Ancient amphibians and problems of reaching land. Landfall
Origin of amphibians
It is believed that the prerequisites for the emergence and formation of amphibians approximately 385 million years ago (in the middle of the Devonian period) were favorable climatic conditions (warmth and humidity), as well as the presence of sufficient nutrition in the form of already formed numerous small invertebrate animals.
And, in addition, during that period, a large amount of organic residues were washed out into water bodies, as a result of the oxidation of which, the level of oxygen dissolved in water decreased, which contributed to the formation of changes in the respiratory organs of ancient fish and their adaptation to respiration atmospheric air.
Ichthyostega
Thus, the origin of amphibians, i.e. the transition of aquatic vertebrates to a terrestrial lifestyle was accompanied by the appearance of respiratory organs adapted to absorb atmospheric air, as well as organs facilitating movement across hard surface. Those. the gill apparatus was replaced by lungs, and the fins were replaced by five-fingered stable limbs that served as support for the body on land.
At the same time, changes occurred in other organs, as well as their systems: the circulatory system, nervous system and sense organs. The main progressive evolutionary changes in the structure of amphibians (aromorphosis) are the following: the development of the lungs, the formation of two circulation circles, the appearance of a three-chambered heart, the formation of five-fingered limbs and the formation of the middle ear. The beginnings of new adaptations can also be observed in some groups of modern fish.
Ancient lobefins
To this day, there is debate in the scientific world about the origin of amphibians. Some believe that amphibians descended from two groups of ancient lobe-finned fishes - Porolepiformes and Osteolepiformes, most others argue in favor of osteolepiform lobe-finned fishes, but do not exclude the possibility that several closely related phyletic lineages of osteolepiform fishes could develop and evolve in parallel.
Armored amphibians - stegocephalians
These same scientists suggest that the parallel lines later became extinct. One of the especially evolved ones, i.e. modified species of ancient lobe-finned fish, was Tiktaalik, which acquired a number of transitional characteristics that made it an intermediate species between fish and amphibians.
I would like to list these features: a movable, shortened head separated from the belt of the forelimbs, reminiscent of a crocodile, shoulder and elbow joints, a modified fin that allowed it to rise above the ground and occupy various fixed positions, and it is possible that it could walk in shallow water. Tiktaalik breathed through the nostrils, and air was probably pumped into the lungs not by the gill apparatus, but by the cheek pumps. Some of these evolutionary changes are also characteristic of the ancient lobe-finned fish Panderichthys.
Ancient lobefins
Origin of amphibians: the first amphibians
It is believed that the first amphibians Ichthyostegidae (lat. Ichthyostegidae) appeared at the end of the Devonian period in fresh water bodies. They formed transitional forms, i.e. something between the ancient lobe-finned fish and the existing ones - modern amphibians. The skin of these ancient creatures was covered with very small fish scales, and along with paired five-fingered limbs they had an ordinary fish tail.
They have only rudiments left of the gill covers, but from the fish they have preserved the cleithrum (a bone belonging to the dorsal region and connecting the shoulder girdle to the skull). These ancient amphibians could live not only in fresh water, but also on land, and some of them crawled onto land only periodically.
Ichthyostega
When discussing the origin of amphibians, one cannot help but say that later, in the Carboniferous period, a number of branches were formed, consisting of numerous superorders and orders of amphibians. So, for example, the Labyrinthodont superorder was very diverse and existed until the end of the Triassic period.
In the Carboniferous period, a new branch of early amphibians formed - Lepospondyli (lat. Lepospondyli). These ancient amphibians were adapted for life exclusively in water and existed until approximately the middle of the Permian period, giving rise to modern orders of amphibians - Legless and Tailed.
I would like to note that all amphibians called stegocephals (shell-headed), which appeared in the Paleozoic, became extinct already in the Triassic period. It is assumed that their first ancestors were bony fish, which combined primitive structural features with more developed (modern) ones.
Stegocephalus
Considering the origin of amphibians, I would like to draw your attention to the fact that lobe-finned fish are closest to shell-headed fish, since they had pulmonary breathing and a skeleton similar to the skeletons of stegocephalians (shell-headed fish).
In all likelihood, the Devonian period, in which shell-headed fish were formed, was characterized by seasonal droughts, during which many fish had a “hard life”, since the water was depleted of oxygen, and numerous overgrown aquatic vegetation made it difficult for them to move in the water.
Stegocephalus
In such a situation, the respiratory organs of aquatic creatures should have been modified and turned into pulmonary sacs. At the beginning of breathing problems, ancient lobe-finned fish simply had to rise to the surface of the water to get the next portion of oxygen, and later, when the water bodies dried up, they were forced to adapt and go to land. Otherwise, animals that did not adapt to new conditions simply died.
Only those aquatic animals that were able to adapt and adapt, and whose limbs were modified to such an extent that they became capable of moving on land, were able to survive these extreme conditions, and eventually develop into amphibians. In such difficult conditions, the first amphibians, having received new, more advanced limbs, were able to move overland from a dried-up reservoir to another reservoir where water was still preserved.
Labyrinthodonts
At the same time, those animals that were covered with heavy bone scales (scaly shell) could hardly move on land and, accordingly, whose skin breathing was difficult, were forced to reduce (reproduce) the bone shell on the surface of their body.
In some groups of ancient amphibians it was preserved only on the belly. It must be said that the shell-headed (stegocephalians) managed to survive only until the beginning of the Mesozoic era. All modern, i.e. The currently existing orders of amphibians were formed only at the end of the Mesozoic period.
On this note, we end our story about the origin of amphibians. I would like to hope that you liked this article, and you will return to the pages of the site for more, immersed in reading amazing world wildlife.
And in more detail, with most interesting representatives amphibians (amphibians), these articles will introduce you to:
About 385 million years ago, conditions formed on Earth that were favorable for the massive development of land by animals. Favorable factors were, in particular, warm and humid climate, the presence of a sufficient food base (an abundant fauna of terrestrial invertebrates has formed). In addition, during that period, a large amount of organic matter was washed into water bodies, as a result of the oxidation of which the oxygen content in the water decreased. This contributed to the appearance of devices for breathing atmospheric air in fish.
Evolution
The rudiments of these devices can be found among a variety of different groups fish Some modern fish are able to leave the water at one time or another and their blood is partially oxidized due to atmospheric oxygen. Such, for example, is the slider fish ( Anabas), which, coming out of the water, even climbs trees. Some representatives of the goby family crawl onto land - mudskippers ( Periophthalmus). The latter catch their prey more often on land than in water. The ability of some lungfish to stay out of water is well known. However, all these adaptations are of a private nature and the ancestors of amphibians belonged to less specialized groups of freshwater fish.
Adaptations to terrestriality developed independently and in parallel in several lines of evolution of lobe-finned fish. In this regard, E. Jarvik put forward a hypothesis about the diphyletic origin of terrestrial vertebrates from two different groups of lobe-finned fish ( Osteolepiformes And Porolepiformes). However, a number of scientists (A. Romer, I. I. Shmalhausen, E. I. Vorobyova) criticized Jarvik’s arguments. Most researchers consider the monophyletic origin of tetrapods from osteolepiform lobe-fins to be more likely, although the possibility of paraphyly, that is, the achievement of the level of organization of amphibians by several closely related phyletic lineages of osteolepiform fishes that evolved in parallel, is accepted. The parallel lines are most likely extinct.
One of the most “advanced” lobe-finned fish was Tiktaalik, which had a number of transitional characteristics that brought it closer to amphibians. Such features include a shortened skull, the forelimbs separated from the belt and a relatively mobile head, and the presence of elbow and shoulder joints. The fin of Tiktaalik could occupy several fixed positions, one of which was intended to allow the animal to be in an elevated position above the ground (probably to “walk” in shallow water). Tiktaalik breathed through holes located at the end of a flat “crocodile” snout. Water, and possibly atmospheric air, was no longer pumped into the lungs by gill covers, but by cheek pumps. Some of these adaptations are also characteristic of the lobe-finned fish Panderichthys.
The first amphibians to appear in fresh water bodies at the end of the Devonian were ichthyostegidae. They were true transitional forms between lobe-finned fish and amphibians. Thus, they had rudiments of an operculum, a real fish tail, and a preserved cleithrum. The skin was covered with small fish scales. However, along with this, they had paired five-fingered limbs of terrestrial vertebrates (see diagram of the limbs of lobe-finned animals and the most ancient amphibians). Ichthyostegids lived not only in water, but also on land. It can be assumed that they not only reproduced, but also fed in the water, systematically crawling onto land.
Subsequently, during the Carboniferous period, a number of branches arose, which are given the taxonomic meaning of superorders or orders. The labyrinthodontia superorder was very diverse. Early forms had relatively small sizes and a fish-like body. Later ones reached very large sizes (1 m or more) in length, their body was flattened and ended with a short thick tail. Labyrinthodonts existed until the end of the Triassic and occupied terrestrial, semi-aquatic and aquatic habitats. The ancestors of anurans are relatively close to some labyrinthodonts - the orders Proanura, Eoanura, known from the end of the Carboniferous and from the Permian deposits.
The second branch of primary amphibians, the Lepospondyli, also arose in the Carboniferous. They were small in size and well adapted to life in water. Some of them lost limbs for the second time. They existed until the middle of the Permian period. It is believed that they gave rise to orders of modern amphibians - tailed (Caudata) and legless (Apoda). In general, all Paleozoic amphibians became extinct during the Triassic. This group of amphibians is sometimes called stegocephalians (shell-headed) for the continuous shell of dermal bones that covered the skull from above and from the sides. The ancestors of stegocephalians were probably bony fish, which combined primitive organizational features (for example, weak ossification of the primary skeleton) with the presence additional organs breathing in the form of pulmonary sacs.
Lobe-finned fish are closest to stegocephals. They had pulmonary breathing, their limbs had a skeleton similar to that of stegocephals. The proximal section consisted of one bone, corresponding to the shoulder or femur, the next segment consisted of two bones, corresponding to the forearm or tibia; Next there was a section consisting of several rows of bones; it corresponded to the hand or foot. Also noteworthy is the obvious similarity in the arrangement of the integumentary bones of the skull in ancient lobe-fins and stegocephalians.
The Devonian period, in which stegocephals arose, was apparently characterized by seasonal droughts, during which life in many fresh water bodies was difficult for fish. The depletion of oxygen in the water and the difficulty of swimming in it were facilitated by the abundant vegetation that grew during the Carboniferous era along swamps and the banks of reservoirs. Plants fell into the water. Under these conditions, adaptations of fish to additional breathing through pulmonary sacs could have arisen. In itself, the depletion of water in oxygen was not yet a prerequisite for reaching land. Under these conditions, lobe-finned fish could rise to the surface and swallow air. But with severe drying out of reservoirs, life for fish became impossible. Unable to move on land, they died. Only those aquatic vertebrates that, at the same time as the ability for pulmonary respiration, acquired limbs capable of moving on land, could survive these conditions. They crawled onto land and moved to neighboring bodies of water, where water still remained.
At the same time, movement on land was difficult for animals covered with a thick layer of heavy bony scales, and the bony scaly shell on the body did not provide the possibility of skin respiration, so characteristic of all amphibians. These circumstances apparently were a prerequisite for the reduction of the bony armor on most of the body. In certain groups of ancient amphibians, it was preserved (not counting the skull shell) only on the belly.
Stegocephalians survived until the beginning of the Mesozoic. Modern orders of amphibians were formed only at the end of the Mesozoic.
Notes
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A lot of work had to be done in searching for fossil traces of extinct creatures in order to clarify this question.
Previously, the transition of animals to land was explained as follows: in the water, they say, there are many enemies, and so the fish, fleeing from them, began to crawl onto land from time to time, gradually developing the necessary adaptations and transforming into other, more advanced forms of organisms.
We cannot agree with this explanation. After all, even now there are such amazing fish that from time to time crawl ashore and then return to the sea. But they do not add water at all for the sake of salvation from enemies. Let us also remember about frogs - amphibians that, living on land, return to the water to produce offspring, where they spawn and where young frogs - tadpoles - develop. Add to this that the oldest amphibians were not at all defenseless creatures suffering from enemies. They were clad in a thick, hard shell and hunted other animals like cruel predators; it is incredible that they or others like them would be driven out of the water by danger from their enemies.
They also expressed the opinion that aquatic animals that overflowed the sea seemed to be suffocating in sea water, felt the need for fresh air, and were attracted by the inexhaustible reserves of oxygen in the atmosphere. Was this really so? Let's remember the flying sea fish. They either swim near the surface of the sea, or rise out of the water with a strong splash and rush through the air. It would seem that it would be easiest for them to start using the air of the atmosphere. But they just don’t use it. They breathe with gills, i.e., respiratory organs adapted for life in water, and are quite content with this.
But among freshwater there are those that have special adaptations for air breathing. They are forced to use them when the water in the river or river becomes cloudy, clogged and depleted of oxygen. If it gets clogged sea water some streams of mud flowing into the sea, then the sea fish swim away to another place. Sea fish and do not require special devices for air breathing. They find themselves in a different position freshwater fish when the water around them becomes cloudy and rots. Some are worth watching tropical rivers to understand what is happening.
Instead of our four seasons, the tropics have a hot and dry half of the year followed by a rainy and damp half of the year. During heavy rains and frequent thunderstorms, rivers overflow widely, the waters rise high and are saturated with oxygen from the air. But the picture changes dramatically. The rain stops pouring. The waters are receding. The scorching sun dries up the rivers. Finally, instead of flowing water Chains of lakes and swamps remain, in which stagnant water is overflowing with animals. They die in droves, the corpses quickly decompose, and when they rot, oxygen is consumed, so that it becomes less and less in these bodies of water filled with organisms. Who can survive such drastic changes in living conditions? Of course, only those who have the appropriate adaptations: he can either hibernate, burying himself in the silt for the entire dry time, or switch to breathing atmospheric oxygen, or, finally, he can do both. All the rest are doomed to extermination.
Fish have two types of adaptations for air breathing: either their gills have spongy outgrowths that retain moisture, and as a result, air oxygen easily penetrates the blood vessels that wash them; or they have a modified swim bladder, which serves to hold the fish at a certain depth, but at the same time can also serve as a respiratory organ.
The first adaptation is found in some bony fish, that is, those that no longer have a cartilaginous, but a completely ossified skeleton. Their swim bladder is not involved in breathing. One of these fish, the “crawling perch,” lives in tropical countries and now. Like some
other bony fish, it has the ability to leave the water and crawl (or jump) along the shore with the help of fins; sometimes it even climbs trees in search of slugs or worms on which it feeds. No matter how amazing the habits of these fish are, they cannot explain to us the origin of those changes that allowed aquatic animals to become land dwellers. They breathe with the help of special devices 9 gill apparatus.
Let us turn to two very ancient groups of fish, those that lived on Earth already in the first half of the ancient era of Earth’s history. We are talking about lobe-finned and lungfishes. One of the remarkable lobe-finned fish, called polypterus, still lives in rivers tropical Africa. During the day, this fish likes to hide in deep holes on the muddy bottom of the Nile, and at night it becomes animated in search of food. She attacks both fish and crayfish, and does not disdain frogs. Lying in wait for prey, polypterus stands at the bottom, leaning on its wide pectoral fins. Sometimes he crawls along the bottom on them, as if on crutches. Once taken out of the water, this fish can live for three to four hours if kept in wet grass. At the same time, its breathing occurs with the help of a swim bladder, into which the fish continually takes in air. This bladder is double in lobe-finned fish and develops as an outgrowth of the esophagus on the ventral side.
We do not know Polypterus in fossil form. Other lobe-finned fish close relative polyptera, lived in very distant times and breathed with a well-developed swim bladder.
Lungfish, or pulmonary fish, are remarkable in that their swim bladder has turned into a respiratory organ and works like lungs. Of these, only three genera have survived to this day. One of them, the horntooth, lives in the slow-flowing rivers of Australia. In silence summer nights The grunting sounds that this fish makes as it swims to the surface of the water and releases air from its swim bladder can be heard far and wide. But usually this big fish lies motionless at the bottom or slowly swims among the water thickets, plucking them and looking for crustaceans, worms, mollusks and other food there.
She breathes in two ways: both with gills and with a swim bladder. Both organs work simultaneously. When the river dries up in the summer and small reservoirs remain, the cattail feels great in them, while the rest of the fish die en masse, their corpses rot and spoil the water, depriving it of oxygen. Travelers to Australia have seen these pictures many times. It is especially interesting that such pictures unfolded extremely often at the dawn of the Carboniferous Age across the face of the Earth; they give an idea of how, as a result of the extinction of some and the victory of others, a great event in the history of life became possible - the emergence of aquatic vertebrates on land.
The modern horntooth is not inclined to move to the shore to live. He all year round spends in water. Researchers have not yet been able to observe that it hibernates during hot periods.
Its distant relative, the ceratod, or fossil horntooth, lived on Earth in very distant times and was widespread. Its remains were found in Australia, Western Europe, India, Africa, North America.
Two other pulmonary fishes of our time - Protopterus and Lepidosirenus - differ from the cattail in the structure of their swim bladder, which has turned into lungs. Namely, they have a double one, whereas the horntooth has an unpaired one. Protoptera is quite widespread in the rivers of tropical Africa. Or rather, he lives not in the rivers themselves, but in swamps that stretch next to the river beds. It feeds on frogs, worms, insects, and crayfish. On occasion, protopters also attack each other. Their fins are not suitable for swimming, but serve for support on the bottom when crawling. They even have something like an elbow (and knee) joint approximately halfway along the length of the fin. This remarkable feature shows that lung fish, even before leaving the water element, could have developed adaptations that were very useful to them for life on land.
From time to time, the protopter rises to the surface of the water and draws air into its lungs. But this fish has a hard time in the dry season. There is almost no water left in the swamps, and the protopter is buried in the silt to a depth of about half a meter in a special kind of hole; here he lies, surrounded by hardened mucus secreted by his skin glands. This mucus forms a shell around the protopter and prevents it from drying out completely, keeping the skin moist. There is a passage running through the entire crust, which ends at the fish’s mouth and through which it breathes atmospheric air. During this hibernation, the swim bladder serves as the only respiratory organ, since the gills then do not work. What is the reason for life in the body of a fish at this time? She is losing a lot of weight, losing not only her fat, but also some of her meat, just as they live off accumulated fat and meat during hibernation and our animals - bear, marmot. Dry time in Africa it lasts a good six months: in the homeland of the protopter - from August to December. When the rains come, life in the swamps will be resurrected, the shell around the protopter will dissolve, and it will resume its vigorous activity, now preparing to reproduce.
Young protoptera hatched from eggs look more like salamanders than fish. They have long external gills, like tadpoles, and their skin is covered in colorful spots. At this time there is no swim bladder yet. It develops when the external gills fall off, just as it happens in young frogs.
The third lung fish - lepidosiren - lives in South America. She spends her life almost the same as her African relative. And their offspring develop very similarly.
No more lungfish survive. And those that still remained - the horntooth, protopterus and lepidosirenus - were approaching the end of their century. Their time has long passed. But they give us an idea of the distant past and are therefore especially interesting to us.
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Chapter 8. Early Paleozoic: “exit of life to land.” The appearance of soils and soil formers. Higher plants and their environment-forming role. Tetrapodization of lobe-finned fishes
Until very recently, people learned from a school biology textbook and popular books on the theory of evolution this approximately picture of an event usually called the “Exit of life to land.” At the beginning of the Devonian period (or at the end of the Silurian), thickets of the first terrestrial plants - psilophytes (Figure 29, a) appeared on the shores of the seas (more precisely, sea lagoons), the position of which in the system of the plant kingdom remains not entirely clear. Vegetation made it possible for invertebrate animals to appear on land - centipedes, arachnids and insects; invertebrates, in turn, created a food base for terrestrial vertebrates - the first amphibians (descending from lobe-finned fish) - such as ichthyostega (Figure 29, b). Terrestrial life in those days occupied only an extremely narrow coastal strip, beyond which stretched vast expanses of absolutely lifeless primary deserts.
So, according to modern ideas, almost everything in this picture is incorrect (or at least inaccurate) - starting with the fact that sufficiently developed terrestrial life reliably existed much earlier (already in the Ordovician period following the Cambrian), and ending with , that the mentioned “first amphibians” were probably purely aquatic creatures that had no connection with land. The point, however, is not even in these particulars (we will talk about them in our turn). Another thing is more important: most likely, the formulation itself is fundamentally incorrect - “Exit of living organisms to land.” There are serious reasons to believe that land landscapes modern look in those days they were completely absent, and living organisms not only came to land, but in a sense created it as such. However, let's take it in order.
So the first question is when; When did the first undoubtedly terrestrial organisms and ecosystems appear on Earth? However, here a counter question immediately arises: how can we determine that a certain extinct organism that we encountered is terrestrial? This is not at all as simple as it seems at first glance, because the principle of actualism here will work with serious malfunctions. A typical example: starting from the middle of the Silurian period, scorpions appear in the fossil record - animals that seem to be purely land animals in modern times. However, it is now quite firmly established that Paleozoic scorpions breathed with gills and led an aquatic (or at least amphibiotic) lifestyle; terrestrial representatives of the order, whose gills are transformed into “book-lungs” characteristic of arachnids, appeared only at the beginning of the Mesozoic. Consequently, finds of scorpions in Silurian deposits in themselves do not prove anything (in the sense of interest to us).
It seems more productive here to track the appearance in the chronicle not of terrestrial (in modern times) groups of animals and plants, but of certain anatomical signs of “landness”. So, for example, a plant cuticle with stomata and the remains of conducting tissues - tracheids must surely belong to terrestrial plants: under water, as you might guess, both stomata and conducting vessels are useless... However, there is another - truly wonderful! - integral indicator of existence in given time terrestrial life. Just as free oxygen is an indicator of the existence of photosynthetic organisms on the planet, soil can serve as an indicator of the existence of terrestrial ecosystems: the process of soil formation occurs only on land, and fossil soils (paleosols) are clearly distinguishable in structure from any type of bottom sediments.
It should be noted that soil is not preserved in a fossil state very often; Only in recent decades have paleosols stopped being looked at as some kind of exotic curiosity and their systematic study began. As a result, in the study of ancient weathering crusts (and soil is nothing more than a biogenic weathering crust), a genuine revolution took place, literally upending previous ideas about life on land. The most ancient paleosols were found in the deep Precambrian - early Proterozoic; in one of them, 2.4 billion years old, S. Campbell (1985) discovered undoubted traces of the vital activity of photosynthetic organisms - carbon with a shifted isotope ratio of 12 C / 13 C. In this regard, we can mention the recently discovered remains of cyanobacterial buildings in Proterozoic karst cavities: karst processes - the formation of basins and caves in water-soluble sedimentary rocks (limestones, gypsum) - can only occur on land.
Another fundamental discovery in this area should be considered the discovery by G. Retallak (1985) in Ordovician paleosols of vertical burrows dug by some fairly large animals - apparently arthropods or oligochaetes (earthworms); in these soils there are no roots (which are usually very well preserved), but there are peculiar tubular bodies - Retallak interprets them as the remains of non-vascular plants and/or terrestrial green algae. In somewhat later, Silurian, paleosols, coprolites (fossilized excrement) of some soil-dwelling animals were found; Their food, apparently, was the hyphae of fungi, which make up a significant portion of the substance of coprolites (however, it is possible that fungi could have developed secondarily on organic matter contained in coprolites).
So, by now two facts can be considered quite firmly established:
1. Life appeared on land a very long time ago, in the middle Precambrian. It was represented, apparently, by various variants of algal crusts (including amphibiotic mats) and, possibly, lichens; all of them could carry out the processes of archaic soil formation.
2. Animals (invertebrates) existed on land at least since the Ordovician, i.e. long before the appearance of higher vegetation (whose reliable traces still remain unknown until the Late Silurian). The algal crusts mentioned above could serve as habitat and food for these invertebrates; at the same time, the animals themselves inevitably became a powerful soil-forming factor.
The last circumstance makes us recall an old discussion - about two possible ways for invertebrates to colonize land. The fact is that non-marine fossils of this age were very rare, and all hypotheses on this subject seemed only more or less convincing speculations, not subject to real verification. Some researchers assumed that the animals came out of the sea directly - through the littoral zone with algal discharges and other shelters; others insisted that freshwater bodies of water were settled first, and only from this “bridgehead” did the “offensive” on land subsequently begin. Among the supporters of the first point of view, M.S.’s constructions stood out for their persuasiveness. Gilyarov (1947), who, based on a comparative analysis of the adaptations of modern soil-dwelling animals, argued that it was the soil that should have served as the primary habitat of the earliest inhabitants of the land. It should be taken into account that the soil fauna is really very poorly included in the paleontological record and the absence of fossil “documents” here is quite understandable. These constructions, however, had one truly vulnerable point: where did this soil itself come from, if in those days there was no terrestrial vegetation yet? Everyone knows that soil formation occurs with the participation higher plants- Gilyarov himself called real soils only those associated with the rhizosphere, and everything else - weathering crusts... However, now - when it became known that primitive soil formation is possible with the participation of lower plants only - Gilyarov’s concept has gained a “second wind”, and was recently directly confirmed by Retallak's data on Ordovician paleosols.
On the other hand, undoubted freshwater faunas (which contain, among other things, tracks of traces on the surface of the sediment) appear much later - in the Devonian. They include scorpions, small (about palm-sized) crustacean scorpions, fish and the first non-marine mollusks; Among the mollusks there are also bivalves - long-living organisms that are unable to tolerate death and drying out of water bodies. Faunas with such indisputably soil animals as trigonotarbs (“shell spiders”) and herbivorous bipedal centipedes already existed in the Silurian (Ludlovian age). And since aquatic fauna always ends up in burials an order of magnitude better than terrestrial fauna, all this allows us to draw another conclusion:
3. Soil fauna appeared significantly earlier than freshwater. That is, at least for animals, fresh waters could not play the role of a “springboard” in the conquest of land.
This conclusion, however, forces us to return to the very question with which we began our reasoning, namely: did living organisms come to land or actually create it as such? A.G. Ponomarenko (1993) believes that all the communities discussed above are, in fact, difficult to definitely call “terrestrial” or “communities of inland water bodies” (although at least the mats should have been in the water for a significant part of the time). He believes that "the existence of true continental bodies of water, both flowing and stagnant, seems highly problematic before vascular vegetation somewhat reduced the rate of erosion and stabilized the shoreline in the Devonian." The main events had to take place in the already familiar flattened coastal amphibiotic landscapes without a stable coastline - “neither land nor sea” (see Chapter 5).
A no less unusual (from the point of view of today) situation should have developed on the watersheds occupied by “primary deserts.” Nowadays, deserts exist in conditions of lack of moisture (when evaporation exceeds precipitation) - which prevents the development of vegetation. But in the absence of plants, the landscape paradoxically became more deserted (in appearance) the more precipitation fell: water actively eroded the mountain slopes, cutting deep canyons, when reaching the plain it gave rise to conglomerates, and further along the plain psephytes scattered across the surface spread, which called plain proluvium; Nowadays such deposits form only the alluvial fans of temporary watercourses.
This picture allows us to take a fresh look at one strange circumstance. Almost all known Silurian-Devonian terrestrial floras and faunas are found at various points on the ancient Continent of Red Sandstone, so named for its characteristic rocks - red flowers; all locations are associated with deposits considered deltaic. In other words, it turns out that this entire continent (uniting Europe and eastern North America) is, as it were, one continuous giant delta. A reasonable question: where were the corresponding rivers located - after all, there are simply no drainage areas for them on a continent of that size! It remains to be assumed that all these “deltaic” deposits, apparently, arose precisely as a result of erosion processes in the “wet deserts” described above.
So, life on land (which, however, is not yet completely dry) seems to have existed since time immemorial, and at the end of the Silurian, another group of plants simply appears - vascular plants (Tracheophyta)... However, in fact, the appearance of vascular plants is one of the key events in the history of the biosphere, because in its environment-forming role this group of living organisms has no equal, at least among eukaryotes. It was vascular vegetation that made, as we will see later, a decisive contribution to the formation of terrestrial landscapes of the modern appearance.
The generally accepted point of view is that some algae that lived near the shore first “stuck their heads into the air,” then populated the tidal zone, and then, gradually turning into higher plants, completely came out onto the shore. This was followed by their gradual conquest of the land. Most botanists consider one of the groups of green algae - Charophyta - to be the ancestors of higher plants; They now form continuous thickets on the bottom of continental water bodies - both fresh and salty, while in the sea (and even then only in desalinated bays) only a few species are found. Characeae have a differentiated thallus and complex reproductive organs; They are similar to higher plants by several unique anatomical and cytological features - symmetrical sperm, the presence of a phragmoplast (a structure involved in the construction of the cell wall during division) and the presence of the same set of photosynthetic pigments and reserve nutrients.
However, a serious - purely paleontological - objection was raised against this point of view. If the process of transformation of algae into higher plants really occurred in coastal waters (where conditions for entering the fossil record are most favorable), then why do we not see any of its intermediate stages? Moreover, the characeae themselves appeared in the Late Silurian - simultaneously with vascular plants, and the peculiarities of the biology of this group do not give grounds to assume a long period of “hidden existence” for it... Therefore, a paradoxical, at first glance, hypothesis appeared: why , in fact, the appearance of macroremains of higher plants at the end of the Silurian should be unambiguously interpreted as traces of their emergence onto land? Perhaps, quite the opposite - these are traces of the migration of higher plants into water? In any case, many paleobotanists (S.V. Meyen, G. Stebbins, G. Hill) actively supported the hypothesis about the origin of higher plants not from aquatic macrophytes (such as Characeae), but from terrestrial green algae. It was these terrestrial (and therefore having no real chance of being buried) “primary higher plants” that could belong to the mysterious spores with a three-rayed slit, which were very numerous in the Early Silurian and even in the Late Ordovician (starting from the Caradocian age).
However, it recently became clear that, apparently, supporters of both points of view are right - each in their own way. The fact is that some of the microscopic terrestrial green algae have the same complex of subtle cytological characters that charophytes and vascular algae (see above); these microalgae are now included in Charophyta. Thus, a completely logical and consistent picture emerges. Initially, there existed - on land - a group of green algae ("microscopic characeae"), from which two closely related groups emerged in the Silurian: the "true" characeae, which populated continental water bodies, and higher plants, which began to colonize the land, and only after some time (in full according to Meyen's scheme) appearing in coastal habitats.
From your botany course you should know that higher plants (Embryophyta) are divided into vascular plants (Tracheophyta) and bryophytes (Bryophyta) - mosses and liverworts. Many botanists (for example, J. Richardson, 1992) believe that it is liverworts (based on their modern life strategies) are the main contenders for the role of “land pioneers”: they now live on terrestrial algal films, in shallow ephemeral reservoirs, in the soil - together with blue-green algae. Interestingly, the nitrogen-fixing blue-green alga Nostoc is able to live inside the tissues of some liverworts and anthocerotes, providing its hosts with nitrogen; this was probably very important for the first inhabitants of primitive soils, where this element could not but be in severe deficiency. The above-mentioned spores from the Late Ordovician and Early Silurian deposits are most similar to the spores of liverworts (reliable macrofossils of these plants appear later, in the Early Devonian).
However, in any case, bryophytes (even if they really appeared in the Ordovician) hardly changed the appearance of continental landscapes. The first vascular plants - rhinophytes - appeared in the Late Silurian (Ludlovian Age); up to the Early Devonian (Zhedino Age), they were represented by extremely monotonous remains of the only genus Cooksonia, the simplest and most archaic of the vascular species. But in the deposits of the next century of the Devonian (Siegen) we already find a wide variety of rhyniophytes (Figure 30). Since that time, two evolutionary lines have separated among them. One of them will go from the genus Zosterophylum to the lycophytes (their number also includes tree-like lepidodendrons - one of the main coal-formers in the next, Carboniferous, period). The second line (the genus Psilophyton is usually placed at its base) leads to horsetails, ferns and seed plants - gymnosperms and angiosperms (Figure 30). Even the Devonian rhinophytes are still very primitive and, frankly speaking, it is unclear whether they can be called “higher plants” in the strict sense: they have a vascular bundle (though composed not of tracheids, but of special elongated cells with a peculiar relief of the walls), but lack stomata . This combination of characteristics should indicate that these plants have never encountered water deficiency (we can say that their entire surface is one large open stomata), and, most likely, were helophytes (that is, they grew “knee-deep in water ", like today's reeds).
The appearance of vascular plants with their rigid vertical axes caused a cascade of ecosystem innovations that changed the appearance of the entire biosphere:
1. Photosynthetic structures began to be located in three-dimensional space, and not on a plane (as was the case until now - during the period of dominance of algal crusts and lichens). This sharply increased the intensity of the formation of organic matter and, thereby, the total productivity of the biosphere.
2. The vertical arrangement of the trunks made the plants more resistant to being washed away by fine earth (compared, for example, to algal crusts). This reduced the irreversible loss of unoxidized carbon (in the form of organic matter) by the ecosystem - improving the carbon cycle.
3. Vertical trunks of terrestrial plants must be quite rigid (compared to aquatic macrophytes). To provide this rigidity, a new tissue arose - wood, which decomposes relatively slowly after the death of the plant. Thus, the carbon cycle of the ecosystem acquires an additional reserve depot and, accordingly, is stabilized.
4. The emergence of a permanent supply of difficult-to-decomposable organic matter (concentrated mainly in the soil) leads to a radical restructuring of food chains. From now on most of matter and energy circulate through detritus, and not through grazing chains (as was the case in aquatic ecosystems).
5. To decompose the difficult-to-digest substances that make up wood - cellulose and lignin - new types of destroyers of dead organic matter were required. Since that time, on land, the role of the main destructors has passed from bacteria to fungi.
6. To maintain the trunk in a vertical position (under the influence of gravity and winds), a developed root system arose: rhizoids - like algae and bryophytes - are no longer enough. This led to a noticeable decrease in erosion and the appearance of fixed (rhizosphere) soils.
S.V. Meyen believes that the land should have been covered with vegetation by the end of the Devonian (Siegenian), since from the beginning of the next, Carboniferous, period, almost all types of sediments now deposited on the continents were formed on Earth. In pre-Sigen times, continental precipitation is practically absent - apparently due to its constant secondary erosion as a result of unregulated runoff. At the very beginning of the Carboniferous, coal accumulation began on the continents - and this indicates that powerful plant filters stood in the way of water flow. Without them, plant remains would continuously mix with sand and clay, so that the result would be clastic rocks enriched in plant remains - carbonaceous shales and carbonaceous sandstones, and not real coals.
So, a dense “brush” of helophytes that has arisen in coastal amphibiotic landscapes (one can call it “rhiniophyte reed”) begins to act as a filter regulating raincoat runoff: it intensively filters (and deposits) the debris carried from the land and thereby forms a stable coastline . Some analogue of this process can be the formation of “alligator ponds” by crocodiles: animals constantly deepen and expand the swamp reservoirs they inhabit, throwing soil onto the shore. As a result of their many years of “irrigation activities,” the swamp is transformed into a system of clean, deep ponds separated by wide forested “dams.” Thus, vascular vegetation in the Devonian divided the notorious amphibiotic landscapes into “real land” and “real freshwater bodies.” It would not be a mistake to say that it was vascular vegetation that became the true executor of the spell: “Let there be firmament!” - having separated this firmament from the abyss...
It is with the newly emerged freshwater bodies that the appearance in the Late Devonian (Famennian Age) of the first tetrapods (quadrupeds) - a group of vertebrates with two pairs of limbs - is associated; it combines amphibians, reptiles, mammals and birds (simply put, tetrapods are all vertebrates, except fish and fish-like creatures). It is now generally accepted that tetrapods originate from lobe-finned fishes (Rhipidistia) (Figure 31); this relict group now has a single living representative, coelacanth. The once quite popular hypothesis of the origin of tetrapods from another relict group of fish - lungfish (Dipnoi), now has practically no supporters.
It should be noted that in previous years, the emergence of a key feature of tetrapods - two pairs of five-fingered limbs - was considered their unambiguous adaptation to a terrestrial (or at least amphibiotic) lifestyle. Nowadays, however, most researchers are inclined to believe that the “problem of the appearance of four-legged animals” and the “problem of their emergence onto land” are different things and not even related to each other by a direct causal relationship. The ancestors of tetrapods lived in shallow, often drying up, abundantly overgrown with vegetation reservoirs of variable configuration. Apparently, the limbs appeared in order to move along the bottom of reservoirs (this is especially important when the reservoir has become so shallow that your back begins to stick out) and push through dense thickets of helophytes; The limbs turned out to be especially useful for crawling on dry land to another neighboring one when the reservoir dried up.
The first, Devonian, tetrapods - primitive amphibians labyrinthodonts (the name comes from their teeth with labyrinth-like folds of enamel - a structure directly inherited from lobe-finned animals: see Figure 31), such as Ichthyostega and Acanthostega, are always found in burials together with fish, which, Apparently, they were eating. They were covered with scales like fish, had a caudal fin (similar to what we see in catfish or burbot), lateral line organs and - in some cases - developed gill apparatus; their limb is not yet five-fingered (the number of fingers reaches 8), and according to the type of articulation with the axial skeleton, it is typically swimming, and not supporting. All this leaves no doubt that these creatures were purely aquatic (Figure 32); if they appeared on land under certain “fire” circumstances (drying out of a reservoir), then they most certainly were not a component of terrestrial ecosystems. Only much later, in the Carboniferous period, small terrestrial amphibians appeared - anthracosaurs, which, apparently, fed on arthropods, but more on this later (see Chapter 10).
Particularly noteworthy is the fact that in the Devonian a number of unrelated parallel groups of stegocephalopod-like lobe-finned fishes appeared - both before and after the appearance of “true” tetrapods (labyrinthodonts). One of these groups were panderichthids - lobe-finned fish, lacking dorsal and anal fins, which is not the case in any other fish. In terms of the structure of the skull (no longer “fish”, but “crocodile”), the shoulder girdle, the histology of the teeth and the position of the choanae (internal nostrils), panderichthids are very similar to Ichthyostega, but acquired these characteristics clearly independently. Thus, we have before us a process that can be called parallel tetrapodization of lobe-fins (it was studied in detail by E.I. Vorobyova). As usual, the “order” for the creation of a four-legged vertebrate capable of living (or at least surviving) on land was given by the biosphere not to one, but to several “design bureaus”; Ultimately, the group of lobe-finned fish that “created” the modern type of tetrapods known to us “won the competition.” However, along with “real” tetrapods, for a long time there existed a whole spectrum of ecologically similar semi-aquatic animals (such as panderichthids), combining the characteristics of fish and amphibians - so to speak, “waste products” of the process of tetrapodization of lobe-finned animals.
Notes
Scorpios form a specialized group of marine crustacean scorpions already familiar to us (from Chapter 7) - eurypterids, whose representatives moved from swimming to walking along the bottom and, having acquired small sizes, first mastered the sea littoral zone, and then the land.
With the discovery of Cambrian marine centipede-like arthropods, their existence on the Early Paleozoic land seems quite probable, although reliable finds of millipedes in continental sediments appear only in the Late Silurian.
It is possible that macroscopic plants already existed on land in the Vendian. At this time on thallum some algae ( Kanilovia) mysterious, complex microstructures appear in the form of a spiral chitinoid ribbon breaking in a zigzag manner. M. B. Burzin (1996) quite logically suggested that they serve to scatter spores, and such a mechanism is necessary only in the air.
Psephites are loose sediments of clastic material, coarser than “clay” (pelites) and “sand” (psammites).
None of the higher plants is capable of nitrogen fixation, i.e. to convert nitrogen from atmospheric gas N2 into an assimilable form (for example, NO3– ions). This is an additional argument in favor of the fact that by the time higher plants appeared on land, prokaryotic communities had long existed there, which enriched the soil with nitrogen in an accessible form.
A more common name is psilophytes– are not used now for nomenclatural reasons. In literature recent years you may come across another name - propteridophytes.
Representatives of almost all the main divisions of higher plants appeared, not only spore(lycophytes, pteridophytes, horsetails), but also gymnosperms ( ginkgo).
Widely known truly romantic story the discovery of this “living fossil”, described in the wonderful book by J. Smith “Old Quadruped”. It should, however, be noted that the lifestyle of the coelacanth has nothing in common with that of the Devonian rhipidistia: it lives in the Indian Ocean at depths of several hundred meters.
Old name " stegocephalians”, which you can find in books, is not used now.
We don’t call an eel a “terrestrial creature,” which is capable of crawling at night through dewy grass from one body of water to another, covering a distance of several hundred meters!
But, perhaps, an equally important event should be considered the appearance on Earth of land organisms and, above all, land plants. When, how and why did this happen?
First half Paleozoic era There were three large continents on Earth. Their outlines were very far from modern ones. A huge continent stretched in the northern half globe from the middle of modern North America to the Urals. To the east of it was another, less large continent. It occupied the territory of Eastern Siberia, Far East, parts of China and Mongolia. In the south, from South America A third continent, Gondwana, extended through Africa to Australia.
The climate was warm almost everywhere. The continents had a flat, uniform topography. Therefore, the waters of the oceans often flooded the land, forming shallow seas, which often became shallow, dried up, and then filled with water again. Thus, nature itself seemed to force some types of aquatic plants - green algae - to adapt to life outside of water. During periods of shallow waters and droughts, some of them survived. Obviously, mainly those whose roots had developed better by that time. Millennia passed, and plants gradually settled in the coastal strip of land, giving rise to the terrestrial plant world.
The first sushi plants were very small, only about a quarter of a meter high, and had a poorly developed root system. They were called "psilophytes", that is, "naked" or "bald", since they did not have leaves. From psilophytes arose horsetail, clubmoss and fern-like plants.
Research by Soviet scientists A. N. Krishtofovich and S. N. Naumova established that the settlement of land by plants occurred more than four hundred million years ago.
Following the plants, animals began to move to land - first invertebrates, and then vertebrates. Apparently, the first to get out of the water were annelids(the ancestors of modern earthworms), mollusks, as well as the ancestors of spiders and insects, which already breathed through trachea - a complex system of tubes that penetrate the body. Some invertebrates of that time, such as crustaceans, reached a length of three meters.
Second half of the era ancient life, which began about three hundred and twenty million years ago, includes the Devonian, Carboniferous and Permian periods. It lasted approximately one hundred and thirty-five million years. It was an eventful time in the history of life on Earth. Living creatures that emerged from the water then spread widely over land, giving rise to numerous and diverse terrestrial organisms.
In the middle of the era of ancient life on the border of the Silurian and Devonian periods, our Earth underwent great changes. In several places the earth's crust rose. Significant areas of the seabed were exposed to water, which led to the expansion of land. Ancient mountains were formed in Scandinavia, Greenland, Ireland, North Africa, Siberia. Naturally, all these changes greatly influenced the development of life. Finding themselves far from water, the first land plants adapted to existence on land. Under new conditions, plants could better absorb energy sun rays, photosynthesis and oxygen release in the atmosphere increased. Moss-like psilophytes, and later lycophytes, horsetails and fern-like plants, spreading into the interior of the continents, spread out into dense forests. This was facilitated by the damp and warm, greenhouse-like climate of continuous summer. The ancient forests were majestic and gloomy. Giant tree-like horsetails and mosses, reaching thirty meters in height, stood close to each other. The undergrowth consisted of small horsetails, ferns and the ancestors of conifers that arose from them - gymnosperms. From the accumulations of the remains of ancient vegetation in the layers of the earth's crust, powerful deposits were subsequently formed coal, for example in the Donbass, the Moscow region, the Urals and other places. It is not for nothing that one of the periods of this time is called Carboniferous.
Representatives of the animal world developed no less intensively at this time. The changing conditions led primarily to the fact that some ancient invertebrates began to die out. Archaeocyaths disappeared, trilobites, ancient corals and others almost became extinct. But they were replaced by organisms that were more adapted to the new conditions. New forms of mollusks and echinoderms emerged.
The rapid spread of terrestrial vegetation increased the amount of oxygen in the air, promoting the formation of nutrient-rich soils, especially in forests. It is not surprising that relatively soon life in the forests was already in full swing. Various centipedes and their descendants appeared there - ancient insects: cockroaches, grasshoppers. Then the first flying animals appeared. These were mayflies and dragonflies. By flying, they could see food better and approach it faster. Some dragonflies of that time were different large sizes. Their wingspan reached seventy-five centimeters.
How did life at sea develop at this time?
Already in the Devonian period, fish became widespread and changed greatly. Some of them developed bones in their skin and formed a shell. Naturally, such “armored” fish could not swim quickly and therefore mostly lay on the bottom of bays and lagoons. Due to their sedentary lifestyle, they were unable to develop further. The shallowing of the reservoir led to mass death armored fish, and they soon became extinct.
A different fate awaited other fish that lived in those days - the so-called lungfishes and lobe-finned fish. They had short fleshy fins - two pectoral and two abdominal. With the help of these fins, they swam and could also crawl along the bottom of reservoirs. But the main difference between such fish is their ability to exist out of water, since their thick skin retained moisture. These adaptations of lungfish and lobe-finned fish allowed them to live in bodies of water that periodically became very shallow and even dried out.
Ichthyostega - the oldest terrestrial vertebrate
It is interesting to note that lungfish still exist today. They live in the rivers of Australia, Africa and South America that dry up in summer. More recently, lobe-finned fish were caught in the Indian Ocean off the coast of Africa.
How did these fish breathe out of water? In the hot summer, their gills were tightly covered with gill covers and a swim bladder with highly branched blood vessels was used for breathing.
In those places where reservoirs became shallow and dried up especially often, fish’s adaptations to life outside of water became more and more improved. The paired fins turned into paws, the gills with which the fish breathed in the water became smaller, and the swim bladder became more complex, grew and gradually turned into lungs with which one could breathe on land; The sense organs necessary for life on land also developed. This is how fish transformed into amphibious vertebrates. At the same time, the fins of lobe-finned fish also changed. They became more and more comfortable for crawling and gradually turned into paws.
Recently, paleontologists managed to discover very interesting fossils. These new discoveries help shed light on the earliest stages of the transformation of fish into land animals. In the sedimentary rocks of Greenland, scientists have found the remains of four-legged animals, the so-called Ichthyostega. Their short, five-fingered paws looked more like fins or flippers, and their body was covered with small scales. Finally, the skull and vertebral column of Ichthyostegus are very similar to the skull and vertebral column of lobe-finned fishes. There is no doubt that ichthyostegas originated from lobe-finned fish.
This, in brief, is the history of the origin of the first four-legged animals that breathed with lungs, the history of a process that lasted millions of years and ended about three hundred million years ago.
The first four-legged vertebrates were amphibians and were called stegocephalians. Although they left the water, they could not spread overland into the interior of the continents, since they continued to spawn in the water. The juveniles developed there, where they obtained food for themselves, hunting for fish and various aquatic animals. In their lifestyle, they were similar to their close descendants - the modern newts and frogs familiar to us. Stegocephalians were very diverse, ranging from a few centimeters to several meters in length. Stegocephals became especially widespread during the Carboniferous period, whose warm and humid climate favored their development.
The end of the Carboniferous period was marked by new strong geological changes in the earth's crust. At that time, the rise of the land began again, and the mountains of the Urals, Altai, and Tien Shan appeared. The redistribution of land and sea changed the climate. And it is quite natural that in the subsequent, so-called Permian period, huge swampy forests disappeared, ancient amphibians began to die out, and at the same time new plants and animals appeared, adapted to a cooler and drier climate.
Here, first of all, it is necessary to note the development of coniferous plants, as well as reptiles, which originated from some groups of ancient amphibians. Reptiles, which include living crocodiles, turtles, lizards and snakes, differ from amphibians in that they do not spawn in water, but lay their eggs on land. Their scaly or horny skin protects the body well from moisture loss. These and other features of reptiles helped them quickly spread on land at the end of the Paleozoic era.
The found remains of small animals with characteristics of both amphibians and reptiles helped to present a picture of the origin of reptiles. These are Seymuria discovered in North America, Lantnosuchus and Cotlassia in our country. For a long time there was a debate in science: which class should these animals belong to? Soviet paleontologist Professor I.A. Efremov managed to prove that they are all representatives of an intermediate group of animals that seem to stand between amphibians and reptiles. Efremov called them batrachosaurs, that is, frog-lizards.
Many remains of ancient reptiles have been found in our country. The richest collection of them - one of the best in the world - was collected on the Northern Dvina by the Russian paleontologist Vladimir Prokhorovich Amalitsky.
At the end of the Permian period, that is, about two hundred million years ago, there was another big river. In the sands, silts and clays that it deposited, the skeletons of amphibians, reptiles, and the remains of ferns were buried. Many years of research by our scientist made it possible to quite completely restore ancient look the region where the Northern Dvina now flows.
We see the shore big river, densely overgrown with horsetails, coniferous plants, ferns. Various reptiles live along the banks. Among them are large, up to three meters in length, hippopotamus-like pareiasaurs that fed on plant foods. Their massive body is covered with bony scutes, and their short paws have blunt claws. A little further from the river live predatory reptiles. Attracting attention are the large animal-like Inostrantsevia, named after the Russian geologist A. A. Inostrantsev. They have a long, narrow body and dagger-shaped teeth protruding from their mouths. Long paws armed with sharp claws. But here are small reptiles similar to foreigners. They already have features inherent in animals or mammals. The molars became multitubercular; These teeth are comfortable to chew on. The paws have become very similar to the paws of modern animals. It was not for nothing that these animals were called beast-like reptiles; it was from them that animals later evolved. There is no fantasy in the picture painted here. For a paleontologist, this is the same reality as the fact that spruce and pine trees now grow in the Northern Dvina basin, squirrels and bears, wolves and foxes live.
So, during the era of ancient life, plants and animals finally spread over the entire surface of the land, adapting to the most different conditions existence. Then the era begins average life- Mesozoic - the era of further development of living nature on our planet.
