Paleozoic Devonian. Devonian period of the Paleozoic era

Middle Devonian landscape

Devonian(Devonian)- fourth geological period since the beginning of the Paleozoic era. It began about 416 million years ago and ended 360 million years ago. Duration - 50 million years. The Devonian system as a stratigraphic unit is subdivided into 3 superdivisions, 3 divisions and 7 stages.

Tectonics and magmatism

The Devonian period, in contrast to other periods of the Paleozoic, is characterized by a relatively small scale of major structural transformations of the earth's crust.

The beginning of the period is characterized by the completion of the Caledonian tectogenesis; in a number of areas the formation of mountain-fold structures is coming to an end. However, the stabilized Caledonian structures do not pass to the platform stage of development, but so-called superimposed depressions or inherited troughs form within the Caledonian structures. With the completion of the Caledonian era of tectogenesis, a new era of tectogenesis begins to develop - Hercynian. The vast majority of the Hercynian geosynclinal troughs, like the Caledonian ones, arose on the Baikal folded basement. The Hercynian tectogenesis embraced all geosynclinal belts known since the beginning of the Paleozoic.

In the early Devonian era, ancient platforms were almost everywhere elevated above sea level. The continental regime is always established after the end of the tectonic stage of development, in this case Caledonian.
In the Middle Devonian, a new transgression set in, which manifested itself most strongly on the East European Platform.

On other platforms, the Middle-Late Devonian transgression of the sea either manifested itself in relatively small areas, or was absent altogether. At the end of the Devonian period, the rise of the platforms again occurred and, as a result, some regression of the sea. In the section of Devonian deposits, on platforms and in depressions, saline and variegated terrigenous strata are widespread, indicating arid conditions.

Animal and plant world

During the Devonian period, lycopsform, horsetail, fern and gymnosperms originated from rhinophytes, many of them were represented by woody forms (eg Archeopteris). The first terrestrial vertebrates appeared. Paleontologists suggest that the lungs that terrestrial creatures breathe originally arose from fish that lived in swamps. Amphibians arose from such lobe-finned fish. One of the first amphibians - ichthyostegi and acanthosteg, had many features of fish, but had well-formed limbs. They were closely related to water, perhaps even more closely than modern frogs. Spiders, ticks, insects appeared - living organisms took on new forms and mastered the land. Bottom predators of racoscorpions - eurypteroids reach 1.5 - 2 m in length in the Devonian. In the seas in the Devonian period, the first ammonites appeared, which were to experience their heyday in the Mesozoic. The Devonian is often called the "age of fish", because it was during this period of geological time that jawless and maxillary animals inhabit almost all marine and freshwater basins and achieve great diversity.

Devonian deposits were first described in the English county of Devonshire. The Devonian period is subdivided into three divisions: lower, middle and upper. In the Devonian, the northern continents formed a single large continent, Atlantia, to the east of which was Asia. Gondwana continues to exist. Huge continents were blocked by mountain ranges, which, collapsing, filled the hollows between the mountains with fragments. The climate became dry and hot. Lakes and lagoons dried up, and the salts and gypsum that were part of their waters precipitated, forming saline and gypsum-bearing strata. Volcanic activity is intensifying.

In the middle Devonian, the sea again advances on land. There are numerous depressions. They are gradually flooded by the sea. The climate becomes warm and humid. In the Upper Devonian, the seas become shallow again, small mountains appear, which were later almost completely destroyed. The most characteristic deposits of the Devonian period are continental red sandstones, shales, gypsum, salt, limestones.

Physical and geographical conditions have changed significantly, leading to a change in flora and fauna.

Numerous algae lived in the waters of the Devonian seas and oceans: siphon, blue-green, red, in the lagoons - char.

The psilophytes that appeared in the Silurian period in the early Devonian already had a more complex organization. Their body was quite clearly divided into root, stem and branches. Primitive ferns originated from them in the Middle Devonian. Psilophytes already had a woody stem. The branches of these plants begin to perform various functions, and their end parts gradually turn into dissected leaves, with the help of which photosynthesis is carried out. Other descendants of psilophytes also grow:
lycopsform and arthropods, with a more complex organization than psilophytes. They gradually displace their ancestors, seizing their places and settling in wet areas, in shallow lagoons and swamps. In the Upper Devonian, psilophytes disappear. The first seed ferns, cordaites and true ferns appear.

Spore psilophytes, primitive ferns, club mosses, and arthropods grew in moist and swampy places, forming dense thickets. They reached 30 m in height and one meter in thickness. Plants reproduced by spores that germinated into sprouts only in a humid environment.

The first seed plants had seed primordia at the tops of specialized leaves lying openly on the leaves. Therefore, plants are called gymnosperms. They were already real trees with real leaves and reproductive organs in the form of cones. Gymnosperms could reproduce directly on land, since for seed germination water environment need not. In addition, seeds are a multicellular organ with a significant amount of reserve nutrients that provide the embryo with everything necessary at the beginning of its life, and the seed coat protects it well from adverse conditions. All this made it possible for gymnosperms to spread widely on land. And although spore plants continued to exist, gymnosperms gradually occupy a dominant position among plants.

The dry, hot climate on the continents has led to the drying up of many rivers, lakes, marshes, lagoons, and shallow inland seas. Of the aquatic animals, only those survived that, in addition to the gills that allowed them to live in the water, also possessed lungs. When the reservoirs dried up, they could breathe atmospheric air. These include primarily lungfish, which had horny teeth and sharp ribs. In 1870, living specimens of lungfish were found in two small rivers in Australia, the structure of which strongly resembled their fossil ancestors. Later in Africa and South America live lungfish have also been found. In addition to them, in the drying reservoirs of the Devonian period, lobe-finned fish were found. With the help of fins resembling a brush, the lobe-finned fish were able to crawl. Their swim bladder was enriched with blood vessels and played the role of lungs. Thus, lobe-finned fish could breathe air and crawl from lagoon to lagoon in search of food and water. The lobe-finned skeleton is almost completely ossified. The skull consisted of bones present in the skulls of higher vertebrates. Consequently, the lobe-finned fishes were the ancestors of all terrestrial vertebrates, including amphibians, which appeared in the Upper Devonian. These were already real land animals. They lived on land, although they still had much in common with fish - the shape of the skull, scales, gill covers.

In 1938, in the waters of the Indian Ocean off the southeastern coast of Africa, living fossil lobe-finned fish were found. They are called coelacanthus, or coelacanths. Coelacanthus live at considerable depths. They are predators. Of particular interest are fossilized paw prints found in Pennsylvania. Three of the five fingers had claws. The trace of the tail, which stretched behind the body of the animal, is clearly visible. Probably, this footprint belongs to a lobe-finned fish moving in search of water bodies along the Devonian land.

Devonian - Devonian period or Devonian system. Devon is the fourth period of the era. It began 419 million years ago and ended 358 million years ago, that is, it continued for 60-61 million years. In order not to get confused in eons, eras and periods, use the geochronological scale, which is located as a visual clue.

The name of the period was given by the name of the English county of Devonshire, on the territory of which geological rocks of this period were found. Sixty million years of the Devonian had a serious impact on the development of life on planet Earth. The Devonian period is known for events precisely from the animal and flora. During this period, many new species, genera and families of flora and fauna appeared. The most striking and most important event of this period can be called the first appearance of animals on the surface of the land.

Devonian period plants

Life in the Devonian period developed new niches and actively populated the land. If in previous periods only rare species plants, and animals continued to live under water, then in the Devonian land became much more inhabited. Also, changes and rapid development took place under water. As for the Devonian fauna, at that time such terrestrial plants appeared that belong to the lycopods, horsetails, ferns and gymnosperms. At the same time, the first trees appeared on land, for example, a tree-like plant with fern-like leaves - Archeopteris (an extinct species). Scientists note that, in contrast to the current situation in the fauna, when in different parts Plants grow on the earth, which differ from each other, in those days all plants on almost the entire planet were the same. As a result of the active settlement of the land by plants, the first soil cover appeared.

Devonian period animal world

In the Devonian period, the very first animals appeared that got to the surface of the land. The lungs, which allowed animals to get out of the water, appeared even in lobe-finned fish that live in swamps. The dual mode of breathing in marsh fish appeared as a result of the fact that they experienced a lack of oxygen in the conditions of isolated swamps and shallow water bodies. lobe-finned fish gave rise to the first amphibians. Judging by the archaeological data, the first amphibians were ichthyostegi and acanthostega (extinct animals), which became intermediate between fish and amphibians. They had much in common with fish, but paws were formed and there was pulmonary breathing. Also, the first insects appeared on land - spiders, ticks and other invertebrate arthropods.

Especially actively developed life under water. No wonder this period is called the age of fish. Among the fish, a huge species diversity has appeared and they have populated almost all water bodies on the planet. At this time, ammonite cephalopods first appear. In the Devonian, the decline of the kingdom of trilobites begins, since at this time many predators appear and trilobites begin to die out.

At the end of the Devonian begins mass extinction animals. Devonian extinction considered one of the largest in history. Archaeologists note that almost all jawless animals disappeared about 359 million years ago. The reasons for the extinction have not been reliably elucidated. The main reasons are the fall of a meteorite, a change environment followed by oxygen deficiency, large-scale evolution of plants, climate change and so on. In total, 19% of families and 50% of genera of flora and fauna died out.

Tectonics. Devon is a period of relative tectonic calm. There were four oceans, three large continents and several small ones. The land is significantly fragmented and almost entirely located in the Western Hemisphere (Fig. 15). In the south of the Western Hemisphere, Gondwana is located, in the north - Euramerica, Siberia and small massifs. The continents approached, the oceans lying between them (Rhea, Ural and Paleotethys) decreased in size. Euramerica included Laurentia, Baltica and Avalonia (the east of the North American Platform), as well as the flooded Armorica (the ancient part of Western Europe). Next to Euramerica lay half-flooded Iberia and Siberia. The relief of Euramerica was dissected: large basins were located between the mountain systems of the Caledonian age. These basins were either flooded by the seas, and then limestones were deposited in them, or dried up, which led to the accumulation of salts.

At the end of the Devonian, in the south of the Western Hemisphere, a monolithic Gondwana rose, uniting the Hindustan, Australian, Antarctic, African and South American platforms. The Panthalassa Ocean stretches in the Eastern Hemisphere, from under whose waters fragments of the submerged Chinese platform rose up in islands.

Rice. 15. Oceans and land of the early Devonian

organic world. Devon is the age of fish. The dimensions of the animals increased, the largest marine predator, the armored fish dinichthys, exceeded 10 m in length. By the end of the Devonian, almost all jawless fish had disappeared, only lampreys and hagfish have survived to this day. Jawed fish were divided into groups: armored, ray-finned, lobe-finned, true sharks, lungfish. Among marine invertebrates, mollusks have retained leading importance: brachiopods and cephalopods. New groups appeared in the composition of cephalopods: ammonoids and nautiloids. Giant reefs were built by corals (rugoses, tabulates) and stromatoporates. The structure of terrestrial plants has changed - strong woody tissues and organs similar to roots have arisen. In the middle of the Devonian, about 375 million years ago, forest communities arose. The first tree-like plants were spore-bearing: club-like, fern-like, horsetail. At the end of the Devonian, the first gymnosperms appeared - cordaites. In the late Devonian, possible predecessors of terrestrial tetrapod vertebrates appeared - panderichthyid fish. At the end of the Late Devonian, the first four-legged animals spread - amphibians with gills and lungs - stegocephals. Arthropods, amphibians, and some gastropods (or gastropods) lived on land. In the late Devonian, a mass extinction occurred that covered more than 50% of animal species.

Carboniferous period

T
ectonics. All land lay in the Western Hemisphere. As a result of the convergence of the continents, the Hercynian folding began. Euramerica came close to Gondwana, and the ocean of Rhea practically closed, becoming one of the bays of Paleo-Tethys. From the north, Siberia and Kazakhstan almost joined Euramerica, which led to the disappearance of the Ural Ocean. Thus, the formation of the grandiose supercontinent Pangea began in the Carboniferous - it has not yet included only the Siberian and Chinese platforms, which were represented by scattered islands (Fig. 16).

Figure 16. Oceans and land during the Late Carboniferous

After the unification of the ancient platforms, in fact, only one ocean remained - Panthalassa. From it, the Paleotethys ocean penetrated into Pangea from the east in a gigantic bay. Blocks of Pangea at the end of the Carboniferous were still separated in many places by long and deep straits and bays. The Gondwana massif once again ended up at the South Pole - the ice sheet of Gondwana began. At the same time, a hot and humid climate persisted within Euramerica - gigantic peat reserves were actively accumulated here, which later turned into coals.

organic world. Forests developed rapidly, and the content of atmospheric oxygen reached a peak in the entire history of the Earth - 35%. Spore and gymnosperms grew in the forests. Tree-like ferns, tree-like club mosses - lepidodendrons and sigillaria, tree-like horsetails - calamites stood out in size and variety. Gymnosperms, cordaites, as well as arboreal and shrubby forms of glossopteridians, were widely distributed. A variety of sharks and bony fish grew in the oceans. The earth was inhabited by giant arthropods. Their marine representatives were eurypterids, in particular, Schouler's gibbertopterus, which reached 3 m in length (the largest arthropod in the history of the Earth). Pulmonoscorpions (up to 70 cm long) and ancient haymaking spiders lived on land. The first flying inhabitants of the planet arose - giant dragonflies, with a wingspan of up to 1 m. Amphibians such as anthracosaurs (their representative is sylvanerpeton) spread on land. Signs of amphibians and reptiles combined Eucrit. Almost a real reptile was the Westlothiana lizsia. Finally, the first true egg-laying reptiles arose - the paleotiris and Lyell's hylonomus. In the late Carboniferous, oviparous tetrapods split into two branches: reptiles and synapsids. Among the leading marine invertebrates, representatives of foraminifers (single-celled protozoa) – fusulinids, were distinguished by a variety of forms, the length of their carbonate shells reached 3 cm. Bivalves and gastropods developed. At the end of the Carboniferous, a cover glaciation occurred.

The geological Devonian period (420 - 358 million years ago) is considered the beginning of the Late Paleozoic. At this time, many biotic events occurred that greatly influenced the further development of life on Earth. The Devonian system was established in 1839 by scientists Adam Sedgwick and Roderick Murchison in the English county of Devonshire, after whom it was named.

Flora and fauna

On the eve of the Devonian there was a mass extinction of the organic world. Many species, formerly widespread on Earth, simply died out and disappeared. In their place, new groups of animal plants arose. It was they who determined what the flora and fauna of the Devonian period looked like.

There has been a real revolution. Now life developed not only in the seas and freshwater reservoirs, but also on land. Terrestrial vertebrates and terrestrial vegetation spread widely. The Devonian period, the flora and fauna of which continued to evolve, was marked by the appearance of the first ammonites. Bryozoans, four-beam corals, and some species of castle brachiopods experienced their heyday.

life at sea

The development of the organic world was influenced not only natural evolution, but also the climate of the Devonian period, as well as intense tectonic movements, cosmic impact and (in general) changes in habitat conditions. Life in the sea has become more diverse compared to the Silurian. The Devonian period is characterized by the predominant development of various fish species (some scientists even call it the "fish period"). At the same time, the extinction of cystoids, nautiloids, trilobites and graptolites began.

The number of genera of hinge brachiopods reached its maximum value. The spiriferids, atripids, rhynchonellids, and terebratulids were especially diverse. Brachiopods were distinguished by the richness of species and rapid variability over time. This group is most important for paleontologists and geologists involved in the detailed subdivision of sediments.

The Devonian period, with a greater variety of animals and plants than in previous epochs, proved to be important for the development of corals. Together with stromatoporoids and bryozoans, they began to take part in the construction of reefs. They were helped by a variety of calcareous algae that inhabited the Devonian seas.

Invertebrates and vertebrates

Among invertebrates, ostracods, crustaceans, tentaculites, blastoids, sea lilies, sea ​​urchins, sponges, and conodonts. According to the remains of the latter, experts today determine the age of sedimentary rocks.

The Devonian period was marked by the increasing importance of vertebrates. As mentioned above, it was the "fish age" - armored, bone and cartilaginous fish occupied a dominant position. From this mass emerged a new group. These were fish-like jawless organisms. Why did these vertebrates flourish? For example, in lamellars and armored fish the front part of the body and the head were covered with a powerful protective shell - a decisive argument in the struggle for survival. These creatures differed in a sedentary way of life. In the middle of the Devonian, not only cartilaginous, but also sharks appeared. They took the dominant position later - in the Mesozoic.

Vegetation

At the turn that separated the Devonian from the Silurian, the emergence of plants on land became more active. Their rapid resettlement and adaptation to a new terrestrial way of life began. The Early and Middle Devonian passed under the predominance of primitive vascular plants, rhinophytes, growing in swampy areas on land. By the end of the period they had become extinct everywhere. In the Middle Devonian, spore plants (arthropods, club mosses, and ferns) already existed.

The first gymnosperms appeared. Shrubs have evolved into trees. Heterosporous ferns spread especially vigorously. Basically, terrestrial vegetation developed in coastal regions, where a warm, mild and humid climate developed. The lands remote from the oceans at that time still existed without any vegetation.

Climate

The Devonian period was distinguished by a more distinct climatic zonation compared to the beginning of the Paleozoic. The East European platform and the Urals were in the equatorial belt ( mean annual temperature 28 - 31 ° C), Transcaucasia - in the tropical zone (23 - 28 ° C). A similar situation exists in Western Australia.

An arid climate (dry desert climate) has been established in Canada. At that time, in the provinces of Saskatchewan and Alberta, as well as in the Mackenzie River basin, there was an active process of salt accumulation. Such a characteristic trace in North America was left by the Devonian period. Minerals accumulated in other regions as well. On the Siberian platform, there appeared the largest deposits diamonds.

Wet regions

At the end of the Devonian Eastern Siberia an increase in moisture began, due to which layers enriched in manganese oxides and iron hydroxides appeared there. At the same time, it was characteristic of some areas of Gondwana (Uruguay, Argentina, South Australia). He was different high humidity, at which more precipitation fell than could seep into the soil and evaporate.

In these regions (as well as in the northeast and south of Asia), reef massifs were located, and reef limestones accumulated. Variable humidification has been established in Belarus, Kazakhstan and Siberia. In the Early Devonian, a large number of semi-isolated and isolated basins formed, within the boundaries of which isolated fauna complexes appeared. By the end of the period, the difference between them began to blur.

Minerals

In the Devonian in regions with humid climate formed the oldest coal seams on Earth. These deposits include deposits in Norway and Timan. The oil and gas bearing horizons of the Pechora and Volga-Ural regions belong to the Devonian period. The same can be said about similar deposits in the USA, Canada, the Sahara and the Amazon Basin.

At that time, reserves began to form in the Urals and Tatarstan iron ore. In regions with arid climate powerful strata of potassium salts were formed (Canada and Belarus). Volcanic manifestations led to the accumulation of copper pyrite ores in the North Caucasus and on the eastern slopes of the Urals. Lead-zinc and iron-manganese deposits appeared in Central Kazakhstan.

Tectonics

By the beginning of the Devonian, mountain structures arose and began to rise in the North Atlantic region (Northern Greenland, Northern Tien Shan, Altai). Lavrussia at that time was located in equatorial latitudes, Siberia, Korea and China - in temperate latitudes. Gondwana was entirely in the southern hemisphere.

Lavrussia was formed at the beginning of the Devonian. It was caused by a collision of Eastern Europe and North America. This continent experienced intense uplift (to the greatest extent the watershed range). Its erosion products (in the form of clastic red sediments) accumulated in Britain, Greenland, Svalbard and Scandinavia. From the northwest and south, Laurussia was surrounded by new folded mountain structures (the folded system of the Northern Appalachians and Newfoundland).

Most of the territory of the East European Platform was a lowland with insignificant hilly watersheds. Only in the northwest, in the region of the British-Scandinavian mobile belt, were low mountains and large uplands located. In the second half of the Devonian, the lowest parts of the East European Platform were flooded by the sea. On the coastal lowlands, red flowers spread. Under conditions of increased salinity, deposits of dolomites, gypsum and rock salt accumulated in the central part of the sea basin.