The Paleozoic era following the Precambrian (Archaean + Proterozoic) lasted from 540 to 252 million years ago. The Paleozoic is divided into six periods (in brackets the beginning and end of each of them in millions of years ago).
At the beginning of the Cambrian, a warm climate dominated on Earth: the average surface temperature was relatively high, with a small temperature difference between the equator and the poles. Climatic zonality was relatively weakly expressed. But there were also zones of arid climate, which were common in the northern part of the North American continent, within the Siberian and Chinese continents. In Gondwana, he dominated only the central regions of South America, Africa and Australia.
The main mass of the atmosphere at the beginning of the Cambrian was nitrogen, the amount of carbon dioxide reached 0.3%, and the oxygen content was constantly increasing. As a result, by the end of the Cambrian, the atmosphere acquired an oxygen-carbon dioxide-nitrogen character. At this time, humid hot conditions began to dominate on the continents, the water temperature in the ocean was not lower than 20 ° C.
During the Ordovician and Silurian, climatic conditions become quite diverse. In the late Ordovician, belts of equatorial, tropical, subtropical, temperate and nival climate types are distinguished. Equatorial uniform conditions existed in the European part of Russia, in the Urals, Western Siberia, Central Kazakhstan, Transbaikalia, in the central regions of North America, in the south of Canada, in Greenland. It became very cold at the beginning of the Late Ordovician.
In subtropical areas, average annual temperatures decreased by 10-15°, and in tropical areas by 3-5°. The South Pole at that time was on the elevated land of Gondwana, within which extensive continental glaciers arose. In the second half of the Silurian in high latitudes, the climate again became moderately warm, close to subtropical. By the early Carboniferous, the planet began to be dominated by a tropical and equatorial climate.
In the Urals, the average annual temperatures were 22–24 °C, in Transcaucasia, 25–27 °C, and in North America, 25–30 °C. An arid tropical climate prevailed in the central parts of the Euro-Asian and North American continents, as well as within South America, North Africa and Northwestern Australia. Humid tropical conditions prevailed predominantly in Eurasia, North America and within Gondwana. A more moderate climate existed on the Siberian continent and in the south of Gondwana .
The increase in the volume of plant biomass on the continents led to increased photosynthesis with intensive consumption of carbon dioxide (with a twofold decrease in its content in the atmosphere) and the release of oxygen into the atmosphere. As a result of the formation of the large supercontinent Pangea, sedimentation temporarily ceased over large areas and the connection between the equatorial marine basins and the polar regions was limited.
These processes led to the onset of cooling, with a lower average temperature, a pronounced climatic zonality and a significant temperature difference between the equator and the poles. As a result, in the Late Carboniferous and Early Permian, a powerful ice sheet covered Antarctica, Australia, India, southern parts of Africa and South America.
Land at the South Pole began to play the role of a global refrigerator. In the northern polar pool, the water temperature decreased and probably, like the now-existing Arctic Ocean, was covered by ice for some time. The ice sheet existed for a relatively short time, periodically receding. During the interglacial epochs, the climate became temperate. Thus, in the Late Carboniferous and Early Permian, the formation of many landscape-climatic zones and climatic zones, known at present, took place, and climatic zonality became pronounced.
On the earth’s surface, an equatorial, two tropical, two subtropical, two temperate zones with different humidification regimes were distinguished. By the end of the Permian, the humid cool climate gave way to a warmer one, in areas with moderate conditions, subtropical climates began to predominate, and the tropical and equatorial climate zones greatly expanded. The average temperatures of the tropical seas were 20–26 °C.
Life in the seas and fresh waters
In the Cambrian period, the main life was concentrated in the seas. Organisms have populated the entire variety of available habitats, down to coastal shallow water and, possibly, fresh water. The aquatic flora was represented by a wide variety of algae, the main groups of which arose back in the Proterozoic era. Starting from the Late Cambrian, the distribution of stromatolites gradually decreases. This is due to the possible appearance of herbivorous animals (possibly some forms of worms) eating stromatolite-forming algae.
The bottom fauna of shallow warm seas, coastal shoals, bays and lagoons was represented by a variety of attached animals: sponges, archaeocyates, coelenterates (various groups of polyps), stalked echinoderms (sea lilies), brachiopods (lingula) and others. Most of them fed on various microorganisms (protozoa, unicellular algae, and so on), which they filtered out of the water.
Some colonial organisms (stromatopores, tabulates, bryozoans, archaeocyates), having a calcareous skeleton, erected reefs on the sea floor, like modern coral polyps. Various worms, including hemichordates, have adapted to burrowing life in the thickness of the bottom sediments. Sedentary echinoderms (starfish, brittle stars, holothurians and others) and mollusks with shells crawled along the seabed among algae and corals.
The first free-swimming cephalopods, the nautiloids, appeared in the Cambrian. In the Devonian, more advanced groups of cephalopods (ammonites) appeared, and in the Lower Carboniferous, the first representatives of higher cephalopods (belemnites) arose, in which the shell gradually reduced and turned out to be a prisoner of the soft tissues of the body. In the thickness and on the surface of the water in the seas lived animals drifting with the flow and kept on the surface with the help of special swim bladders or “floats” filled with gas (intestinal siphonophores, hemi-chordal graptolites).
Highly organized animals also lived in the Cambrian seas arthropods: gill-breathers, chelicerae and trilobites. Trilobites flourished in the early Cambrian, at that time accounting for up to 60% of the entire fauna, and finally died out in the Permian. At the same time, the first large (up to 2 meters in length) predatory arthropod eurypterids appeared, which reached their peak in the Silurian and the first half of the Devonian and disappeared in the early Permian, when they were replaced by predatory fish.
Beginning with the Lower Ordovician, the first vertebrates appeared in the seas. The oldest known vertebrates were fish-like animals, devoid of jaws, with a body protected by a shell (armored jawless, or ostracoderms). The first of them belong to the Upper Cambrian. The oldest representatives of fish appeared in the seas and fresh waters of the early and middle Devonian and were dressed in a more or less highly developed bone shell (armored fish). By the end of the Devonian, armored vertebrates die out, being replaced by more progressive groups of jawed animals.
In the first half of the Devonian, there already existed diverse groups of all classes of fish (among the bony fish, ray-finned, lung-puffed, and lobe-finned), with a developed jaw, real paired limbs, and an improved gill apparatus. The subgroup of radiant fish in Paleozoic was small.
The “Golden Age” of the other two subgroups fell on Devon and the first half of the carbon. They were formed in intracontinental reservoirs, well warmed up by the sun, abundantly overgrown with aquatic vegetation and partly swampy. Under such conditions, a lack of oxygen in water has an additional respiratory organ (light), which allows the use of oxygen from the air.
The development of land, as a habitat, could begin in the second half of the Ordovician period, when the content of oxygen in the earth’s atmosphere reached 0.1 from modern. The settlement of lifeless continents was a long process that developed throughout the Ordovician, Silura and Devon.
The first inhabitants of the land were plants, which first settled in shallow waters near the sea coasts and fresh water bodies, and then gradually mastered wet habitats on the shores. The oldest representatives of this amphibious flora were psilophytes, which did not yet have real roots. The colonization of land by plants marked the beginning of soil formation with the enrichment of the mineral substrate with organic substances.
In the early Devonian, other groups of terrestrial vascular plants arose from psilophytes: lycopsids, horsetails, and ferns. Representatives of these groups in the Late Devonian everywhere replaced the psilophytes and formed the first true terrestrial flora, including tree-like plants. The appearance of the first gymnosperms also belongs to this time.
In a humid and warm climate, characteristic of the first half of the Carboniferous period, abundant terrestrial flora, which had the character of dense tropical rainforests, became widespread. Among the tree plants, plainlike lepidodendenrons (up to 40 m high) and sigillers (up to 30 m high), teddy kalamites, various creeping and treelike ferns, gymnosperms permets and cordites were distinguished. The wood of all these trees did not have annual rings, which indicates the absence of a clearly defined seasonal climate.
As the land was settled by plants, the prerequisites appeared for the development of the terrestrial habitat by animals. Most likely, the first among them were small herbivorous forms, which from the early Silurian period began with the use of soil, which, according to habitat conditions, is close to the aquatic environment.
The most primitive groups of modern terrestrial invertebrates (onychophores, centipedes, lower insects apterygotes, many arachnids) are close to such forms. But they left no trace in the fossil record. Representatives of several groups of terrestrial arthropods are known from the Devonian: the Paleozoic group of armored spiders, mites and lower primary wingless insects. In the second half of the Early Carbon era, the highest insects endowed with wings, belonging to the subclass of winged insects.
Diania, onychophora class. Diania is a small animal, 6 cm long. It had an elongated torso and 10 armored legs. Body covered with small spines.
In the Carboniferous on land, herbivorous gastropod mollusks from the group of pulmonary, breathing air, appear. In the Upper Devonian deposits of Greenland, the most ancient representatives of amphibians are known Ichthyostegs. They lived in shallow coastal areas of water bodies (where free swimming was difficult), wetlands and areas with excess moisture on land. In the Carboniferous, the flowering of ancient amphibians begins, represented in the Late Paleozoic by a wide variety of forms, which are united under the name of stegocephals.
Pederpes (Pederpes finneyae, Pederpes finneyi) a primitive tetrapod (“amphibian”) of the early Carboniferous era. The only quadruped of this era known from a fairly complete skeleton.
The most famous representatives of stegocephals: labyrinthodonts, which in the late Paleozoic were one of the most common and abundant species of vertebrate groups. In the Permian period, large crocodile-shaped stegocephalians and legless or caecilians appear. In the early Carboniferous, a group of anthracosaurs separated from the primitive labyrinthodonts, combining the features of amphibians and lizards (Seimurians, Kotlassii).
From them, in the Early Carboniferous, real reptiles arose, which have already become fully terrestrial animals. Small (up to 50 cm long) reptiles fed on insects and their skin respiration disappears. The oldest and most primitive reptiles belonged to the cotylosaur subclass. The emergence of new abundant habitats and food methods available on land contributed to the appearance in the second half of the Carboniferous, in addition to insectivorous groups, herbivorous animals and large predators that feed on vertebrates.
Kotilosaurs: from above nyctifruret (Nyctiphruretus acudens); below limnoscelis (Limnoscelis paludis)
Some reptiles (mesosaurs) returned to water bodies in the Carboniferous, becoming semi-aquatic or fully aquatic animals. At the same time, their limbs were converted into fins, and narrow jaws were seated with many thin and sharp teeth.
Life in the late Paleozoic
Starting from the late carbon in the southern hemisphere, the processes of glaciation associated with the location of the South Pole in Gondwan are intensifying. A moderate cool climate with a pronounced seasonality was established on a supercontinent’s free of glaciers. In the wood of the Gondvan flora, called glossopteric, annual rings appear.
Such a flora was characteristic of the vast territories of modern India, Afghanistan, South Africa, South America, Australia, New Zealand and Antarctica. It contains, in addition to various pheridospers, representatives of other gymnosperms were included: cordites, ginking and coniferous.
On the northern continents, which were part of Laurasia and located in the early Permian time to a large extent in the equatorial zone, vegetation has been preserved that is close to the tropical flora of the Carboniferous, but already depleted in species of lepidodendrons and sigillaria.
In the middle of the Permian period, the climate of these areas (Europe and North America) became more arid, which led to the disappearance of ferns, calamites, tree-like lycopods and other moisture-loving tropical forest plants. Only in the eastern regions of Laurasia (China and Korea) did the climate and flora remain close to those in the Carboniferous.
The fauna during the Permian period underwent significant changes, which became especially dramatic in the second half of the Permian. The number of many groups of marine animals has decreased (brachiopods, bryozoans, sea urchins, brittle stars, ammonoids, nautiluses, ostracods, sponges, foraminifers), as well as their diversity, up to the complete extinction of entire classes (trilobites, eurypterids, blastoids, Paleozoic groups of sea lilies, tetracorals).
Acanthodia, many Paleozoic groups of cartilaginous fishes die out from vertebrates. In fresh inland waters, the number of choan fish is significantly reduced. By the end of the Paleozoic, lepospondylic stegocephalians die out. The Permian extinction in terms of scale belongs to the category of the so-called “great extinctions”.
During this period, 96% of all marine species and 70% of terrestrial vertebrate species became extinct. The catastrophe was the only known mass extinction of insects, which resulted in the extinction of about 57% of the genera and 83% of the species of the entire class of insects. Changes in terrestrial fauna were not so massive. Insectivorous cotylosaurs were divided into several main evolutionary trunks, herbivorous reptiles (pareiasaurs, reaching a length of up to 3 m) and large predators (synapsid reptiles) arose.
In the late Carboniferous, the most ancient animal-like reptiles appeared pelycosaurs, which became extinct already in the middle of the Permian period. They could not compete with representatives of a more progressive group of animal-like reptiles therapsids, which became the dominant group of reptiles in the late Permian period.
Therapsids were very diverse: among them were predators of various sizes (foreigners) and herbivorous animals (deinocephals). In the Late Permian, dicynodonts were widespread, having lost all teeth, except for the huge upper teeth in males and toothless jaws covered with a horny “beak”.