§ 33. Emergence of amphibians on land The most probable biotope of transition from water to land for loached feathers was coastal water-air labyrinths (Fig. II-32; II-33). They contained both sea water and fresh water flowing from the shore, numerous

Exiting the state of hibernation With the onset of spring, which is associated with warming and an increase in the length of daylight hours, hibernating mammals come out of the state of stupor, i.e., “wake up.” Obviously, an increase in body temperature upon awakening

12.3. Way out of the crisis - transition to the noosphere The central theme of the doctrine of the noosphere is the unity of the biosphere and humanity. V. I. Vernadsky in his works reveals the roots of this unity, the importance of the organization of the biosphere in the development of mankind. This allows you to understand

About 385 million years ago, conditions favorable for the mass development of land by animals formed on Earth. Favorable factors were, in particular, a warm and humid climate, the presence of a sufficient food base (formed abundant fauna of terrestrial invertebrates). In addition, at that time, a large amount of organic matter was washed into the reservoirs, as a result of the oxidation of which the oxygen content in the water decreased. This contributed to the appearance in fish of adaptations for breathing atmospheric air.

Evolution

The rudiments of these adaptations can be found among various groups of fish. Some modern fish are able to leave the water for one time or another, and their blood is partially oxidized due to atmospheric oxygen. Such, for example, is a creeper fish ( Anabas), which, leaving the water, even climbs trees. Some representatives of the goby family crawl out onto land - mud jumpers ( Periophthalmus). The latter catch their prey more often on land than in water. The ability to stay out of the water of some lungfish 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 terrestrial life developed independently and in parallel in several lines of evolution of lobe-finned fishes. 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. Shmalgauzen, E. I. Vorobyova) criticized Yarvik's arguments. Most researchers consider the monophyletic origin of tetrapods from osteolepiform brushopterans to be more likely, although this allows the possibility of paraphilia, that is, the achievement of the level of organization of amphibians by several closely related phyletic lines of osteolepiform fish that evolved in parallel. Parallel lines are most likely extinct.

One of the most "advanced" lobe-finned fish was Tiktaalik, which had a number of transitional features that bring it closer to amphibians. These features include a shortened skull, separated from the girdle of the forelimbs and a relatively mobile head, the presence of elbow and shoulder joints. The Tiktaalik's fin could have taken 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" muzzle. 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 loach-finned fish Panderichthys (Panderichthys).

The first amphibians that appeared in fresh water at the end of the Devonian are ichthyostegidae (Ichthyostegidae). They were true transitional forms between lobe-finned fish and amphibians. So, they had the rudiments of the gill cover, a real fish tail, and the kleytrum was preserved. The skin was covered with small fish scales. However, along with this, they had paired five-fingered limbs of terrestrial vertebrates (see the diagram of the limbs of lobe-finned and ancient amphibians). Ichthyostegids lived not only in water, but also on land. It can be assumed that they not only multiplied, but also fed in the water, systematically crawling out onto land.

Later, in the Carboniferous period, a number of branches arose, which are given the taxonomic significance of superorders or orders. The superorder of labyrinthodonts (Labyrinthodontia) was very diverse. The early forms were relatively small and had a fish-like body. Later ones reached very large sizes (1 m or more) in length, their body was flattened and ended in 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 Permian deposits.

In the Carboniferous, a second branch of primary amphibians arose - lepospondyls (Lepospondyli). They were small and well adapted to life in the water. Some of them lost limbs a 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 referred to as stegocephals (shell-headed) for a solid shell of skin bones that covered the cranium from above and from the sides. The ancestors of the stegocephalians were probably bony fish, which combined primitive organizational features (for example, weak ossification of the primary skeleton) with the presence of additional respiratory organs in the form of lung sacs.

The lobe-finned fishes are closest to stegocephals. They possessed pulmonary breathing, their limbs had a skeleton similar to that of stegocephalians. The proximal section consisted of one bone, corresponding to the shoulder or thigh, the next segment consisted of two bones, corresponding to the forearm or lower leg; then there was a section consisting of several rows of bones, it corresponded to the hand or foot. Also noteworthy is a clear similarity in the arrangement of the integumentary bones of the skull in ancient lobe-finned and stegocephalians.

The Devonian period, in which stegocephalians arose, was apparently characterized by seasonal droughts, during which life in many fresh water bodies was difficult for fish. The depletion of water with oxygen and the difficulty of swimming in it was facilitated by abundant vegetation that grew in the Carboniferous time in swamps and on the banks of reservoirs. Plants fell into the water. Under these conditions, adaptations of fish to additional breathing with lung sacs could arise. In itself, the depletion of water with oxygen was not yet a prerequisite for landfall. Under these conditions, lobe-finned fish could rise to the surface and swallow air. But with a strong drying up of reservoirs, life for fish became already impossible. Unable to move on land, they perished. Only those of the aquatic vertebrates, which, along with the ability to pulmonary respiration, acquired limbs capable of providing movement on land, could survive these conditions. They crawled out onto land and crossed into neighboring reservoirs, where water was still preserved.

At the same time, movement on land for animals covered with a thick layer of heavy bone scales was difficult, and the bony scaly shell on the body did not provide the possibility of skin respiration, which is so characteristic of all amphibians. These circumstances, apparently, were a prerequisite for the reduction of the bone armor on most of the body. In separate groups of ancient amphibians, it was preserved (not counting the shell of the skull) only on the belly.

Stegocephalians survived until the beginning of the Mesozoic. Modern detachments of amphibians are formed only at the end of the Mesozoic.

Notes

Wikimedia Foundation. 2010 .

Chapter 8 Appearance of soils and soil formers. Higher plants and their environmental role. Tetrapodization of lobe-finned fish

Until very recently, a person took out from the school textbook of biology and popular books on the theory of evolution such a picture of the event, usually referred to as the "Exit of life to land." At the beginning of the Devonian period (or at the end of the Silurian) on the shores of the seas (more precisely, sea lagoons), thickets of the first terrestrial plants appeared - psilophytes (Figure 29, a), the position of which in the plant kingdom system remains not entirely clear. Vegetation made possible the appearance on land of invertebrates - centipedes, arachnids and insects; invertebrates, in turn, created a food base for terrestrial vertebrates - the first amphibians (originating 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 boundless expanses of absolutely lifeless primary deserts.

So, according to modern ideas, in the aforementioned picture, almost everything 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 details (we will talk about them in our turn). Another thing is more important: most likely, the wording itself is fundamentally wrong - "The exit of living organisms to land." There are good reasons to believe that land landscapes of modern appearance did not exist at that time, and living organisms did not just come to land, but in a sense created it as such. However, let's go 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 to 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 failures. A typical example: starting from the middle of the Silurian period, scorpions appear in the paleontological chronicle - animals at the present time seem to be purely terrestrial. However, it has now been firmly established that Paleozoic scorpions breathed through gills and led an aquatic (or at least amphibious) lifestyle; the terrestrial representatives of the order, in which the gills are transformed into book-lungs characteristic of arachnids, appeared only at the beginning of the Mesozoic. Consequently, the finds of scorpions in the Silurian deposits do not prove anything by themselves (in terms of interest to us).

It seems more productive here, as it seems, to track the appearance in the chronicle not of terrestrial (at present) groups of animals and plants, but of certain anatomical signs of "land". So, for example, a plant cuticle with stomata and the remains of conductive tissues - tracheids, must certainly belong to terrestrial plants: under water, as you might guess, both stomata and conductive vessels are useless ... However, there is another - truly wonderful! - an integral indicator of the existence of terrestrial life at a given time. 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 (paleosoils) are clearly distinguishable in structure from any type of bottom sediments.

It should be noted that the soil is not preserved in a fossil state very often; only in recent decades have they stopped looking at paleosols as some kind of exotic curiosity and have begun to study them systematically. As a result, a real revolution took place in the study of ancient weathering crusts (and soil is nothing but a biogenic weathering crust), which literally turned the previous ideas about life on land upside down. The most ancient paleosols were found in the deep Precambrian - in the early Proterozoic; in one of them, which is 2.4 billion years old, S. Campbell (1985) found undoubted traces of the life of photosynthetic organisms - carbon with a shifted isotope ratio of 12 C / 13 C. In this connection, we can also mention the recently discovered remains of cyanobacterial structures in the Proterozoic karst cavities: karst processes - the formation of depressions and caves in water-soluble sedimentary rocks (limestone, gypsum) - can only take place on land.

Another fundamental discovery in this area should be considered the discovery by G. Retallak (1985) in the Ordovician paleosols of vertical burrows dug by some rather 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 (petrified excrement) of some soil-dwelling animals were found; Apparently, they were fed by fungal hyphae, which make up a significant proportion of the substance of coprolites (however, it is possible that fungi could also develop a second time on organic matter contained in coprolites).

So, by now, two facts can be considered established quite firmly:

1. Life appeared on land a very long time ago, in the average Precambrian. It was represented, apparently, by various variants of algal crusts (including amphibious mats) and, possibly, lichens; all of them could carry out the processes of archaic soil formation.

2. Animals (invertebrates) have existed on land since at least the Ordovician, i.e. long before the emergence of higher vegetation (whose reliable traces remain unknown until the Late Silurian). The aforementioned algal crusts 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 brings to mind one old discussion - about two possible ways of settling the land by invertebrates. The fact is that non-marine fossils of this age were very rare, and all hypotheses on this subject seemed to be 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 with algal emissions and other shelters; others insisted that freshwater reservoirs 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. Gilyarov (1947), who, based on a comparative analysis of the adaptations of modern soil-dwelling animals, proved that it was the soil that should have served as the primary habitat of the earliest inhabitants of the land. At the same time, it should be taken into account that the soil fauna is really extremely poorly included in the paleontological record and the absence of fossil "documents" here is quite understandable. These constructions, however, had one really vulnerable point: where did this soil itself come from, if there was no terrestrial vegetation in those days? After all, everyone knows that soil formation occurs with the participation of 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 only lower plants - Gilyarov's concept gained a "second wind" and was recently directly confirmed by Retallak's data on Ordovician paleosols.

On the other hand, the undoubted freshwater fauna (which contain, among other things, footprints on the surface of the sediment) appear much later - in the Devonian. They include scorpions, small (about the size of a palm) crustacean scorpions, fish and the first non-marine mollusks; among the mollusks there are also bivalves - long-living organisms that are unable to endure the deaths and drying up of water bodies. Faunas with such indisputably soil animals as trigonotarbs ("armored spiders") and herbivorous bipedal centipedes already exist in the Silurian (Ludlovian age). And since aquatic fauna always ends up in burials an order of magnitude better than terrestrial ones, all this allows us to draw one more conclusion:

3. Soil fauna appeared much earlier than freshwater. That is - at least for animals, fresh waters could not play the role of a "bridgehead" 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 created it as such? A.G. Ponomarenko (1993) believes that it is actually difficult to call all the communities discussed above with certainty “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 water bodies, both flowing and stagnant, seems to be very problematic before the vascular vegetation in the Devonian somewhat reduced the rate of erosion and stabilized the coastline." The main events were to take place in the already familiar flattened coastal amphibiotic landscapes without a stable coastline - "not land, not sea" (see Chapter 5).

No less unusual (from the point of view of today) the situation should have developed on the watersheds occupied by "primary deserts". Today, 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 the more deserted (in appearance) the more precipitation fell: the water actively eroded the mountain slopes, cutting through deep canyons, when it entered the plain it gave conglomerates, and further along the plain spread psephites scattered over the surface, which called plain proluvium; now such deposits compose only the alluvial fans of temporary streams.

This picture allows you to take a fresh look at one strange circumstance. Almost all known Silurian-Devonian terrestrial floras and faunas were found at various points of the ancient Continent of red sandstone (Old Red Sandstone), so named for its characteristic rocks - red flowers; all locations are associated with deposits that are considered deltaic. In other words, it turns out that this entire continent (which united Europe and the east of 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 catchment areas for them on the continent of this size! It remains to be assumed that all these "deltaic" deposits, most likely, 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 (Tracheophyta) ... However, in fact, the appearance of vascular plants - one of the key events in the history of the biosphere, because in its environmental role this group of living organisms has no equal, at least among eukaryotes. It was vascular vegetation that made, as we shall see below, a decisive contribution to the formation of modern terrestrial landscapes.

The generally accepted point of view is that some algae that lived near the coast, first "poked their heads into the air", then populated the tidal zone, and then, gradually turning into higher plants, completely came ashore. This was followed by a gradual conquest of land by them. Most botanists consider the ancestors of higher plants to be one of the groups of green algae - Charophyta; they now form continuous thickets at the bottom of continental water bodies - both fresh and salty, while only a few species have been found in the sea (and even then only in desalinated bays). 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 building 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 put forward against this point of view. If the process of transformation of algae into higher plants really took place in coastal waters (where conditions are most favorable for entering the fossil record), then why do we not see any of its intermediate stages? Moreover, the characeae themselves appear in the late Silurian - simultaneously with vascular plants, and the biology of this group does not give grounds to assume a long period of "hidden existence" for it ... Therefore, a paradoxical, at first glance, hypothesis appeared: why , Strictly speaking, the appearance of macroremains of higher plants at the end of the Silurian should be unequivocally interpreted as traces of their emergence on land? Perhaps, quite the contrary - these are traces of the migration of higher plants into the water? In any case, many paleobotanists (S.V. Meyen, G. Stebbins, G. Hill) actively supported the hypothesis of the origin of higher plants not from aquatic macrophytes (such as characeae), but from terrestrial green algae. It is these terrestrial (and therefore having no real chance of getting into burials) "primary higher plants" that could have belonged to mysterious spores with a three-beam slit, which are very numerous in the Early Silurian and even in the Late Ordovician (starting from the Caradocian Age).

However, recently it turned out that, apparently, the supporters of both points of view are right - each in his own way. The fact is that some of the microscopic terrestrial green algae have the same complex of fine cytological features as the char and vascular ones (see above); these microalgae are now incorporated into 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 originated in the Silurian: "real" characeae, which inhabited continental water bodies, and higher plants that began to colonize the land, and only after some time (in full according to Meyen's scheme) that appeared in coastal habitats.

From the course of botany, you should be aware that higher plants (Embryophyta) are divided into vascular (Tracheophyta) and bryophytes (Bryophyta) - mosses and liverworts. Many botanists (for example, J. Richardson, 1992) believe that it is the liverworts (based on their modern life strategies) that 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 algae Nostoc is able to live inside the tissues of some liverworts and anthocerotes, providing their hosts with nitrogen; this must have been very important for the first inhabitants of primitive soils, where this element could not but be in severe deficit. The spores from the Late Ordovician and Early Silurian deposits mentioned above are most similar to the spores of liverworts (reliable macroremains of these plants appear later, in the Early Devonian).

However, in any case, bryophytes (even if they actually appeared in the Ordovician) hardly changed the appearance of continental landscapes. The very first vascular plants - rhinophytes - appeared in the late Silurian (Ludl age); right up to the early Devonian (Zhedinian) they were represented by extremely monotonous remains of a single genus Cooksonia, the simplest and most archaic of the vascular. But in the deposits of the next Devonian (Siegen) age, we already find a variety of rhinophytes (Figure 30). Since that time, two evolutionary lines have been distinguished among them. One of them will go from the genus Zosterophylum to the lycopods (they also include tree-like lepidodendrons - one of the main coal-formers in the next, carbonic, period). The second line (the genus Psilophyton is usually placed at its base) leads to horsetails, ferns and seeds - gymnosperms and angiosperms (Figure 30). Even the Devonian rhinophytes are still very primitive and, to be honest, it is not clear whether they can be called "higher plants" in the strict sense: they have a vascular bundle (though not composed of tracheids, but special elongated cells with a peculiar relief of the walls), but there are no stomata . Such a combination of features should indicate that these plants have never experienced a shortage of water (we can say that their entire surface is one large open stomata), and, apparently, they were helophytes (that is, they grew "knee-deep in water ", like the current reed).

The appearance of vascular plants with their rigid vertical axes caused a whole cascade of ecosystem innovations that changed the face of the entire biosphere:

1. Photosynthetic structures began to be located in three-dimensional space, and not on a plane (as it was until now - during the period of dominance of algal crusts and lichens). This sharply increased the intensity of the formation of organic matter and, thus, the total productivity of the biosphere.

2. The vertical arrangement of the trunks made the plants more resistant to the introduction of washed-out fine earth (compared, for example, with algal crusts). This reduced the irretrievable loss of non-oxidized carbon (in the form of organics) by the ecosystem - the improvement of the carbon cycle.

3. Vertical trunks of terrestrial plants must be sufficiently rigid (compared to aquatic macrophytes). To ensure this rigidity, a new tissue arose - wood, which, after the death of the plant, decomposes relatively slowly. Thus, the carbon cycle of the ecosystem acquires an additional reserve depot and, accordingly, stabilizes.

4. The appearance of a constantly existing stock of hard-to-decompose organic matter (concentrated mainly in the soil) leads to a radical restructuring of food chains. Since that time, most of the matter and energy turns through detritus, and not through pasture chains (as was the case in aquatic ecosystems).

5. For the decomposition of difficult-to-digest substances that make up wood - cellulose and lignin - new types of destroyers of dead organic matter were required. Since that time, the role of the main destructors on land has shifted from bacteria to fungi.

6. To maintain the trunk in a vertical position (under the action of gravity and winds), a developed root system arose: rhizoids - like in algae and bryophytes - are no longer enough here. 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 (Siegen Age), since from the beginning of the next, Carboniferous, period, almost all types of sediments now deposited on the continents have formed on Earth. In Dosigenian times, however, continental sediments are practically absent, apparently due to their constant secondary erosion as a result of unregulated runoff. At the very beginning of the Carboniferous, coal accumulation begins on the continents - and this indicates that powerful plant filters stood in the way of water flow. Without them, the remains of plants would be continuously mixed with sand and clay, so that clastic rocks enriched with plant remains would be obtained - carbonaceous shales and carbonaceous sandstones, and not real coals.

Thus, a dense “brush” of helophytes (one might call it “rhinophyte reeds”) that has arisen in coastal amphibiotic landscapes begins to act as a filter that regulates mantle runoff: it intensively filters (and precipitates) detrital material carried from 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 inhabited by them, throwing soil ashore. As a result of their many years of "irrigation activity", the swamp turns into a system of clean deep ponds, separated by wide forested "dams". So the vascular vegetation in the Devonian divided the notorious amphibious landscapes into "real land" and "real freshwater reservoirs." It will not be a mistake to say that it was the vascular vegetation that became the true performer of the spell: "Let there be firmament!" - having separated this firmament from the abyss ...

It is with the newly emerged freshwater reservoirs that the appearance in the late Devonian (Famenian age) of the first tetrapods (four-legged) is associated - a group of vertebrates with two pairs of limbs; it combines in its composition amphibians, reptiles, mammals and birds (simply speaking, tetrapods are all vertebrates, except fish and fish-like). It is now generally accepted that tetrapods are descended from lobe-finned fish (Rhipidistia) (Figure 31); this relict group now has the only living representative, the coelacanth. The once 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 appearance of the key feature of tetrapods - two pairs of five-fingered limbs - was considered their unambiguous adaptation to a terrestrial (or at least amphibious) way of life. Nowadays, however, most researchers are inclined to believe that the "problem of the appearance of tetrapods" and "the problem of their landing on land" are two different things and are not even connected 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 the reservoirs (this is especially important when the reservoir has become so shallow that your back is already starting to stick out) and wade through dense thickets of helophytes; the limbs turned out to be especially useful in order to crawl over dry land to another, neighboring one, when the reservoir dries up.

The first, Devonian, tetrapods - primitive amphibian labyrinthodonts (the name comes from their teeth with labyrinth-like folds of enamel - a structure directly inherited from the crossopterans: 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 the one we see in catfish or burbot), lateral line organs and - in some cases - a 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 of the reservoir), then they certainly were not a component of terrestrial ecosystems. Only much later, in the Carboniferous period, did small terrestrial amphibians appear - anthracosaurs, which, apparently, fed on arthropods, but more on that later (see Chapter 10).

Particularly noteworthy is the fact that a number of unrelated parallel groups of stegocephalic lobe-finned fish appear in the Devonian, both before and after the appearance of "true" tetrapods (labyrinthodonts). One of these groups were panderichthids - cross-finned, devoid of dorsal and anal fins, which does not happen 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 features clearly independently. Thus, we have before us a process that can be called the parallel tetrapodization of the crossopterans (it was studied in detail by E.I. Vorobieva). 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"; "Win the competition" in the end, that group of lois-finned animals, which "created" the tetrapods of the modern type known to us. However, along with "real" tetrapods, for a long time there existed a whole range of ecologically similar semi-aquatic animals (such as panderichthids), combining the characteristics of fish and amphibians - if I may say so, the "waste" of the process of tetrapodization of crossopterans.

Notes

Scorpions form a specialized group of sea scorpions already familiar to us (in chapter 7) - eurypterid, whose representatives switched from swimming to walking along the bottom and, having acquired small sizes, first mastered the sea littoral, and then land.

With the discovery of Cambrian marine millipede-like arthropods, their existence on the Early Paleozoic land seems quite probable, although reliable finds of centipedes in continental deposits appear only in the Late Silurian.

It is possible that macroscopic plants also existed on land already in the Vendian. At this time on thalli some algae ( Kanilovia) there are mysterious complex microstructures in the form of a zigzag torn along a spiral chitinoid ribbon. 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; to the conversion of nitrogen from atmospheric N2 gas into an usable form (eg NO3– ions). This is an additional argument in favor of the fact that by the time higher plants appeared on land, prokaryotic communities had already existed there for a long time, which enriched the soil with nitrogen in an accessible form.

More common name psilophytes- now do not use for nomenclature reasons. In the literature of recent years, you may come across another name - propteridophytes.

Representatives of almost all major divisions of higher plants appeared, not only spore(lycosform, fern, horsetail), but also gymnosperms ( ginkgo).

The truly romantic story of the discovery of this "living fossil", described in the wonderful book by J. Smith "Old Quadruped", is widely known. It should, however, be noted that the way of life of the coelacanth has nothing to do with what the Devonian ripidistia led: 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 “land creature” that is capable of crawling over dewy grass from one reservoir to another at night, covering a distance of several hundred meters!

Origin of amphibians

It is believed that the prerequisites for the emergence and formation of amphibians about 385 million years ago (in the middle of the Devonian period) were favorable climatic conditions (heat and humidity), as well as the availability of sufficient food in the form of already formed numerous small invertebrates.

And, in addition, during that period, a large amount of organic residues was washed 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 in ancient fish and their adaptation to breathing atmospheric air.

Ichthyostega

Thus, the origin of amphibians, i.e. the transition of aquatic vertebrates to a terrestrial way of life was accompanied by the appearance of respiratory organs adapted to absorb atmospheric air, as well as organs that facilitate movement on a solid surface. Those. the gill apparatus was replaced by lungs, and the fins were replaced by five-fingered stable limbs that serve as a support for the body on land.

At the same time, there was a change in other organs, as well as their systems: the circulatory system, the nervous system and the 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 circles of blood circulation, 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 crossopterans

Until now, there has been controversy in the scientific world about the origin of amphibians. Some believe that amphibians originated from two groups of ancient lobe-finned fish - Porolepiformes and Osteolepiformes, most others argue in favor of osteolepiform lobe-finned fish, but do not exclude the possibility that several closely related phyletic lineages of osteolepiform fish could develop and evolve in parallel.

Shell-headed amphibians - stegocephals

These same scientists suggest that the parallel lineages later died out. One of the specially evolved, i.e. mutated species of ancient lobe-finned fish was Tiktaalik, which acquired a number of transitional characters that made it an intermediate species between fish and amphibians.

I would like to list these features: a movable, shortened head separated from the front limbs, resembling a crocodile, shoulder and elbow joints, a modified fin that allowed it to rise above the ground and occupy various fixed positions, it is possible that walking in shallow water. Tiktaalik breathed through the nostrils, and the air into the lungs, perhaps, was pumped not by the gill apparatus, but by the buccal pumps. Some of these evolutionary changes are also characteristic of the ancient lobe-finned fish Panderichthys.

ancient crossopterans

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. 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.

From the gill covers they have only rudiments left, however, 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 out onto land only periodically.

Ichthyostega

Discussing the origin of amphibians, one cannot 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 superorder Labyrinthodonts was very diverse and existed until the end of the Triassic period.

In the Carboniferous period, a new branch of early amphibians, the Lepospondyli (lat. Lepospondyli), was formed. These ancient amphibians were adapted to life exclusively in water and existed until about the middle of the Permian period, giving rise to modern amphibian orders - Legless and Tailed.

I would like to note that all amphibians, called stegocephals (shell-headed), which appeared in the Paleozoic, died out 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 most of all the armored-headed fish are close to the lobe-finned fish, since they had pulmonary respiration and a skeleton resembling the skeletons of stegocephals (shell-headed).

In all likelihood, the Devonian period, in which the shell-headed ones formed, was distinguished by seasonal droughts, during which many fish lived “hard times”, since the water was depleted of oxygen, and the 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 had to change and turn into lung sacs. At the beginning of the occurrence of breathing problems, ancient lobe-finned fish simply had to rise to the surface of the water to receive the next portion of oxygen, and later, in the conditions of drying up of reservoirs, 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 able to move on land, were able to survive these extreme conditions, and eventually turn 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. I must say that the armor-headed (stegocephals) managed to survive only until the beginning of the Mesozoic era. All modern, i.e. The present 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 again, immersed in reading into the wonderful world of wildlife.

And in more detail, with the most interesting representatives of amphibians (amphibians), you will be introduced to these articles:

Now let's return from the Mesozoic to the Paleozoic - to the Devonian to where we left the descendants of the lobe-finned fish, which were the first of the vertebrates to come ashore.

However, you can't forget about it! - this feat, which I described before (traveling over land in search of water), is a very, very approximate simplified diagram of the motives that forced the fish to leave the drying up reservoirs.

It's easy to say: fish got out of the water and began to live on land . Centuries, thousands of thousands of years passed irrevocably, until the restless descendants of the lobe-finned fish slowly but surely, dying out and surviving in whole clans, adapted to everything that the land met them with, inhospitable as an alien world: sand, dust, stones. And emaciated psilophytes, primeval grasses, hesitantly surrounding damp hollows in some places.

So, shortening the tedious time spent by the ancestors of amphibians to conquer a new element, let's just say: they got out of the water and looked around. What did they see?

There is something, one might say, and nothing. Only near the shores of the seas and large lakes in rotting plants, thrown out by waves on land, crustaceans and worms swarm, and near the edge of fresh water - primitive wood lice and centipedes. Here and at a distance, along the sandy lowlands, various spiders and scorpions crawl. The first wingless insects also lived on land by the end of the Devonian. A little later, winged ones appeared.

It was scarce, but it was possible to feed on the shore.

Landing of half-fish, half-amphibians - ichthyostegs (the first stegocephalians ) - was accompanied by many radical changes in their body, which we will not delve into: this is too specific a question.

To breathe fully on land, you need lungs. They were in lobe-finned fish. In stagnant lakes and marshes, full of decaying plants and depleted of oxygen, the lobe-feathers floated to the surface and swallowed air. Otherwise, they would have suffocated: in musty water, gills alone are not enough to saturate the body with oxygen necessary for life.

But here's the thing: as calculations showed, lobe-finned fish could not breathe with their lungs on land!

“In the resting position, when the animal is lying on the ground, the pressure of the entire body weight is transferred to the belly and floor of the oral cavity. In this position of the fish lung breathing is impossible. Sucking air into the mouth is possible only with difficulty. Suction and even forcing air into the lungs required great effort and could only be carried out by raising the front part of the body (with the lungs) on the forelimbs. In this case, pressure on the abdominal cavity stops, and air can be distilled from the oral cavity into the lungs under the action of the hyoid and intermaxillary muscles ”(Academician I. Schmalhausen).

And the limbs of the lobe-finned fish, although they were strong, however, in order to support the front part of the body for a long time, were not suitable. Indeed, on the shore, the pressure on the fins-paws is a thousand times greater than in the water, when the lobe-finned fish crawled along the bottom of the reservoir.

There is only one way out: skin breathing. The assimilation of oxygen by the entire surface of the body, as well as by the mucous lining of the mouth and pharynx. Obviously, it was the main one. Fish crawled out of the water, at least only half. Gas exchange - the consumption of oxygen and the release of carbon dioxide - went through the skin.

But here at ichthyostegov, the closest evolutionary descendants of the lobe-finned fishes, the paws were already real and so powerful that they could support the body above the ground for a long time. Ichthyostegs are called "four-legged" fish . They were inhabitants of two elements at once - water and air. In the first, they bred and mostly fed.

Amazing mosaic creatures ichthyostegi. They have a lot of fish and frogs. They look like scaly fish with legs! True, without fins and with a single-bladed tail. Some researchers consider ichthyostegi to be a barren side branch of the amphibian family tree. Others, on the contrary, chose these "four-legged" fish as the ancestors of stegocephals, and, consequently, of all amphibians.

Stegocephalians (shell-headed ) were huge, similar to crocodiles (one skull is more than a meter long!) And small: ten centimeters the whole body. The head from above and from the sides was covered with a solid shell of skin bones. It has only five openings: in front - two nasal, behind them - eye, and on the crown of the head one more - for the third, parietal, or parietal, eye. It apparently functioned in Devonian armored fish, as well as in Permian amphibians and reptiles. Then it atrophied and in modern mammals and humans turned into the pineal gland, or pineal gland, the purpose of which is not yet fully understood.

The back of the stegocephalians was bare, and the belly was protected by not very strong armor made of scales. Probably so that, while crawling on the ground, they would not injure their belly.

One of stegocephalians, labyrinthodonts (labyrinth-toothed: the enamel of their teeth was intricately folded), gave rise to modern tailless amphibians. Others, the lepospondyls (thin vertebrates), produced caudate and legless amphibians.

Stegocephalians lived on Earth "a little" - about a hundred million years - and in the Permian period they began to quickly die out. Almost all of them died for some reason. Only a few labyrinthodonts passed from the Paleozoic to the Mesozoic (namely, the Triassic). Soon they came to an end.