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Prototaxites, a huge, 400 million years old, fungus? Or an enormous lichen?

IV. Was Prototaxites a lichen?

I. The fossil
II. A fungus? (Hueber)

III. An alga? (Schweitzer)
V. Miscellaneous
VI. Literature and credits

In 2002 Marc-André Selosse from Paris published a reaction on Hueber's paper on Prototaxites. He valued Hueber's research very much, but he also had 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 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 find 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 or a cyanobacteria with mutual benefit (symbiosis). Fungi can gain minerals and water from the soil with their mycelium, whereas algae and cyanobacteria can produce nutrients from carbon dioxide and water by assimilation. Furthermore the alga/cyanobacteria and the fungus can give each other protection in very difficult circumstances. The alga/cyanobacteria is sometimes called photobiont. Selosse thinks that the fotobiont in Prototaxites was an alga.

According to Selosse, the photobiont 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 hyphae must have been the tubes 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.  

Retallack and Landing have proof!

Retallack & Lanting published a paper in 2014  in which they, describing a trunk with branches and remains of the original surface, show that Prototaxites must have been a lichen.
The fossil they used, is a complete trunk of Prototaxites loganii from the Middle Devonian (approx. 386 million year old), that was found in a small quarry in the State of New York at the end of the 19th century (see the figures below). The trunk is in the New York State Museum in Albany under the name of Schunnemunk Tree. It's length is 8.83 m and in the upper part it has six branches which are each about 1 m long and 9 cm thick.

Skunnemunk Tree

The Schunnemunk Tree: Prototaxites loganii. A. Photo of the branched top at the time of the excavation in 1898; B. The wrinkeled trunk near the top. C. Upper part of the trunk. D. Detail of the bases of the branches. E. Separate basal piece of a branch with wrinkles.

Skunnemunk Tree

Sketches of the fossil. A. The trunk as it lies in the museum. B. The trunk as it was photographed in 1898. C and D. Fossil branches found in 2011.

Reprinted with permission from Mycologia.  ©The Mycological Society of America

The trunk is silicified and at transverse sections it shows the same image of thick and thin tubes (hyphae) as other specimens of Prototaxites. The basal parts of the branches have been well preserved. Still in 2011 coalified remains of three branches were recovered. At the trunk and at the branches there are irregular, coarse wrinkles.

The outermost layer
At some places at the base of the trunk the coalified outside has been preserved rather well. The new information comes from this coalified outer layer.
Under the whole surface there proved to be a narrow gap of about 25 µm, which was only interrupted at small invaginations (see the pictures below). From the transverse sections and the SEMs it turned out that nests with small spheres ("coccoid cells")  were situated under the invaginations. The coccoids are visible in clusters of strongly branched thin hyphae. The diameter of such a sphere is about 6 µm. Some of them are whole, others are penetrated by hyphae. The researchers think that these coccoid cells are the remains of the photobiont.


Transverse sections of the coalified outer layer of the trunk. A. Ribs and invaginations of the surface and underneath an elongated cavity, sometimes interrupted at invaginations. B, C and D. Nests of hyphae with in them badly preserved little spheres. These can be seen more clearly below.


Scanning electron micrographs (SEMs) of clusters of coccoid cells in the middle of and sometimes penetrated by hyphae. These coccoid cells must be the fossil photobiont.

Reprinted with permission from Mycologia.  ©The Mycological Society of America.

As the alga or cyanobacteria needed light, it was necessarily living at the outside of the trunk. The thick and thin hyphae are part of the fungal component. At the concentric rings spots with branching hyphae have been described bij Hueber (2001), but no coccoid cells have been found in these spots. Retallack & Lanting think these are old nests of the photobiont from which the alga/cyanobacteria has disappeared.
Furthermore a kind of radially directed structures have been found, running from those 'old nests' to the outer side. Retallack & Lanting speculate that the photobiont has migrated via these routes to the outer side. The figure below gives an impression of their idea of the structure of a trunk of Prototaxites.

Structuur van Prototaxites

A. Sketch of the structure of a trunk with branch and wrinkles. B. Schematic drawing of the transverse section of the outermost layers of the trunk. C. Transverse section of the silicified trunk with hypae. D. Longitudinal section with hyphae.

Reprinted with permission from Mycologia.  ©The Mycological Society of America

Reconstructie Prototaxites

Prototaxites according to  Retallack & Landing.
The authors also have an idea about the growth rings. The outermost growth layer was probably formed as an wrapping cylinder from the bottom upwards. The base of the trunk could have been a kind of pseudocambium (called hypothallus) from which at regular times a new ring came into existence. This ring extented in the upward direction complete with invaginations and nests with the photobiont as a covering new growth layer. In this process irregularities like scars of old branches were overgrown. In this way remains of land plants could have come into the trunk, as has been observed.

Considering the diameter of the coccoid cells Retallack & Landing think that the phobiont must have been an alga. The diameter of cyanobacteria in recent lichens is commonly larger than 10 µm, whereas the diameter of algae happens to be lesser than 10 µm. The coarse wrinkles at the trunk are explicable as enlarging of the assimilating surface.

Can we say that the enigma of Prototaxites has now been solved definitively? I had that feeling already when I studied the paper by Hueber (2001) in which he defended that it was an enormous fungus. That feeling was still strengthened when Boyce et al. (2006) demonstrated that Prototaxites was a heterotrophic organism. Still some points of doubt remained, like the question of how such a huge organism could feed on a very meager vegetation in that time. And why was the trunk so enormous? These objections have been overcome  by the model of Retallack & Landing (2014). The photosynthesis provides supplementary feeding for the fungal component and an enlarged surface is advantageous for the reception of light.
So this is very convincing. Let us now wait and see if the lichen hypothesis can be confirmed bij other investigations.

Great, such a super enigma!

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