Our planet was very different hundreds of millions of years ago. On dry land there were no animals, no jungles, not even plants that rose more than a few decimeters from the ground. During the Ordovician and Silurian periods, most life was still in the sea, a lush sea teeming with living things. But in the early Devonian there was a species that stood out on land.
locating us temporarily
During the Ordovician period, between 470 and 444 million years ago, the second most relevant evolutionary radiation until then was taking place in the sea, only behind that of the Cambrian. Trilobites diversified widely, and the first cephalopods and crinoids— also known as sea lilies—populated the oceans.
The first fish with jaws appeared and the first ostracoderms, fish without jaws, but armored. Regarding vegetation, green algae covered large areas of the seabed and the first plants, similar to the current liverworts , began as the first known living beings that came out of the sea to dry land.
444 million years ago, a massive extinction in which up to 60% of the species disappeared opened the doors to a new period: the Silurian . From the ashes of extinction, a new evolutionary radiation allowed fish to develop a bony skeleton and strong jaws . Arthropods underwent enormous diversification, from large sea scorpions to the first animals to come ashore . It was already colonized by plants, which at the end of the period, about 420 million years ago, began to have vascular tissues. The largest plants reached up to half a meter in height.
The Devonian period begins, when the huge armored fish appear. But on land, the landscape was green, covered with low vegetation that did not rise more than a foot. Specimens of the genus Cooksonia stood out here and there, plants that reached a meter, and that stood out against the forest of mosses. However, there was another organism that drew even more attention, like a cactus in a desert. Huge silhouettes of something that look like trees up to 8 meters high.
Prototaxites , the tree that wasn’t a tree
Prototaxites was the largest living terrestrial being of its time , and the largest that the planet had known until then. 50 million years before the first arboreal plants populated the terrestrial soil, this being already dominated the mainland.
Its trunk was made up of agglomerations of filaments about 50 microns in diameter, which were intertwined with each other forming a large solid piece. Fossils of Prototaxites were long thought to correspond to a conifer. However, its anatomy precludes this option and its temporary location makes it impossible.
Later it was identified as green, red, brown algae, and even as a lichen . Its filamentous structure is reminiscent of the hyphal structures of certain algae. But none of these explanations was satisfactory, there was no consensus regarding the nature of these fossils.
Solving the unknown
A common feature of all the organisms mentioned, lichens, algae and plants, is that they are autotrophs , that is, they feed on sunlight and carbon dioxide and form their own organic matter by releasing oxygen. It is the opposite behavior to that we have, for example, animals, that we obtain organic matter and, by breathing capturing oxygen, we release carbon dioxide. We are heterotrophs.
To find out if Prototaxites was an autotrophic or heterotrophic organism, a research group led by researcher C. Kevin Boyce, from the University of Chicago, proposed to analyze carbon isotopes . In the atmosphere, naturally, there are several; Among them, carbon 12 and 13 (respectively ¹²C and ¹³C) stand out because they are stable isotopes, that is, they are not radioactive. Terrestrial plants discriminate this carbon and show a preference for the ¹²C isotope, which is much lighter than ¹³C. The ratio of the different isotopes of carbon is called the isotropic range.
When they analyzed the isotropic ranges of Prototaxites , they saw that the results were very difficult to reconcile with autotrophy. It was ruled out that it was an algae or a plant. Quite the contrary, they observed that it must be a heterotrophic organism, which also obtained carbon from many different sources, whether they were plants, animals, or even remains of bacterial decomposition. And that, in fact, it could change its food source over time or depending on where it was.
All this led to the conclusion that Prototaxites was a fungus . A huge fungus, whose body was formed by the agglomeration of a multitude of mycelium hyphae, like modern mushrooms , but in a giant version. And like all fungus, it must have had an underground mycelium, an extension of hyphae that extends through the ground, beyond its fruiting body. These strands also fossilized, and have been found invading the tissues of nearby vascular plants .
Also, as a curiosity, holes have been observed, presumably made by insects, and which later healed, restored by the same hyphae of the fungus. Boring insects already existed tens of millions of years before wood existed , and it is possible that the origin of this type of feeding occurred in giant fungi and that they later transferred this habit to terrestrial plants.
REFERENCES:
Boyce, C. K. et al. 2007. Devonian landscape heterogeneity recorded by a giant fungus. Geology, 35(5), 399-402. DOI: 10.1130/G23384A.1
Hueber, F. M. 2001. Rotted wood–alga–fungus: the history and life of Prototaxites Dawson 1859. Review of Palaeobotany and Palynology, 116(1), 123-158. DOI: 10.1016/S0034-6667(01)00058-6
Munnecke, A. et al. 2010. Ordovician and Silurian sea–water chemistry, sea level, and climate: A synopsis. Palaeogeography, Palaeoclimatology, Palaeoecology, 296(3), 389-413. DOI: 10.1016/j.palaeo.2010.08.001
