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What was life on Earth millions of years ago?
What was life on Earth millions of years ago?
Anonim

A sharp increase in the number of biological species, called the "Cambrian explosion", opened the Phanerozoic - the aeon of "explicit life". However, the "secret" life in the previous era was also very diverse, giving rise to, among other things, gigantic forms. Discovering the secrets of this incredibly ancient fauna was made possible by discoveries made in Russia.

The first traces of macroscopic soft-bodied multicellular creatures, which could be carefully attributed to the Precambrian, were found in the 1860s in the Newfoundland region. In the 20th century, significant finds were made in Namibia and Australia. On the territory of our country, separate fossils were found in the core extracted from wells (Ukraine, Crimea, Ural).

These were small prints that resembled either discs or cakes, which were not even immediately recognized as imprints of living beings: some believed that we were talking about traces of geological processes. The trouble was that it was not initially possible to reliably determine the age of the rock, and some researchers attributed the finds to the Cambrian, Sillurian, or Ordovician.

Certainty appeared only in 1957, when the prints of a creature called charnia found in Great Britain were unequivocally dated to the Precambrian era.

Vendian fauna

It is interesting not only the very fact of the discovery of the remains of a large group of Precambrian animals, but also the fact that their appearance and structure turned out to be extremely unusual, as if they were talking about alien life. But this life, called the Vendian, or Ediacaran, biota, became the first mass appearance of multicellular organisms in the fossil record, inhabiting the ocean more than 600 million years ago.

The history of the most extensive and unique location of footprints of the Vendian fauna began in 1972, when student-trainee A.V. Stepanov found several footprints of organisms on the Onega Peninsula at the mouth of the Syuzma River (Arkhangelsk Region) and delivered the find to the Geological Institute of the ANSSSR.

An employee of the institute, Professor B.M. Keller, examined the prints and noted their similarity with the prints of the Precambrian fauna from Namibia. Soon an expedition was sent to the shores of the White Sea, by the bushes of Syuzma. It was not possible to find anything at the site of the finds made by the student, however, about five kilometers up the river, the expedition met an outcrop on a steep bank.

New prints were found on protruding sandstone blocks. The next year, at the steep 15-meter "wall" of the discoverers, a new expedition was replaced, which included N.M. Chumakov and the author of these lines.

Lying and crawling

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Everything that we know about the Vendian fauna has come to us in the form of thin reliefs on the surface of the sandstone. There are both negative and positive representations of these beings.

Wend was the realm of three-beam symmetry. Tribrachidium is a classic example of such a creature (photo below). In the absence of natural enemies (the representatives of the Vendian fauna did not eat each other), the tribrachidium lay peacefully at the bottom, and in order not to miss the nutrient microparticles brought by the current from different sides, it acquired three mouth openings. Then, through the three branches of the intestine, food entered the body.

Another type of animals in Vendian consisted of organisms with a bilateral structure, however, unlike later animals such as trilobites, the right and left parts of the bodies of Vendian creatures did not have perfect symmetry.

They were characterized by the so-called grazing reflection symmetry, when opposing "rays" are placed opposite each other in a "checkerboard" pattern. The bottom photo shows a print of the Dickinsonia animal.In some organisms of this type, for example, in Andiva, cephalization is clearly visible - isolation of the head region, probably with sensitive cells.

Protected by ice and chalk

The cliff on the river bank became for us a window into the unimaginably distant past. I came there for several years in a row, and every year I gave us new finds. In the spring, the melting ice tore from the coast new sand slabs with imprints of the Vendian era. All this was the first time in Russia - in such quantity, in such complexity and in such variety.

It seemed that after incredible scientific success, it was difficult to expect anything more. But we nevertheless decided to look around: the White Sea is large - suddenly there will be new promising locations on its shores. The choice fell on the Winter Mountains, located about 200 km of the sea route from Syuzma. Here, the outcrops were not a piece of river bank 10-15 m high, but deposits of clay and sandstone layers with an apparent thickness of about 120 m that protruded onto the surface.They went into the depths of the earth for another 700 m.

The era in which we live is characterized by an unusually low level of the ocean: a large amount of water is bound by polar caps. In warmer and longer times, there was so much water that there was no land between the present Black and White seas.

Greetings from ancestors

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One of the most promising hypotheses concerns a Vendian animal called Ausia fenestrata - only 2 prints came from it from the shores of the White Sea (two more similar prints were found in Namibia).

Fenestrata means "fenestrated", and, indeed, according to the prints, the appearance of this animal was originally restored as a kind of bag, the surface of which is dotted with large holes. It looked like a sponge, but the size of the holes was not very consistent with this hypothesis. Later, another thought came: what if the print did not retain the full appearance of the animal, but only a part of it? The sack with "windows" strikingly resembled the gill basket of chordates like ascidians, belonging to the tunicata type (tunicates).

In Tunicats, the basket is inside, covered with a tunic-like shell, consisting of a substance similar to cellulose. Ascidians are related to lancelet - primitive chordate animals that are found at the base of the tree of all vertebrates, including, of course, humans.

Thus, if the hypothesis regarding the kinship of Ausia fenestrata with tunicates is correct, this means that in sediments aged 550 Ma, we groped an evolutionary branch going from the Vendian fauna to humans.

And 25,000 years ago the Russian Plain was covered with ice up to the latitude of Kiev - it was a huge mass that was constantly freezing from above. And the earth's crust began to bend under the weight of the ice. When the ice left, the opposite process began: as if "springing", the crust began to bulge upward, lifting the bottom of the ancient oceans to the sky.

The winter mountains, to which we arrived, are still growing upward, lifting higher and higher layers of clay and sand that once accumulated at the bottom. And here's what's interesting: in some places, almost kilometer-long strata of these deposits are pierced by kimberlite pipes - vents through which magma escaped to the surface.

These vents are filled with partly remelted, partly altered ancient matter. And in it, oddly enough, there are blocks of limestone, which is not in the district. And in the blocks - fossils with the Cambrian and Ordovician fauna. Where does all this come from?

The answer turned out to be simple: over the clay-sand strata, over millions of years, other sediments from the later oceans accumulated, but all these sediments were subsequently eaten away, preserving individual fragments of the calcareous bottom in kimberlite pipes. Lumps of limestone fell there after being thrown up by a volcanic explosion. Having destroyed the bottom sediments of the Cambrian and Ordovician seas, nature has laid bare for us the sediments of the Precambrian ocean.

Moreover, due to the fact that these deposits were covered by other rocks for millions of years, the ancient strata in which clay and sandstone alternate are very fresh: the clays have not lost their elasticity, there are no traces of strong deformations, and therefore the Winter Mountains ended up as a unique location with thin and clear prints of the Vendian fauna.

Ascidia and her

Ascidia and its "basket"

Scrap as a tool of knowledge

When we started researching the Vendian biota (by the way, the term “Vendian” was proposed back in 1952 by Academician BS Sokolov), we had only a few samples of prints of these mysterious animals. Today, thanks to expeditions to the Winter Mountains, which did not stop even in the 1990s, a collection of about 10,000 samples has been collected in Russia, and the priority in describing them belongs to Russian paleontologists.

This is a collection of world significance, which includes, in particular, specimens of those animals, whose prints were also found in Newfoundland, the Urals, Australia and Namibia.

How does the fingerprint search work? At the height of the cliff, a sandstone slab sticks out. It is not clear whether there is anything on it or not. To find out, it is necessary to remove several tons of sediment with crowbars and shovels and free a part of the slab surface. Then the slab is split open and piece by piece is lowered down.

Heavy sandstone blocks have to be dragged on the back. Below, on the shore, fragments of the slab are numbered and put together. Then they turn it over. The prints, if any, are on the side of the plate that was facing down. But they still cannot be seen, since the sandstone is covered with clay.

Now you need to wash off the clay with a brush and water very carefully and find the desired prints. The finds must be photographed in the rays of the setting sun, so that the relief appears better in low light. Already from this short story, it is clear that the extraction of samples is physically hard work. But the harsh conditions of the expeditions compensate for the insane excitement of the discoverers who had a chance to look into the mysterious page of the history of life on Earth.

Canyon

In a world of non-obviousness

Paleontologists working with Phanerozoic fauna often deal with real fossils - shells, shells, teeth, bones, fossilized eggs. The Vendian fauna was born before the era of active biomineralization inherent in the Cambrian.

Most of these strange creatures had no skeletons, no hard shells, no hard shells. Their bodies were soft, often jellyfish-like, and few species boasted a paper-thin dorsal shield or tubular chitinous sheath.

Therefore, all that Vendian fauna specialists deal with are reliefs on the cemented sand, which once enveloped the gelatinous body, which disappeared almost without a trace. Hence the incredible difficulty in interpreting these tracks. Here are just a few examples.

One of the characteristic types of prints is the so-called radial-tine discs. Initially, they were interpreted as traces of jellyfish-like organisms, which received the corresponding names such as "cyclomedusa". It was assumed that these jellyfish did not swim freely, but constantly sat on the bottom (like some modern species).

This interpretation prevailed until near the discs they began to find prints of some creatures similar to a feather, after which a completely different picture was drawn: "cyclomedusa" are just traces of the so-called attachment discs. The organism developed in the following way: the larva sank to the bottom, its base grew, which was gradually covered with sand.

And already from the base a stem grew with lateral branches, with the help of which the animal fed. When the creature died, the disc imprint remained more often than the stem imprint, although the latter could reach cyclopean sizes for primitive fauna - up to 3 m in height with a disc diameter of about 1 m.

Jellyfish

Another textbook example is Dickinsonia.The prints left by this creature resemble the leaves of veined plants. So maybe this is the plant? Or a mushroom? Or something else? If this is an animal, then where is its mouth opening, and where is the anal? The author of these lines defended the hypothesis that we are talking about a representative of the fauna, but for about two decades I had to resist misunderstanding on the part of many colleagues.

One of my main arguments boiled down to the fact that the imprint, which we tend to take for the trace of the entire animal, is actually formed only by a thin, paper-like shell through which the elements of the internal structure "shine through". At the same time, there are several prints, which clearly show that something like a halo, similar to an imprint of soft tissue, extends beyond the ribbed zone.

However, it was possible to finally prove that Dickinsonia belongs to animals only when the traces of crawling of these creatures were found and studied. The traces of the moving belly are more blurred. If at the end of the path the Dickinsonia died, the trace of the shell is completely different - clear. Thus, this is an animal: it moved independently, apparently absorbing food from the bottom in the form of bacteria by the surface of its abdomen.

Fractal and Symmetry Oddities

One of the first specimens of the Vendian fauna described by domestic scientists was the Vendia. The print was found in a core from a well in the Arkhangelsk region. The animal had a bilateral, two-sided, body structure with obvious segmentation, which made it possible to even call this creature "naked trilobite" (true trilobites appeared, as is known, in the Cambrian).

But even then, B.M. Keller noticed that the left and right parts of the segments are not opposite each other, but, as it were, in a checkerboard pattern. This phenomenon, which I called "grazing reflection symmetry", turned out to be very common among Vendian animals, which is another mystery, since nothing of the kind is observed in the Cambrian.

Apparently, such a strange symmetry of bilateral creatures is associated with some peculiarities of the growth and development of the organism - perhaps there was a so-called spiral growth, characteristic, for example, of plants and consisting in alternating division of one or another group of cells.

In rankomorphs - feather-like organisms of the cyclomedusa type (they were discussed above) - not only the symmetry of the glancing reflection is observed, but also the fractality of the structure. Tubes extend from the main stem, which then branch out in the same way, and new branches branch again.

Vendian fauna

Charnia is one of the long-known forms of the Vendian fauna. It belongs to the so-called feather-like organisms, and is, most likely, an animal that led an attached lifestyle. Charnia, as well as some other similar forms, resembled in their appearance fern bushes growing from the seabed.

The branching of the vessels extending from the main stem had a fractal structure, which is one of the characteristic features of the Vendian fauna. There were also tubular creatures in the Vendian, similarly "holding on" to the bottom.

In addition to bilateral creatures with gliding reflection symmetry, interesting organisms with three-beam symmetry are noted in the Vendian, which is also atypical for subsequent eras. These include, for example, the tribrachidium, the imprint of which resembles a three-rayed swastika inscribed in a circle (most likely, these are traces of the channels of the digestive system leading to the three mouth openings).

This also includes ventogiruses - these are ovoid creatures with a complex system of internal cavities based on three chambers.

Cold for the giants

The more data on the diversity of the Vendian fauna the fossil record brings us, the more acute is the question of the place of the Vendian biota on the evolutionary tree.Who were the ancestors of this amazing aquatic life, and can you find its descendants among the animals of subsequent eras?

Obviously, Vendian organisms were not the first multicellular animals. In the National Glacial Park in Montana (USA) and in Australia, chains of prints of multicellular creatures that lived 1600-1200 million years ago have been found. The imprints, which look like a necklace of small beads, are believed to be from a colonial marine animal of the hydroid polyp type.

This life is a billion years older than the Vendian, but … no other pre-Vendian traces of multicellular organisms, especially any ancestral forms, have yet been found. This makes one think that, perhaps, the emergence of multicellularity in animals was not a one-time evolutionary leap, but some kind of strategy. For example, even today there are some flagellate protozoa, which either live as separate unicellular organisms, or gather in colonies that act as a single organism. If the sponge is rubbed into individual cells on a sieve, the cells can come together again.

Even experiments were carried out during which, when the parameters of the environment (temperature, salinity) changed, the cells of the embryo of a multicellular organism disintegrated, becoming unicellular. So, it is possible that there is no continuous lineage of multicellular organisms from "beads" from Montana to the Vendian fauna, but generations of unicellular forms may lie between them.

Vendian gigantism probably finds its explanation in the special natural conditions of that environment and that era. The fact is that the richest localities of this fauna are found where carbonates have not accumulated on the bottom. And this is a property of cold-water basins - it is in them that the main sediments are silts, clay and sand.

Cold water contains more oxygen, it constantly mixes, bringing up the nutrient organics from the bottom. Vendian animals did not eat each other - they absorbed microparticles from the water or from the bottom, which provided them with a long life and the ability to develop into large forms.

However, most likely, it was the warming on the planet and the reduction in the number of cold seas that caused the extinction of the Vendian fauna. In the Cambrian, we see a completely different life - in particular, adapted to living in water with a lower oxygen content. But the process of biomineralization began actively, and animals began to acquire strong skeletons, shells and shells.

The question of whether there are descendants of the Vendian fauna among the Cambrian animals today should be answered positively, although it remains the subject of heated scientific discussions. In particular, we find these descendants among molluscs, arthropods, coelenterates. There are many extinct classes of animals that lived in the Cambrian but have roots in the Vendian.

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