Paleozoic Devonian period. Devon, Devonian

Began about 416 million, ended 360 million years ago. The duration of the Devonian is 56 million years. This period is rich in biotic events. Life developed rapidly and developed new ecological niches.

On earth, lycopsform, horsetail, fern and gymnosperms originated from rhinophytes, many of them were represented by woody forms. Soil appeared, plants all over the Earth were the same, there was no geographical differentiation.

The first terrestrial vertebrates appeared. Paleontologists suggest that the lungs that land creatures breathe originally originated in swamp-dwelling fish. From such lungfish, amphibians arose. The first amphibians known to us, ichthyostegi, had many fish features, but had well-formed limbs. They were closely related to water, maybe even more closely than modern frogs.

Spiders, mites, insects arose - life mastered the land. Changes also took place in the seas in the Devonian period. The first ammonites appeared - cephalopods with spirally twisted shells, which had yet to experience their heyday in the Mesozoic. Bottom predators of crustacean scorpions - eurypteroids reach 1.5-2 meters in length. Trilobites are beginning to die out, apparently it is already difficult for them to live in such an abundance of predators.

The Devonian is often referred to as the age of the fish, however, it can also be called the age of the cephalopods. In the Devonian period, ammonoids appeared, cephalopods and fish began to actively explore the water column. Some paleontologists write about the "Devonian nekton revolution", about the mass appearance of nekton - animals actively swimming in the water column.

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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 hot and humid climate remained 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. Shark diversity grew in the oceans and bony fish. 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.

Devonian is a period of time on the geological time scale that began about 419 million years ago and ended about 360 million years ago. This time period is so named because of the area where scientists studied the fossilized beds of this period: Devon, England. Scientists believe that it was during this period of time that life on land began to actively evolve. During the Silurian period, life colonized the land, but did so predominantly along the coasts. It was only during this period that the first “deeper” colonization of land by living organisms took place.

Before the colonization of land, the soil on the planet had red tints, which indicated a high content of minerals, organic matter was practically absent. Only algal or bacterial layers lived in the soil. Changes in soil composition began during this period as ground vegetation began to take over the land and spread. Early plants, however, were not like most of the plants we are familiar with today. They didn't have a root or leaf system, and many of them didn't even have a vascular system (although, of course, some species did). Animal life, which existed in parallel with these primitive plants, was mainly represented by families of arthropods. There were trigonatarbids, myriapods, mites and wingless insects. Most likely, there were other types of fauna, but they, most likely, have not yet been found by paleontologists.

By the end of this period, the plants had learned to grow roots. These were plants with root systems and leaves, and most of them also had vascular systems. It was also the time when the first seed plants began to appear, life became more diverse. This has led many paleontologists to call this increase in the expansion of life the "Devonian explosion". Although this event is not as well known in the scientific world as the Cumbrian Explosion, it is just as important.

As life spread across the land, the Devonian seas began to expand. At the beginning of the period, jawed and armor-piercing fish predominated in the seas. Then, by the middle of the period, the first species of jawed fish appeared. As a consequence, many of these species have evolved into the most vicious predators our planet has ever seen. Other marine animals were represented by the following species: spiriferids, tabulated corals and sea ​​corals, blastoid echinoderms, bivalves, graptolites and, of course, trilobites.

The climate during this period was quite warm, and scientists believe that at that time glaciers were completely absent on earth. Along the equator, the weather was driest, but very dry climates also prevailed around the world. During the first part of this period, the temperature environment is believed to have been around 30 degrees Celsius (86 degrees Fahrenheit). When this period arrived, CO2 levels began to decline, and as a result, the Earth cooled slightly: by about 5 degrees Celsius (9 degrees Fahrenheit). This did not last long, and by the end of this period the temperature returned to its previous values. Many paleontologists believe that this warming led to the eventual extinction of stromatoporoids.

However, stromatoporoids were not the only animals to go extinct. So did the silent fish species known as the Agnathans, all but a subclass called the Heterostraci. At the end of the Devonian period, another extinction occurred, which many scientists consider one of the five largest extinction events in the history of the planet. This extinction affected species such as lean fish, trilobites, akritarchams, placoderms, ammonites and brachiopods. At the time of this writing, scientists are still unsure what cataclysm caused this disappearance. It was during this extinction that the Devonian period ended.

The Devonian Period, often referred to as the "Age of the Fish", was the time when vertebrates began to exercise their superiority in the sea and took their first steps on land.
began 408 million years ago, at a time when major changes occurred that led to the formation of the world as we know it now. The giant continent of Gondwana was still close to south pole, but continued to move north, while parts of Europe, North America and Greenland united and formed a single continent that stretched to the equator. The climate was warm and the first low-growing Silurian plants gradually gave way to those that settled best on land. By the end Devonian period the first forests appeared.

Jaws again.

In the process of evolution of the animal world, the same adaptation was often "invented" several times. This is what happened in the Devonian period with a group of fish known as placoderms.
Placoderms had powerful jaws - blade-like plates with tooth-like protrusions. But since placoderms were not direct descendants of the first jawed fish, most experts believe that this valuable adaptation developed independently in different fish. Along with the jaws, these fish had two hard shields - one covered the head, and the other the front of the body. The shields were connected by a pair of "loops" that allowed the overhead shield to rise when the fish bit the prey.
Some placoderms lived on seabed, where they ate mollusks and other shell animals, but by the end of the Devonian, some of them began to hunt in the open sea. Here they were the largest predatory fish. One of the types is dunkleosteus- reached almost 4 meters in length and could bite any other fish in half with its mouth plates.
Giant armored fish dunkleosteus(picture below) is approaching the cladoselachia, the primordial shark. In Dunkleosteus, the dental plates did not change during life, and in Cladoselachia, like in today's sharks, by inner edge jaws, dozens of triangular teeth continued to grow. Both of these primitive fish swam by undulating tails; their fins were stiff and stabilized her position in the water, helping her stay on course.

"branched" fish.

AT Devonian Placoderms shared the seas with several other groups of jawed and jawless fish. There were jawless species with bizarrely armored bodies, but there were also unarmored species that resembled modern ones in many ways. Shellless fish were divided into two groups: in some, the skeleton consisted of cartilage, and in others, of real bones.
Cartilaginous fish were the ancestors of modern sharks and rays. Their bodies were covered with small, coarse scales called skin denticles, and in their mouths those same denticles enlarged and became an endless row of sharp teeth. From the very beginning of their existence, many of these fish resembled modern sharks, and by the end of the Devonian, representatives of one of the groups, cladoselachia already grown up to two meters. Bony fish were usually small, and the scales that covered them became thinner and lighter. These fish developed a gas-filled swim bladder that gave them buoyancy and movable fins to help them maneuver.
In one group of bony fish called lobed-finned or sarcopterygia developed fleshy fins. These fish are of particular interest to scientists, because it is from them that four-legged vertebrates descend. Not all lobe-finned animals could leave the water: several species, including lungfish and coelacanths, lived in fresh and salty waters where they live to this day.

Life on dry land.

Despite many years of research, experts still do not know exactly which of the sarcopterygium fish is the progenitor of the first amphibians - that is, the first vertebrates that began to spend part of their time on land. But by the end of the Devonian, landfall had already taken place. When the first amphibians appeared, they seemed slow and clumsy compared to other land creatures, but it was a breakthrough that changed the entire course of life on Earth. Although, like the first land plants, amphibians could not completely break away from the water: in order to lay eggs, they returned to the water.

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, there were lobe-finned fish. 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.