The Paleozoic era left its mark on the history of the development of the Earth: violent geophysical processes, the formation of the main continents and seas, the emergence of marine and land animals... The Ordovician period - the second from the bottom of six periods in the historical geological system of division - follows the Cambrian and precedes the Silurian.
History of the study
The history of the geological and biological development of the Earth can be traced through the study of radiological studies of the strata that were formed in each period. The Paleozoic era is no exception. The Ordovician period was identified and described by English researchers Murchison and Lapworth, who proposed its name. The Ordovicians were the name given to an ancient tribe that lived in what is now Wales. It is in this place, in the area of Areninga and Bala, that geological strata corresponding to the period that occurred 500 million years ago were identified for 60 million years.
At the XXI session of the International Geological Congress, the Ordovician period acquired the status of an independent system.
Ordovician divisions
The Ordovician system is divided into three periods: lower, middle and upper. According to the generally accepted international classification, they correspond to certain tiers:
- Lower Ordovician: Tremadocian, Floian.
- Middle Ordovician: Dalingian, Darriwilian.
- Upper Ordovician: Sandbian, Katian, Hirnantian.
The geological history of the Ordovician period is studied in many countries; paleontologists put forward their own systems for dividing periods, somewhat different from the international version.
Continents and oceans
At the border of the Cambrian and Ordovician periods, the distribution of ocean and land looked like this: North America and Greenland formed a single continent, Laurentia. To the south of it was the Brazilian mainland. The African continent included Madagascar and Arabia. North - Russian continental platform, to the east of it Angarida, the Chinese and Australian continents. Tectonic movements during the Ordovician period led to the convergence of the Russian, Siberian, Chinese and North American platforms and the formation of a single large continent of Laurasia in the north. The southern platforms - Hindustan, African, Antarctic, South American, Australian - united into the huge continent of Gondwana. The two large continents were separated by the Tesis Sea - a place where in modern times the mountain systems of Europe, North Africa, Asia, and South America are located. High seismic activity is characteristic of these regions to this day.
Climate
Volcanoes continued to rage throughout the Ordovician period. The climate became warmer and wetter. The current hottest part of the world - Africa - in those days was located under the very pole, as evidenced by traces of glaciations on its territory. The increase in sea area led to a decrease in the northern deserts and the complete disappearance of the southern dry zone. Active seismic conditions lead to the accumulation of thick volcanic rocks. Tuffs and lava fill the sea trenches. Black volcanic ash and sand accumulate on the seabed, which gradually sinks.
Minerals
Climatic conditions, tectonic and seismic activity of this period led to the formation of special accumulations in the earth's crust, characteristic only of the Ordovician. The Ordovician period is distinguished by deposits of rocks of marine origin: limestone, shale, sandstone. Volcanic lava and magmatic processes lead to the formation of phosphorites, oil shale, oil, iron and other ores, granite, marble, and shell rock.
Ordovician period: flora
In the Ordovician, as in the previous Cambrian period, the class of plants is represented mainly by red and green algae. Plants that appeared on land are observed in very small numbers in the studied sections; apparently, these were the first examples of small tubular representatives of the flora, which were just beginning their continental development. For a more extensive conquest of the soil, optimal conditions were formed only in the subsequent Devonian period, when outbreaks of volcanic passions subsided somewhat and suitable humidity was established. The first seaweeds are unicellular lower forms of plants, which, however, have survived to this day. Algae coexisted with plant forms such as fungi and bacteria.
Animals of the Ordovician period
As you know, the origin of the animal world began in the ocean depths. In this regard, the Ordovician is characterized by explosive progress in the development of many new species and forms of major marine organisms.
Trilobites, peculiar marine arthropods that appeared in Cambrian times, experienced unprecedented progress. During the Ordovician period, the number of their genera increased to 77; they differed in size and lifestyle. Some species of trilobites were blind, but most of them had eyes with facets ranging from 10 to 1200. The body of these arthropods consisted of several segments (from 2 to 29) and was covered with spines, which protected them from enemies and kept the trilobite on the surface of the water.
During the period we are considering (Ordovician), cephalopods received a surge in development. The ancestors of modern cuttlefish, squid and octopuses formed in those distant times many species and subspecies, ranging in size from a few centimeters to several meters.
In the sandy layers of the Ordovician period, remains of foraminiferal, radiolarian, and graptolite nuclei were found.
Bryozoans, echinoderms, tabulates, and corals have made significant progress in development. The Ordovician is associated with the appearance of the most ancient vertebrates - jawless fish agnates. In their structure, the first cartilaginous fish resembled modern hagfish. It was flexible, like a loach, a creature with a cartilaginous spine along its slippery body. The first vertebrates still had a cover in the form of a hard shell, which protected them from external damage, but they were already more mobile than their predecessors, since the spine served as a support for developing muscles. At the very end of the period, the gills of some species turned into jaws, and the horny plates into teeth. The first ferocious predators - placoderms - looked terrifying, reached three meters in length, and moved quickly.
Ordovician catastrophe
Many people know the historical fact about the mass extinction of dinosaurs on the planet, but few know that the first significant catastrophe occurred about 200 years before the appearance of dinosaurs. In the late Ordovician period, the animal world lost approximately 60% of the species that existed at that time. Most living organisms belonged to primitive forms and existed primarily in an aquatic environment, however, in terms of the number of dead individuals, this event is second in the list of similar mass extinctions and third in the percentage of species and genera that disappeared forever from the face of the Earth.
Versions of the causes of the Ordovician catastrophe
As with dinosaurs, the extinction of Ordovician protists is hotly debated among paleontologists. Experts are discussing the reasons that caused the tragedy. There are five main versions:
- First cosmic: a powerful burst of gamma waves near the Solar System.
- The second is cosmic: the fall of large meteorites or the collision of large asteroids.
- One of the reasons could be the intensive formation of mountain systems, in particular the Appalachians, which leads to intense weathering and changes in the composition of soil and water.
- Climate version. The shift of Gondwana towards the South Pole led to a sharp cooling. The concentration of carbon dioxide decreased, the level of the World Ocean dropped, and glaciation followed.
- Opponents of the above version are researchers of the chemical composition of fossil microorganisms. They are sure that the reason lies in a sharp change in the ratio of carbon dioxide and oxygen in the air, and the saturation of water with the elements iron, lead, copper, and manganese. Living organisms underwent metamorphosis, and subsequently entire species and genera became completely extinct.
Major events of the Ordovician period
To summarize, we can highlight several main and most important events of that era, which significantly influenced the course of the further history of the development of life on Earth.
- The area of the seas has increased (compared to the previous period).
- The formation of the main platforms has been completed.
- There was an accumulation of thick layers of tuff, lava, sedimentary rocks, and clastic deposits.
- Deposits of iron and manganese ores, gold, oil, and building materials are being established.
- The emergence of folded regions and mountain systems determined certain boundaries of regions, which explains the difference in the development of vegetation in different parts of the globe.
- Plants of the Ordovician period are represented by rapidly developing algae, the first of which appear on land, albeit in small quantities. Presumably, they became the harbingers of the first land tubular plants - rhiniophytes.
- Rapid development of marine inhabitants: trilobites, echinoderms, mollusks, branchapods, bryozoans, corals.
- The most important and important event of the Ordovician was the appearance of the first vertebrates - cartilaginous jawless fish.
; on the stratigraphic (geochronological) scale it follows (period) and precedes the Silurian system (period). The beginning of the Ordovician period is dated by radiological methods at 490 ± 15, and the end - at 435 ± 10 million years from the present; the total duration of the period is about 65 million years.
The Ordovician system was established by the English geologist C. Lapworth in 1879 on the territory of Great Britain; as a type section - deposits of the Arenig and Bala region in Wales. The question of the independence of the Ordovician system was finally resolved only in 1960 at the 21st session of the International Geological Congress. Prior to this, in many countries the Ordovician system was considered as the lower (Ordovician) division of the Silurian system. In the territory, deposits of the Ordovician system were studied by F. B. Schmidt, V. V. Lamansky, B. S. Sokolov, V. N. Weber, T. N. Alikhova, O. I. Nikiforova, B. M. Keller, A. M. Obut, R. M. Myannil, A. K. Ryymusoks and many others. etc. From foreign studies, the works of English geologists C. Laworth, R. Murchison, H. B. Whittington, A. Williams, Czech - J. Barrand and V. Havlicek, American - J. Hall, G. Cooper, M. Kay are known , Swedish - V. Jaanusson, S. Bergström, Japanese - T. Kobayashi and others.
Divisions. There is still no generally accepted division of the Ordovician system into departments and stages. The most common division in the CCCP and other countries is into 3 departments and 6 tiers (table). With the two-member division of the Ordovician system, the boundary of departments is drawn between the Llanvirnian and Llandalean stages. In Great Britain, the lower boundary of the Ordovician system is drawn at the base of the Arenigian stage, and the Tremadocian stage belongs to the Cambrian. The most fractional units used in the subdivision and correlation of Ordovician deposits are graptolite and conodont zones.
general characteristics. The Ordovician system is identified on all continents and on many islands. It participates in the structure of the platform cover, and is exposed along the western, northern and eastern frame of the ancient Gondwana platform - in Bolivia and Argentina, in the north and south of Africa, in eastern Australia, in Antarctica, and is widespread in all folded systems located between these platforms. In most areas, Ordovician sediments are closely related to Cambrian ones, but in places at the Cambrian-Ordovician boundary, breaks in sedimentation are established due to short-term regression of the sea. The maximum expansion of marine spaces—sea transgression—occurs in the Middle Ordovician. Subsequently, a regressive stage begins. In relatively shallow epicontinental seas, which spread on platforms, predominantly thin (on average up to 500 m) calcareous, less often sandy-clayey sediments accumulated. In transitional areas between platforms and geosynclines - in the mirgeosynclinal zones of the Appalachians, the western slope of the Urals, Altai-Sayan region, etc., sediment thickness increases to 3500 m; Along with carbonate deposits, clastic sediments are widespread.
In the CCCP, the Ordovician system is widespread on the East European and Siberian platforms, on Taimyr, in the folded systems of the Urals, Pai-Khoi, Novaya Zemlya, on the islands of Severnaya Zemlya and on the New Siberian Islands, in the Middle, Altai-Sayan region and in the North east CCCP.
On the East European Platform, the Ordovician system is best exposed and studied in Northern Estonia and the Leningrad region, i.e. along the cliff (classical sections of the Ordovician system).
Organic world. In the Ordovician system, representatives of almost all types and most classes of marine invertebrates are found, and bacteria and algae are widespread among plants, and the first vertebrates and terrestrial plants appear. Radiolarians and foraminifera, graptolites, chitinozoans and conodonts lived in the water column of the Ordovician oceans and seas. Numerous and diverse trilobites, brachiopods, crinoids, bryozoans, sponges, elasmobranchs, gastropods and cephalopods, as well as calcareous green and red algae, settled on the bottom of shallow seas, in coastal zones and on shallows. In shallow seas, in areas of reefs and organic structures, in shallow water, solitary and colonial corals lived. The most important faunal groups for the stratigraphic division of the Ordovician system are graptolites, conodonts, trilobites, brachiopods and colonial corals.
Minerals. Among the mineral deposits occurring in the Ordovician system, the most important are deposits of oil and gas (especially in North America), oil shale, phosphorites, and ores, the formation of which is due to Ordovician magmatism. Industrial accumulations and manifestations of oil and gas are associated mainly with platforms and with their folded framing: on the East European platform (in the CCCP - Baltic and), on the North American platform (in the arched uplifts of Kansas, Seminole, Chattaqua, Cincinnati, Bend, Forest depressions City, Salina, Dodge City, Michigan, Illinois, Permian; in Canada - Eastern oil and gas province) and in its frame (folded structures of the Rocky Mountains, Ouachita - Washita), in the north of the African platform (Libya, Algeria, Morocco) and on Australian platform (the depression of the central part of the Australian plate). Of industrial importance are the Middle Ordovician
The Ordovician period, or Ordovician (485 - 444 million years ago) is one of the least known geological periods in the history of the Earth. It did not witness the same burst of evolutionary activity that characterized the previous one; rather, it was a time when the earliest arthropods and vertebrates expanded their presence in the world's oceans. The Ordovician is the second period (542-252 million years ago), which was preceded by the Cambrian, and then it was replaced by and periods.
Climate and geography
During most of the Ordovician period, global climate conditions were as warm as during the previous Cambrian; the average air temperature in the world was about 50 ° C, and the water temperature in the seas reached 45 ° C. However, by the end of the Ordovician, the climate was much colder, as an ice cap formed at the south pole and glaciers covered the adjacent land areas. Plate tectonics has taken the Earth's continents to some strange places; for example, most of the land that later became Australia and Antarctica was in the northern hemisphere! These early continents were biologically significant: their coastlines provided protected habitats for shallow-water marine organisms.
Sea life
Invertebrates
During this period, the Great Ordovician Radiation occurred, an event of significant biodiversity (biodiversification) that was second only to the Cambrian Explosion in importance for the early history of life on Earth.
Over the course of about 25 million years, the number of marine organisms around the world increased significantly, with new species, trilobites, brachiopods, and (early starfish) appearing. One theory is that the formation and migration of new continents helped maintain biodiversity along their shallow coastlines, although climatic conditions likely also played a role.
On the other side of the evolutionary coin, the end of the Ordovician period marked the first great thing in the history of life on Earth (or, let's say, the first for which scientists have sufficient fossil evidence). Changing global temperatures, accompanied by a sharp drop in sea level, destroyed a huge number of species, although the whole recovered quite quickly by the beginning of the next Silurian period.
Vertebrates
Almost everything there is to know about life during the Ordovician period lies in Arandaspis and Astraspis. These were two genera of the first jawless, lightly armored prehistoric fish, measuring 12 to 14 cm in length and vaguely resembling giant tadpoles. The bone plates of Arandaspis and its ilk later developed into true skeletons. Some paleontologists also believe that the numerous, tiny, worm-like conodonts found in Ordovician deposits are true vertebrates; if so, they may have been the first vertebrates on Earth to have teeth.
Vegetable world
As in the previous Cambrian, evidence of terrestrial plant life in the Ordovician is elusive. If land plants existed, they consisted of microscopic green algae floating on or below the surface of the water. However, it was not until after the Silurian period that the first land plants of which there is hard fossil evidence appeared.
This period was a time when invertebrates were still the undisputed rulers of the ocean floor. Some of them were able to move, others lived alone or in groups, being tied to the bottom. These sedentary or immobile animals collected food that was within their reach and did not require a developed brain. But life made more severe and demanding demands on moving animals. When searching for food, they relied on their senses and quick reactions to avoid attacks from other predators.
Found in South Africa in the early 1990s, the Promissum specimen was a giant conodont-bearing specimen, reaching 40 cm in length. Its bulging eyes indicate that it actively hunted for its prey.
Armed arthropods.
When the first arthropods appeared (in the beginning), their bodies were very small, and their shells (exoskeleton) were no thicker than a sheet of paper. But by the beginning of the Ordovician period, some arthropods developed shells that turned into real armor for protection against enemies. One of the groups of arthropods that had such shells and was numerous in the Ordovician period was the “horseshoe crabs”, or horseshoe crabs.
Despite the name, these animals were not actually crabs. They belonged to the chelicerates, which include spiders and scorpions. The front part of their body was protected by a dome-shaped shield, which completely hid the mouth opening and paws of these animals. The back of the body was protected by a second, smaller shield and ended in a long, sharp spike. Their shells are well preserved in sediment, but there is a much easier way to see these animals, because they have survived to this day. These are not the same species that existed in the Ordovician period, but over 400 million years these animals have changed very little.
They fed on small animals, using their claw-ended limbs to capture prey. These claws were hidden deep under the front shield of the body, which limited their size. Some close relatives of horseshoe crabs - eurypterids, or crustaceans, had claws pointed forward. During the Ordovician, most crustaceans were relatively small in size, but in the subsequent Silurian period they became the largest arthropods. 
Arandaspis belonged to heterostracans, or heteroscutaneous fish-like creatures that did not have jaws. He moved in the water by moving his tail. It had no fins.
The first horseshoe crabs walked along the seabed on five pairs of legs. Today, five species of these "living fossils" exist on the east coast of North America and Asia.
Mysterious conodonts.
For more than a century, scientists have collected and cataloged scores of tiny tooth-like fossils that date back to Ordovician period or even considered even more ancient. They are known under the name conodonts because they are often cone-shaped. These formations obviously belonged to some kind of animal. Over time, the shape of conodonts changed. Almost every type of conodont corresponds to a specific time period, so geologists can determine the age of rocks by the appearance of the conodonts. Despite many years of searching, it was not possible to find the animals to which these conical miniature teeth belonged.
But in 1993, fossilized corpses of animals with conical teeth were found in Scotland. Then the same fossils were found in North America and South Africa. One of the species found is promissum. The mysterious animal had a thin, serpentine body and well-developed eyes. Some fossils contain traces of V-shaped muscles and notochord. This is already a feature of vertebrates and animals related to vertebrates.
Many scientists think that conodont-bearing animals were among the first vertebrates in the process of evolution. However, unlike other vertebrates from which four-legged animals evolved, conodont-bearing animals did not survive.
During the Ordovician period, something very unusual happened: an entirely new group of animals arose, one of very few to appear after the Cambrian extinction. These animals, called bryozoans, were tiny invertebrates protected by a skeleton made of cells. They lived in colonies next to each other and their shape often resembled plants. Bryozoans turned out to be a very successful addition to the animal world and have not only survived to this day, but are also widespread.
Sea bottom Ordovician period was home to a variety of larger plant-like animals known as crinoids, or sea lilies. They belong to the same phylum of animals as the starfish, sea urchin; The sea lily has a long stem consisting of calcareous segments and a “crown” of fragile branching tentacles that grab food. Later, some crinoids moved from a static existence to a mobile lifestyle in the sea, where they not only wait for food, but search and fight for it. Currently, sea lilies attached to the bottom still exist in nature. 
This Ordovician reef has been reconstructed from fossils found on the island of Newfoundland that are almost 500 million years old. Two nautiloids search the seafloor while trilobites and gastropods, or gastropods, crawl along the surface of the bottom below them. 1. Nautiloids with straight shells; 2. Nautiloids with spirally twisted shells; 3. Trilobites; 4. Gastropods; 5. Corals; 6. Sea lilies.
Ordovician period
Ordovician period- the second period of the Paleozoic era (named after the Celtic Ordovician tribe that inhabited the territory of Wales). During this period, the continents again experienced subsidence, as a result of which geosynclines and low-lying basins turned into shallow seas. At the end of the Ordovician ca. 70% of North America was flooded by the sea, in which thick layers of limestone and shales were deposited. The sea also covered large areas of Europe and Asia, partly Australia and the central regions of South America.
All Cambrian invertebrates continued to evolve into the Ordovician. In addition, corals, pelecypods (bivalves), bryozoans and the first vertebrates appeared. In Colorado, in Ordovician sandstones, fragments of the most primitive vertebrates were discovered - jawless (ostracoderms), which lacked real jaws and paired limbs, and the front part of the body was covered with bone plates that formed a protective shell.
Based on paleomagnetic studies of rocks, it has been established that throughout most of the Paleozoic, North America was located in the equatorial zone. Fossil organisms and widespread limestones from this time indicate the dominance of warm, shallow seas in the Ordovician. Australia was located near the South Pole, and northwestern Africa was located in the region of the pole itself, which is confirmed by signs of widespread glaciation imprinted in the Ordovician rocks of Africa.
At the end of the Ordovician period, as a result of tectonic movements, continental uplift and marine regression occurred. In some places, the native Cambrian and Ordovician rocks experienced a process of folding, which was accompanied by the growth of mountains. This ancient stage of orogenesis is called the Caledonian folding.
Silurian
Silurian. For the first time, rocks of this period were also studied in Wales (the name of the period comes from the Celtic tribe of Silures who inhabited this region).
After the tectonic uplifts that marked the end of the Ordovician period, a denudation stage began, and then at the beginning of the Silurian the continents again experienced subsidence, and the seas flooded the low-lying areas. In North America, in the Early Silurian the area of seas decreased significantly, but in the Middle Silurian they occupied almost 60% of its territory. A thick sequence of marine limestones of the Niagara formation was formed, which received its name from the Niagara Falls, the threshold of which it forms. In the Late Silurian, the areas of the seas were greatly reduced. Thick salt-bearing strata accumulated in a strip stretching from modern Michigan to central New York.
In Europe and Asia, the Silurian seas were widespread and occupied almost the same territories as the Cambrian seas. The same isolated massifs as in the Cambrian, as well as significant areas of northern China and Eastern Siberia, remained unflooded. In Europe, thick limestone strata accumulated along the periphery of the southern tip of the Baltic Shield (currently they are partially submerged by the Baltic Sea). Small seas were common in eastern Australia, northern Africa and central South America.
In the Silurian rocks, in general, the same basic representatives of the organic world were found as in the Ordovician. Land plants had not yet appeared in the Silurian. Among invertebrates, corals have become much more abundant, as a result of whose vital activity massive coral reefs have formed in many areas. Trilobites, so characteristic of Cambrian and Ordovician rocks, are losing their dominant significance: they are becoming smaller both in quantity and in species. At the end of the Silurian, many large aquatic arthropods called eurypterids, or crustaceans, appeared.
The Silurian period in North America ended without major tectonic movements. However, in Western Europe at this time the Caledonian belt formed. This mountain range extended across Norway, Scotland and Ireland. Orogenesis also occurred in northern Siberia, as a result of which its territory was raised so high that it was never flooded again.