible to overcome these objections by assuming that Prototaxites was a lichen. That is a cohabitation of a fungus and an alga with mutual benefit (symbiosis). Fungi can gain minerals and water from the soil with their mycelium, whereas algae can produce nutrients from carbon dioxide and water by assimilation. Furthermore the alga and the fungus can give each other protection in very difficult circumstances.
According to Selosse the alga must have been situated near
the outside of the youngest growth increment, because there light could
be caught. The alga in the older (inside) parts died for lack of light, but
the tubes stayed intact and because of their firm structure they gave
stiffness to the trunk. The so-called skeletal hypha must have been the tube
of the alga in Selosse's view.
Nowadays algae which form tubes without septa do exist. In them growth is concentrated in the top and to prevent the flowing back of the protoplasm a temporary septum is formed. The rest of the tube is empty. It is possible that the alga belonged to this type. Moreover, green algae can produce chemicals which strengthen the walls.
The big size of the organism could have been an advantage because
of the larger surface for the food production by the alga. Selosse mentions
also the competition with plants but this argument certainly does not make
sense for the Early Devonian.
An interesting and attractive idea (of Hans-Jürgen Weiss) is the possibility that the big size points at succulence. Cactuses can grow very tall and thick to collect as much water as possible in the scarce moments of rainfall.
The absence of reproductive structures can be explained by assuming that Prototaxites used a vegetative form of reproduction. This occurs also in living lichens, e.g. when detached pieces develop to new individuals.
In this case the extinction could have been the consequence of the competition as a consequence of densification of the vegetation.
All in all it can be concluded that the enigma has not yet been solved. Prototaxites will not be an alga, but perhaps it is a giant fungus and, who knows, the truth lies midway and it is a combination of a fungus and an alga, namely a giant lichen. At this moment the fungus theory is on the winning hand thanks to Boyce at al (2007).
The idea of these giants is a bit uneasy, but we have now also accepted that dragonflies with a wingspread of 70 cm and dinosaurs sized up to 35 m have existed. And a time span of 400 million years is no negligible factor.
Moreover, Jules Verne has already predicted in the Voyage to the center of the Earth that giant mushrooms could grow ...