Paleozoic era

The beginning of the Paleozoic era is marked by the Cambrian explosion. During this relatively rapid period of evolution and development of species, many new and more complex organisms appeared than the Earth had ever seen. During the Cambrian, many of the ancestors of today's species appeared, including arthropods and echinoderms. The Paleozoic is divided into 6 periods:

1. Cambrian Period
2. Ordovician Period
3. Silurian Period
4. Devonian Period
5. Carboniferous Period
6. Permian Period

During this period, the supercontinent disintegrated. In the south, the continent of Gondwana arose, which was formed by the South American, African-Arabian, Hindustan, Australian and Antarctic platforms. To the north were the continents of Laurentia, Baltica and Siberia. Between Gondwana and the northern continents there was an ocean basin, and the Paleopacific Ocean surrounded all the continental blocks. There were also several microcontinents in the ocean - Central Kazakhstan, Tuva-Mongolian, Barguzin-Vitim, Central Mongolian, on the outskirts of which island volcanic arcs were formed. Oceanic transgressions were frequent, significant areas were covered by shallow, warm seas rich in fauna 25 ° C. The climate was 22 ° C. Organisms with mineral skeletons arose. There were many trilobites, gastropods, and brachiopods. Reef-building organisms were archaeocyaths, which existed only in the Cambrian, and algae that secreted lime. The first soil invertebrates, worms, and millipedes appeared. Coral polyps, cephalopods, arthropods, and chordates appeared.

Mollusks and arthropods remained the dominant marine animals. The first jawed fish appear. The land remained deserted, but the first land plants began to appear in coastal zones. Bacteria dominated. Blue-green algae continued to develop. Calcareous green and red algae, which lived in warm seas at depths of up to 50 m, reached their peak development. Life lived in seas, oceans, fresh and brackish waters. There was no terrestrial life (with the exception of some arthropods). Representatives of almost all types and most classes of marine invertebrates existed. The first jawless vertebrates (arandaspids) also lived at that time. Planktonic radiolarians and foraminifera lived in the water column of the oceans and seas. Corals and other coelenterates lived in warm-water seas. Among the echinoderms, sea buds, sea bladders, sea lilies, starfish, and hedrioasteroids flourished. Mollusks were widespread - bivalves, gastropods, and cephalopods (orthoceras, endoceras, oncoceras, and ascoceras). Among the snails, there were many pteroshells and bellerophons. Eurasian scorpions, trilobites, brachiopods, bryozoans, sponges, graptolites, and horseshoe crabs were common.

The Late Ordovician-Early Silurian glaciation continued, during which a significant part of Gondwana was covered by an ice sheet. By the end of the Ruddanian Age, the glaciation ended. By the end of the period, the climate became warm and dry. At the end of the period, the average air temperature exceeded 20 °C. The formation of the protective ozone screen continued. The oxygen content in the atmosphere was 65% of the modern level, at the end of the period it decreased to 35%. The continental platforms were occupied by shallow sea basins. Their area underwent some transgression, and at the end - regression. At the beginning of the Silurian, part of the East European and most of the East Siberian platforms were occupied by such basins. Carbonate sediments of the shallow shelf and lagoon deposits predominated. By the end of the period, the sea left the East European platform and a significant part of the East Siberian platform. Some groups of jawless fish appeared - bony-armored and armorless. The rise of graptolites and straight-shelled nautiloids. The diversity of brachiopods increased. In the late Silurian, cartilaginous ray-finned fishes from the order Paleoniscoformes appeared. The bony fish up to a meter long is considered the first vertebrate predator.

Fish reach great diversity, placoderms occupy almost all marine habitats. The first tetrapods evolve from lobe-finned fish. The climate is tropical. Plants spread over land, the first representatives of ferns and seed plants appeared. The period ended with extinction, which, in particular, destroyed placoderms and almost all trilobites. On land, lycopsids, horsetails, ferns and gymnosperms evolved from rhyniophytes. In the middle of the Devonian, the oldest forest formed by cladoxyleal trees Calamophyton grew on the territory of modern England. The first land vertebrates appeared. Amphibians arose from lobe-finned fish - ichthyostegs, acanthostegs - had "fish" features, but had formed limbs. Millipedes spread, spiders, ticks, insects appear. The first ammonites appeared - cephalopods with spirally twisted shells. Predatory euryscorpions flourish. Trilobites begin to die out. Agnatha and gnathostomes inhabit almost all marine and freshwater basins and reach great diversity. The diversity of plants increases significantly. Among the groups widespread in the second half of the Devonian are tree-like lycopods, the first horsetails, protoferns, progymnosperms and the first gymnosperms. Soil cover appeared. At the end of the period, there is a mass extinction of animals.

Among the invertebrates: new orders of foraminifera, prosobranchs and pulmonate gastropods; among the vertebrates: reptiles (cotylosaurs) and synapsids; among the higher plants, various gymnosperms: conifers, cordaites and cycads. Cephalopods, echinoderms and graptolites became extinct. Foraminifera - fusulinids, corals - tabulatoides, chaetetids and rugoses flourish, among the mollusks - gastropods, nautiloids, ammonoids (goniatites), bryozoans, brachiopods - productids and spiriferids, sea lilies and ancient sea urchins, amphibians (stegocephalians), arthropods (insects) and plants (club mosses and horsetails) also flourish. In the seas there were cartilaginous fish (sharks and bradyodonts). Freshwater lobe-finned fish and rhipidistia survived. Rhipidistia-rhizodonts that lived in swamps were the highest predators. A great diversity of amphibians is noted. large groups of "amphibians": batrachomorph, reptiliomorph, lepospondyls, loxommatids, microsaurs, crassigyrhinids. primitive forms of reptiles arise, they lived near water until they became land animals. spore plants became widespread: sigillaria, lepidodendron (lycopods), calamites (horsetails), stauropteris, various adder's tongues (ferns), seed horsetails, cordaites (gymnosperms). The seed plants that emerged could settle in drier habitats. Warm swamps were abundant with insects and amphibians, giant flying cockroaches, dragonflies (meganeuras) and mayflies, arthropleura. Arachnids were found in the undergrowth: pulmonoscorpius, spiders and distant ancestors of ticks. In the territories of coastal plains, deposits of coal and peat were formed from the remains of dead plants.

The diversity of sauropsids (reptiles) and synapsids (ancestors of mammals) increased. As a result of the Carboniferous forest crisis, the tropical forests of the Carboniferous period gave way to deserts. Since amniotes survived the crisis better due to their lesser dependence on moisture, they diversified greatly, unlike amphibians. Pangea continued to exist in the Permian. The period ended with the Permian–Triassic extinction. As a result of the eruption of the Siberian Traps, 81% of all marine and 70% of all terrestrial species of organisms became extinct. It took about 30 million years for the biosphere to recover from this catastrophe. At the boundary with the Triassic, about 90% of marine species (including the last trilobites) and 70% of terrestrial species disappeared. One explanation for this extinction is that a large asteroid impact caused a significant climate change. Another (more common) version is that the extinction was caused by a global increase in volcanic activity due to all the continents joining into one continent, Pangea.