Ordovician period - Period Paleozoic era, following the Cambrian, lasting 70 - 80 million years. Animal world The Ordovician period is very rich and diverse compared to the Cambrian period. In the Ordovician there is a change in the trilobite fauna that existed in the Cambrian. This fauna is represented in the Ordovician mainly by postobuccal forms. The number of representatives of brachiopods increases sharply, among which the castle ones, which have a calcareous shell, dominate. By the end of the period, groups of corals - tabulates and bryozoans - trepostomata, become highly developed. Graptolites, which are one of the widespread and leading groups, are represented by the order Axonolipa. Cephalopods (nautiloids) are also very numerous. From Echinodermata the first hedgehogs and crinoids appear, and cystoids reach their peak, forming many genera and species. Gigantostracans appear among arthropods. The remaining groups of animals - protozoa (radiolaria), worms, elasmobranchs, gastropods, etc. - were poorly developed. Vertebrates were represented by primitive jawless animals. In the Ordovician period, apparently, the first highly developed terrestrial organisms appeared - centipedes and scorpions. The Ordovician flora was represented by bacteria, algae, which often formed bioherms among carbonate strata, as well as primitive psilophytes. But there is reason to assume that plant organisms were more diverse, as indicated by the varied and numerous spores found in Ordovician deposits.
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Ordovician or Ordovician system - second period. The Ordovician lasted from 485 million years ago to 443 million years ago, that is, for 42 million years. To avoid getting confused about eons, eras and periods, use the geochronological scale, which is located, as a visual clue.

The name of this period was given according to the typical geological section, which clearly indicates the history of this period. The section is located in the region of Wales, on the territory of which the Celtic tribe of the Ordovicians lived in ancient times.

Life continued to develop during the Ordovician period. During the study of the geological layer of the period, numerous discoveries were made regarding the Ordovician biota. It is noted that during this period green and red algae achieved great development and distribution. Moreover, in addition to marine plants, the first land plants appeared during this period. In the period from 485 to 443 million years ago, remains of spores of land plants were discovered, as well as imprints of stems that apparently belonged to vascular plants.

Concerning animal world, then, unlike plants, they had not yet reached land and lived only under water. The seas and oceans were inhabited by unicellular radiolarians, unicellular foraminifera, jawless vertebrates arandaspids (extinct), echinoderm sea buds (blastoidea, extinct), echinoderm globuloids (sea bladders, cystodea, extinct), sea lilies, starfish. In addition, bivalves, gastropods and cephalopods, crustaceans, trilobites, brachiopods, bryozoans, sponges, graptolites, and horseshoe crabs lived in the Ordovician. The Ordovician is also characterized by animals that lived only during this period, that is, they arose in the Ordovician and became extinct in the Ordovician. Scientists note that by the end of the Ordovician and the onset of the next period, unique groups of echinoderms, which were not observed in other periods, became extinct. In addition, by the beginning of the Silurian, the extinction of many families of graptolites, brachiopods, corals, cephalopods and trilobites occurred, since a mass extinction of animals occurred at the Ordovician-Silurian boundary.

Ordovician-Silurian extinction considered one of the five worst extinctions in history and the second largest loss among living organisms (the first is the Permian extinction, when 96% of all marine species and 70% of terrestrial vertebrate species). The main reason for the extinction of animals during this period is considered to be the movement of the supercontinent Gondwana, which moved towards the south pole, which led to global cooling, glaciation and a drop in sea levels. In total, about 100 families of marine animals, or 49% of all animals on Earth, became extinct.

Animals of the Ordovician period

Cincinnetina meeki

Platystrophia ponderosa

Rhynchotrema dentatum

Arandaspidae

Blastoidea

Graptolites

Horseshoe crabs

Sea stars

sea ​​lilies

Orthoceras

Radiolarians

Cancerscorpios

Trilobites

Foraminifera

Ball joints

Endoceras

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The Ordovician - the Ordovician period - began around 488 and ended 444 million years ago. It lasted 44 million years. This was the heyday of trilobites and cephalopods with straight shells - endoceras and orthoceras. Horseshoe crabs appeared, and corals replaced archaeocyaths.

Trilobites achieve enormous diversity - they occupy a variety of ecological niches - in the Ordovician there were trilobites crawling along the bottom, burrowing in silt, swimming in the water column. Most of trilobites were detritivores, but some species, according to paleontologists, were predatory.

Vertebrates are still few in number, but there are already two classes of jawless animals. Brachiopods, gastropods, and echinoderms felt good in the seas. People well known to everyone appear sea ​​stars and less familiar, but very widespread in some periods, crinoids - crinoids. However, throughout the Ordovician itself, crinoids were not widespread; other echinoderms, cystoids (Echinospherites), were much more common. At the end of the Ordovician, many ancient groups of invertebrates became extinct.

It should be noted that although the name “Ordovician” was proposed as early as 1879, the Ordovician period itself was separated from the Silurian only in 1960. Therefore, do not be surprised by the absence of the Ordovician in books and reference books published before 60, where it was part of the Silurian.

I first visited the Alekseevsky quarry in 1984. Back then it was a busy quarry. Crushed stone from it was transported throughout the North-West. Pieces of hard bluish limestone with brownish inclusions of Kuker shales traveled to cities and towns, taking with them the Ordovician fauna: brachiopod shells, trilobite fragments, mollusk cores... Today the Alekseevsky quarry does not work. I haven’t been to it for a long time, and this summer I finally got around to it. For a long time I couldn’t figure out the minibuses... >>>

Identifying finds is often as much fun as searching for them, but it can take much more time. Below follows the history of the identification of one of the trilobites from the Ordovician of the Leningrad region, known to trilobite lovers under the name “Valdaites” limatus Jaanusson. What could be the connection between detective stories and paleontology? It turns out that it is the most direct - the essence is in the method of determination, an attempt to get to the roots, to find out why the trilobite you are holding in your hands is called exactly... >>>

ORDOVICIAN PERIOD

Ordovician deposits were identified in England and described by the English geologist R. Murchison. - By decision of the 21st session of the International Geological Congress, the Ordovician was identified as an independent system.

During the Ordovician period, the Laurentian mainland broke up into four large and a number of smaller islands. In place of the Russian continent, two large islands were formed, separated by a narrow strait. Almost half of the territory of the Siberian and Chinese continents was flooded by a shallow sea.

In the southern hemisphere, a huge continent was formed - Gondwana, which included modern South America, the southern part Atlantic Ocean, Africa, Indian Ocean, Australia, Northern Asia. The Northern Tien Shan, Altai, Australian Cordillera, and West Siberian ranges begin to form.

In the sea basins that existed in the Urals, Chukotka and Cordillera, thousands of volcanoes were active, producing powerful deposits of volcanic rocks.

Among the Ordovician rocks, marine sediments predominate - sandstones, limestones, and shales. Compared to Cambrian deposits, there are fewer lagoonal formations among Ordovician deposits - gypsum, salts, limestones, and dolomites. The climate during the Ordovician period becomes warmer and milder, as evidenced by the wide distribution of limestones: stromatoporoid, coral, crinoid, trilobite and cephalopod. The area of ​​the sea has increased significantly. The equatorial primordial sea flooded vast areas of the Cambrian continents.

The southern dry zone completely disappears. The area of ​​northern deserts is shrinking. As a result of these changes, the animal and vegetable world. Mountain continents wedged between sea basins prevented the spread of animals and plants around the globe. This is why the fauna and flora of the European Ordovician are different from the Indian and East Asian ones.

At the end of the Cambrian period, volcanic eruptions filled sea basins with tuffs and lavas. At the same time, the seabed drops significantly. All this led to the accumulation of thick layers of sedimentary rocks, in particular black silt, consisting of volcanic ash, sand, and clastic rocks.

The algae underwent almost no changes during this period. The marine fauna was characterized by such a wealth of forms that the Ordovician period seems to us to be the most important era in the entire history of the Earth. It was in the Ordovician that the main types of marine organisms were formed. Compared to the Cambrian, the number of trilobites increases significantly. In the Ordovician, many large trilobites (up to 50-70 cm) also appear in Europe. This indicates that they felt good in the new conditions.

Thanks to the migration of fauna from west to east and adaptation to new conditions, 77 new genera of trilobites appear in the Ordovician seas. External structure bodies indicates that trilobites led different lifestyles. Their eyes had from 10 to 1200 facets. There were also blind trilobites. The number of body segments (segments) varied among different species from 2 to 29. The body was covered with spines for protection from enemies or completely smooth, well adapted to crawling in mud. Sometimes the body was covered with long sharp spines, increasing its surface, which allowed the animal to float freely in the water.

All the most important groups of animals that lived in the seas at a later time were found in Ordovician deposits. In the loose green sandstones near Leningrad, many foraminiferal cores are found. Radiolarium is found in black shales. Sponges with silica needles in their skeletons are quite numerous in Ordovician deposits: Cyathophicas, up to 12 cm high, and Brachiospondia, up to 30 cm high with 12 root shoots.

Sea sponges were four- and six-rayed. The four-armed eutaxidima and six-armed receptaclelitida had especially beautiful needles. The body of the first, the size of a cherry, had a fibrous structure. Each of the fibers was a hexagonal tube consisting of small four-ray needles, intertwined so closely that it is very difficult to separate at least one of them. Six-rayed sponges first appeared in the Early Ordovician. The round, flat, pear- or saucer-shaped body of this creature was covered with a shield of rhombic plates. Under each plate there was an empty pointed column. The columns were connected to the internal plates. All this made up the inner shell.

The first corals appeared, but they did not yet have much significance in nature. The most common mollusks were nautiloids and gastropods. Nautiloid shells were straight. The mollusk itself was placed in the living chamber, the remaining chambers were filled with gas. By filling these chambers with water, the mollusk could dive to considerable depths, and displacing the water with gas, float to the surface. Graptolites appeared that looked like branches, spirals, and loops. They lived in colonies, attached to algae or swam freely with the help of a bladder.

In the Ordovician period, bryozoans and tabulates first appeared, which became especially widespread in the Silurian period.

Brachiopods are rapidly developing. If in the Cambrian there were 18 genera, then in the Ordovician there were already 41 genera of these animals.

Echinoderms in the Ordovician were represented by many species of cystoids, whose bodies were covered with a calcareous shell. The round mouth opening was protected by a plate. The significant distribution of cystoid forms gives grounds to consider them the ancestors of crinoids, sea ​​urchins and starfish, since various cystoids had much in common in structure with these large groups of animals.

The Ordovician period lasted 60 million years. Its deposits contain polymetallic and iron ores, phosphorites, oil shale, building materials, and oil.

During the Ordovician period, the Laurentian mainland broke up into four large and a number of smaller islands. On the site of the Russian continent, two big islands separated by a narrow strait. Almost half of the territory of the Siberian and Chinese continents was flooded by a shallow sea. In the southern hemisphere, a huge continent was formed - Gondwana, which included modern South America, the southern part of the Atlantic Ocean, Africa, the Indian Ocean, Australia, and Northern Asia. The Northern Tien Shan, Altai, Australian Cordillera, and West Siberian ranges begin to form. In the sea basins that existed in the Urals, Chukotka and Cordillera, thousands of volcanoes were active, producing powerful deposits of volcanic rocks.

Organic world

The algae underwent almost no changes during this period. The marine fauna was characterized by such a wealth of forms that the Ordovician period seems to us to be the most important era in the entire history of the Earth. It was in the Ordovician that the main types of marine organisms were formed.

Compared to the Cambrian, the number of trilobites increases significantly. In the Ordovician, many large trilobites (up to 50-70 cm) also appear in Europe. This indicates that they felt good in the new conditions. Thanks to the migration of fauna from west to east and adaptation to new conditions, 77 new genera of trilobites appear in the Ordovician seas.

All the most important groups of animals that lived in the seas at a later time were found in Ordovician deposits. In the loose green sandstones near Leningrad, many foraminiferal cores are found. Radiolarium is found in black shales. (There should also be pictures here, they are named after animals).

The first corals, bryozoans and tabulates appeared. Brachiopods and blue-green algae, calcareous and brown algae are rapidly developing. There were representatives of almost all types and most classes of marine invertebrates. At the same time, jawless fish-like creatures appeared - the first vertebrates. Planktonic radiolarians and foraminifera lived in the water column of oceans and seas; The graptolites reached their peak. At the bottom of shallow seas, in coastal areas and the shallows contained numerous and varied trilobites, brachiopods, echinoderms, bryozoans, sponges, elasmobranchs, gastropods and cephalopods. Corals and other coelenterates lived in warm-water seas.

At the end of the Ordovician period, some fish developed jaws and became active predators. Scientists believe that some of the rigid arches that supported the gills gradually turned into jaws, and teeth were formed from the plates surrounding the mouth opening. One of the new groups - the so-called placoderms (plate-skinned fish) - included the largest marine fish of that period, including the ferocious predators Dunkleostea, up to 3.3 m long. In the upper jaw, instead of teeth, they had rows of small plates. Constantly in contact with the lower jaw, these plates sharpened its edge so much that the fish were able to bite and crush prey with both jaws.