About 420 million years ago, the largest inhabitants of the land were not plants or even animals, but strange organisms - prototaxites. Their bodies, similar to columns or elongated cones, gained up to a meter in diameter and up to eight in height, towering above the "forests" of primitive plants that resembled thickets of tall moss.
Numerous invertebrates found shelter in the "trunks" of prototaxites, and green algae settled on the surface. For a century and a half, these unusual creatures, which scientists have found in the fossils of the Paleozoic era, remained a complete mystery. Only at the turn of the 21st century did it become clear that the giant prototaxites were ... mushrooms.
Prehistory of predecessors
Let us recall that the current (Cenozoic) era in the history of the earth was preceded by the era of "middle life" - the Mesozoic, when conifers and reptiles, including dinosaurs, dominated on land. It began about 250 million years ago with the Permian extinction, which, in turn, ended the era of the Paleozoic - “ancient life”.
It was in the Paleozoic that most modern types of animals appeared, including mollusks, arthropods and vertebrates, and the development of land began. The earliest finds of representatives of the mushroom kingdom, such as Tortotubus, date from the beginning of this period (about 440 million years ago). Tortotubuses grew along the shores of the Silurian seas and rivers that washed the shores of the supercontinent of that time, Gondwana and Laurentia.
Life here was still not very confident: vertebrates practically did not get out of the water, and only bacteria and algae, primitive plants such as mosses, the first terrestrial arthropods and worms lived on land. And then mushrooms began to appear here, immediately proceeding to their main duty: to process dead matter and almost any organic matter that came to hand.
The unusual fossils were first discovered in 1843, in the Canadian province of Quebec, while exploring for coal deposits. They belong to deposits about 420 million years old - about 20 million years younger than the earliest tortoirubuses. However, at that time all this, of course, did not know, and the find did not attract much attention, for a long time remained in the museum's storerooms.
It was only in the 1850s that the hands of the local paleontologist John Dawson reached the fossils, who examined the 8-meter smooth, branchless pillars, considering them to be the trunks of early conifers, with separate fragments of mushroom mycelium growing in them. He gave the "plants" a name that has survived to this day: Prototaxitaceae - that is, "primitive yew".
20 years later, Scottish botanist William Carruthers, who studied the structure of the fossils, questioned the coniferous nature of prototaxites. In his opinion, these creatures were closer to algae and could grow in shallow water, like some kind of kelp. Despite the fact that everything pointed to the terrestrial nature of the deposits, where the "trunks" were found, the Carruthers hypothesis became the main one for many decades. The scientist even advocated changing the name of prototaxites to something more suitable for algae.
The British Arthur Church was the first to suggest that we are talking about mushrooms. However, its publication remained unnoticed, and throughout the twentieth century. prototaxites are so customarily considered algae, while naming them after conifers. But discussions among experts did not subside, and in 2001, the American paleontologist Francis Hueber finally placed prototaxites on the correct branch of the "tree of life."
Indeed, a cut of these fossils can be viewed as something like annual rings. Unlike real tree rings, in prototaxites they are uneven, often merge and merge into each other. Examining them under a microscope, scientists discovered long and branching tubular cell structures, almost the same as those of the mycelium of the familiar mushrooms. This assumption was confirmed by the chemical analysis of the samples, which was carried out already in the late 2000s.
Huber and his colleagues examined the carbon isotope abundance preserved in prototaxite fossils. The fact is that plants receive small amounts of it from the atmosphere, including it in their own tissues. The rate of biochemical reactions of carbon -13 and carbon -12 is slightly different due to the different mass of the nuclei, which makes it possible to distinguish a photosynthesizing plant from a saprophyte.
At the same time, one more version is preserved: it is possible that prototaxites were hybrids of algae and fungi - colossal lichens - and it remains to be proved or refuted. However, even in this case, we can rightfully compare the prototaxites of the Paleozoic with tyrannosaurs and diplodocus of the Mesozoic period, or with the people of the Cenozoic: this was the time of their dominance.
The land landscape in early Devon - about 400 million years ago - had little resemblance to today's Earth. Plants, still devoid of a vascular system, covered the moist lowlands with a dense "forest" that rarely reached more than half a meter in height. Smooth mushroom columns of prototaxites rose above them to a height of several meters.
They were not yet as "decentralized" as the mycelium of modern fungi, and under the surface of the earth branching hyphae were branching out from the "trunks" in all directions, which digested dead organic matter and absorbed nutrients. Like today's trees, prototaxites in the Paleozoic nourished entire ecosystems. They served as food and home for the first sushi invertebrates, as indicated by numerous holes, as if gnawed by small pests.
Their dominance lasted about 70 million years, and such giant mushrooms are no longer found in the fossil record of later periods. The reason for this is not fully understood: perhaps they grew too slowly, and the animals grew too fond of the "mushroom diet" - and the prototaxites simply did not have time to recover. But most likely, they were supplanted by plants, competing with them, if not for food, then for water and space. One way or another, the mushrooms themselves prepared such an outcome.
All fungi are organic destructors, and prototaxites, apparently, were no exception. However, the substances that fungi release into the environment for the decomposition of various molecules gradually destroy even the rock. This is how a long and important process of forming a fertile soil layer starts in nature.
It is not surprising that the activity of early Paleozoic fungi paved the way for the future triumph of vascular land plants. Their victorious march began in the Devonian period and soon led to the disappearance of giants like prototaxites. But by this time, a close symbiosis had already formed between mushrooms and plants, and they were forever satisfied with their modest, mostly underground and surface way of life.
Without them, modern plants are unable to survive in nature - just like animals without symbiotic microflora in their intestines. Relying on this union, the plants raise their crowns by tens of meters. The mushrooms look at them from the bottom up, recalling the era when the pillars of prototaxites grew many times higher than the tallest ancestors of trees.