Ordovician, Ordovician period, Ordovician extinction. Ordovician period (Ordovician) Ordovician period fauna and flora

general characteristics, stratigraphic units and stratotypes.

The Ordovician system got its name from the Ordovician tribe, who inhabited Wales (Great Britain) in antiquity. Initially, the Ordovician deposits were included in the previously distinguished Silurian system. For a long time the Ordovician was considered as the lower section of the Silrian system, and its upper section was the Gotlandian (along the island of Gotland in the Baltic Sea). The name "Ordovician system" was first proposed by C. Lapworth in 1879. In the domestic geological literature, A.F. Lesnikova and D.V. Nalivkin in the 30s advocated the independence of the Ordovician and Silurian systems (the latter in the volume gotland). In 1951, the Ordovician system was officially identified on the state geological maps of the USSR. However, only in 1960 the independence and the name of the Ordovician and Silurian systems were approved at the XXI session of the International Geological Congress in Copenhagen. The Ordovician was established from typical sections in the Arenig Bala region in northern Wales. Initially, the boundaries of the Ordovician and its subdivisions were determined by changes in shell fauna complexes, and much later, rapidly evolving graptolites began to be used for these purposes. Therefore, to date, there are two stratigraphic scales that have not yet been finally linked to each other, and this makes it difficult to correlate sections of Wales with sections of other regions, dissected by the remains of graptolites.

In Great Britain, the lower boundary of the Ordovician is drawn along the foot of the Arenig, since the trema-dock here is closely related to the Cambrian. In other European countries and in Russia lower tier Ordovician is considered Tremadocian. The upper boundary of the system formally coincides with the top of the Ashgilian Stage. However, it should be noted that the position of both the lower and upper boundaries of the Ordovician is debatable and has not received official international recognition.

The Ordovician tiered and zonal scales are based on graptolites, and the beginning of the Ordovician division was laid down by the works of C. Lapworth and G. Elles. C. Lapworth proposed a three-term division of the Ordo-Vic, but more often used a two-term one. This is associated with certain difficulties of dismemberment and is the subject of discussion. In Russia, a three-term division of the Ordovician is accepted, although the boundaries between the divisions are in some cases not well defined.

The stratotype of the Tremadocian Stage is located in Caernarvonshire. Its volume was established by A. Sedgwick, who attributed this stage to the Cambrian. The stratotype section of the Arenigian Stage is located in the Arenig Mountains of North Wales. It is also installed by A. Sedgwick. The stratotype is incomplete and poorly characterized by fauna.

The Llanvirn Stage has been described from Pembrokeshire in West Wales. Shales with numerous graptolites are common here. Among them, the genus Didymograptus is the most characteristic. The Llandale Stage of the Middle Ordovician in Caernarvonshire, Wales, is composed of platy limestones with Glyptograptus and Nemagraptus.

In the West of England, in Shropshire, there is a stratotype of the Caradocian Stage. Quartz sandstones and quartzites containing Dicranagraptus, Climacograptus are developed here.

The Ashgill Stage takes its name from the Ash Gill Creek in Lancashire in Northern England. Here a series of slates with Dicellagraptus is exposed. Characteristic sections of the Ordovician and Silurian are shown in Scheme III, col. incl.

organic world

In contrast to the Cambrian, life in the Ordovician was much more diverse. The plant kingdom was dominated by algae, including green ones. A representative of green algae (or cyanobionts?) - the genus Gloeocapsomorpha played an important role in the formation of kukersite oil shales.

Of great importance for the zonal stratigraphy of the Ordovician are graptolites belonging to the Hemichordata type (semi-chordate). Graptolites in the Ordovician evolved rapidly, had significant ranges and are therefore guide fossils. The early Ordovician is characterized by axisless forms (Phyllograptus, Didymograptus), while the middle and late Ordovician are characterized by autumn two-row graptolites (Diplograptus, Climacograptus).

Conodonts, which appeared in the middle Cambrian, were very widespread in the Ordovician. Conodonts belong to the primitive chordates and represent a semblance of the jaw apparatus of these animals in the form of teeth of microscopic size and various shapes: simple ("fangs"), rod-shaped and platform. Conodonts lived in a wide variety of marine environments, from deep water (preferably) to shallow water.

Animal world The seas are represented by invertebrates, as well as jawless fish-like organisms (thelodonts). Especially widespread were trilobites, sea bladders, brachiopods, cephalopods from the subclasses of endoceratoids and nautiloids, coral polyps from the subclasses of four-beams (rugoses) and tabulatomorphs.

Trilobites are represented mainly by new genera. The most important of them are Asaphus, Trinucleus, Megistaspis, Illaenus and others. Trilobites acquired the ability to fold due to the fact that predators appeared - cephalopods. As a consequence of this - the development of equal-sized and close in shape head and tail shields. Brachiopods are represented by both hingeless forms with chitin-phosphate and hinge forms with a calcareous shell. The genus Obolus is known from the non-locking ones (but other species than in the Cambrian); from castle-Porambonites.

Of the endoceratoids, orthoceratoids, and other similar straight-shelled cephalopods that lived in seas with normal salinity, the Ordovician is especially characterized by large forms of the genus Endoceras, as well as Orthoceras, Actinoceras, and representatives of various orders of nautiloids. They led a near-bottom, active lifestyle. The shells of these predators reached a length of 2-3 m (Endoceras). In the Ordovician, the development of intestinal - tabulate (genus Syringopora) and four-beam corals (rugos) began, which, together with hydroid polyps - stromatoporates (Stromatoporata) - were not only guiding, but also rock-forming organisms. Together with bryozoans and corals, they built reefs. From the echinoderms in the bottom biocenoses, sea bladders (cystoids) developed, which were joined by crinoids (crinoids) from the Middle Ordovician.

These are the main groups of invertebrates. In addition to them, other groups of fauna existed in the seas of the Ordovician, which did not enjoy such a wide development. These include fora minifers, radiolarians, ostracods, sponges, worms, bivalves, gastropods, bryozoans, etc.

structures earth's crust and paleogeography

The same platforms and geosynclinal belts existed in the Ordovician as at the end of the Cambrian period. In geosynclinal troughs, intensive subsidence continued, which favored the accumulation of many kilometers of strata of predominantly terrigenous marine sediments and effusives.

At the end of the Ordovician, in a number of geosynclinal regions, the second phase of the Caledonian epoch of tectogenesis, the Taconian, began. It manifested itself approximately in the same areas of the Northern Hemisphere, where the Salair phase of folding took place. In connection with the Taconian phase of folding, some parts of the geosynclinal regions turned into high-elevated mountain structures, some of which existed for a very long time (the Northern Appalachians, the northern ranges of the Tien Shan), and others at the beginning of the Silurian again sank below sea level (Wales in Great Britain).

The regression of the seas of the end of the Cambrian with the onset of the Ordovician was replaced by a new general transgression. The area of ​​the epicontinental seas expanded so much that the Ordovician transgression on the platforms turned out to be the largest in the entire history of the Paleozoic (thalassocratic epoch). However, this transgression was not the same on all ancient platforms. If the Ordovician transgression on the North American platform exceeded the Cambrian by many times and almost covered the entire territory, then on the Siberian and East European it was weaker than the Cambrian. The expansion of the epicontinental seas also occurred in Gondwana.

By the end of the period, due to mountain building in a number of geosynclinal systems, and especially those adjacent to platforms, both geosynclinal and epicontinental seas are shrinking.

For the Ordovician, according to the study of paleomagnetism of rocks, the same plan for the location of the poles is preserved and, accordingly, climatic zones, as in the Cambrian. Obviously, the wide development of transgressions in the Northern Hemisphere softened the climatic conditions here. Tropical wet zone was located in a strip stretching from southern Greenland through Novaya Zemlya to Western Siberia. It is characteristic that all the warm zones at that time were shifted far to the north in comparison with the modern position of the equator.

The position of the continents in the Late Ordovician, according to the concept of a new global tectonics, is shown in Scheme XVI, col. incl.

History of platform development

East European (Russian) platform

The Ordovician deposits are distributed in the same place as the Cambrian, that is, in the Baltic, Dniester and Moscow syneclise, and are represented by all three divisions. They occur with stratigraphic unconformity in the Cambrian. These are horizontally lying strata of marine shallow-water sediments of small thickness (~300 m), containing rich marine fauna of the Western European type, which indicates a transgression coming from the west (see diagram III, col. inc.). It begins. The section of Estonia in coastal cliffs and ledges with obol sandstones. A huge amount of phosphate valves of the genus Obolus turned this sequence into a phosphorus-bearing horizon, developed in Estonia and the Leningrad region. Above - black graptolitic mudstones with dicyonems. Both sequences make up the Tremadocian Stage. The upper part of the Lower Ordovician - the Arenigian Stage - is composed of glauconite sandstones and limestones with numerous remains of brachiopods and trilobites (Asaphus, Megistaspis). The Middle Ordovician (up to 160 m thick) is represented by limestones with a rich fauna of brachiopods, trilobites, graptolites, and ostracods. There are strata of combustible shale - kukersites. They testify to temporary uplifts and shallowing of the warm Ordovician sea, its overgrowing with blue-green algae (cyanobionts), from which combustible shale - kukersites were formed. The Upper Ordovician is again composed of limestones with fauna. Ordovician limestones are widely used for a variety of building purposes. The Estonian section is typically platform-like, composed of marine shallow-water sediments, which are good building materials (the Old City Hall and other buildings in Tallinn).

Siberian platform

The Ordovician occupies the western part of the platform and is exposed along the margins of the Tunguska syneclise and in the southwest of the platform. The sections differ in lithology and paleontological characteristics. There is a dominance of carbonate rocks with the remains of a diverse marine fauna, especially brachiopods. On the margins of the basin, shallow-water sediments were deposited: dolomite silts, variegated sands and clays, sometimes with interlayers of gypsum. The sections show a regional hiatus before the Middle Ordovician. The thickness of the deposits is several hundred meters.

Chinese platform

Lower and Middle Ordovician sandy-argillaceous and carbonate deposits several hundred meters thick with remains of brachiopods, gastropods, and nautiloids are widespread here.

North American Platform

At the beginning of the Ordovician, the greatest transgression occurred here, during which carbonate sediments accumulated. At the beginning of the Middle Ordovician there was a short-term regression and islands appeared. In the late Ordovician, the platform began to sink again, and limestone and dolomite oozes were deposited. In the east, clastic material began to enter the sea - the products of destruction of the Taconian uplifts in the Appalachian geosyncline. The thickness of the Ordovician is the first hundreds of meters.

gondwana

In the South American part of Gondwana, uplifts dominated in the Ordovician. Marine clastic deposits are found in the extreme west along the border with the East Pacific geosynclinal region. Sandy-argillaceous deposits of small thickness are known in the Amazon Basin. The African part of Gondwana began to sink in the north at the end of the Cambrian. In the Ordovician, marine quartz sands with interlayers of pebbles and clays were deposited on the territory of the Sahara. They lie directly on the Precambrian basement. The thickness of the strata is 500-800 m, in avla-cogens 2-2.5 km. On the Arabian Peninsula, the Ordovician is represented by sandy-clayey formations of considerable thickness. In the Australian part of Gondwana, a large area of ​​the sea occupied little in the Ordovician. It flooded the central region and spread in the latitudinal direction. Sands were deposited here, less often calcareous silts.

North Atlantic geosynclinal belt

Grampian geosynclinal region. Grampian geosyncline. Within this geosyncline, thick strata of sedimentary and volcanogenic rocks accumulated. The Ordovician section of Wales is stratotypical and is exposed in many parts of this area (see Scheme III, col. inc). The lowest, Tremadocian, schistose mudstones with Dictyonema and trilobites, are overlain by Areniga rocks with distinct unconformity. Therefore, English geologists attribute the tremadoc to the Cambrian. The Arenig Stage is composed of volcanic rocks with intercalations of limestones with trilobites and brachiopods (the thickness of the Lower Ordovician is 1.2 km).

Llanvirn consists of shales with remains of trilobites, brachiopods, and graptolites. Shales are sometimes replaced by effusives along the strike. The Llandale Stage is the most carbonate part of the Ordovician section - platy limestones with brachiopod and trilobite shells. The Caradoksian Stage - carbonate-argillaceous deposits with brachiopods and graptolites or effusives (Middle Ordovician thickness - 2 km). At the end of the Ordovician, volcanic activity ceased and Ashgill is represented by polymictic sandstones, cross-bedded, with signs of ripples and shale (thickness - 1 km).

Altai-Sayan geosynclinal region. The Salair cycle of tectogenesis, which manifested itself in this area in the Middle Cambrian, did not fully stabilize it. Geosynclinal conditions in the Ordovician are restored in the West Sayan and Gorno-Altai troughs, which are separated by the Gorno-Shor uplift. But flysch formations are already accumulating in the Ordovician troughs (thickness - 7-8 km).

On the uplifts, there is a different type of sections: less thickness, sediments - carbonate silts, sands with an abundance of shallow-water fauna. In the Ordovician deposits, breaks in sedimentation are known (these are manifestations of Caledonian movements). It should be noted that in the Altai-Sayan region there is a sharp angular unconformity between the Cambrian and Ordovician deposits. This is the result of the Salair phase of folding.

In general, not the entire territory of the Ural-Mongolian geosynclinal belt in the Ordovician was occupied by the sea. Within ere there were Early Caledonian uplifts and islands that supplied terrigenous material to depression zones. Such uplifts were in Central Kazakhstan and in the east of the belt - in the Altai-Sayan region and Mongolia. In the end Ordovician period in the west of the Central Asian part of the Ural-Mongolian belt, the Taconian phase of the Caledonian folding was actively manifested. Its consequence was the formation of extensive uplifts in Kazakhstan, traced from Kokchetav south to the Tien Shan and were the area of ​​removal of terrigenous material in the Silurian. Caledonian folding in the southern and eastern parts of the Ural-Mongolian geosynclinal belt was accompanied by intrusive magmatism. The Takonian phase is associated with the intrusion of large granitoid intrusions over a vast territory from the Northern Tien Shan to Petropavlovsk and Omsk.

In the European geosynclinal region, the Ordovician deposits are more widespread than the Cambrian ones. They are known in the north of Europe, where they are represented by marine sandstones, shales with intercalations of limestones or effusive horizons. The Franco-Czech massif (Moldanub block) in the Ordovician was a huge island, and marine terrigenous deposits with interlayers of siliceous and effusive rocks accumulated on its eastern margin in the Czech Republic. The section of these strata became classic in the 19th century thanks to the works of I. Barrand. The section begins with conglomerates, Diccyonema schists, and sandstones unconformably overlying Cambrian rocks (Scheme III, col. inc). Above, there are sandstones and shales with trilobites, graptolites, and quartzites with brachiopod shells. In southern Europe, the Ordovician conformably overlies the Cambrian and is represented by marine terrigenous facies of a typical geosynclinal appearance, but there are no effusives. In the Asian region, geosynclinal conditions are also observed with a set of corresponding facies.

Pacific geosynclinal belt

The entire Early Paleozoic is represented by marine facies. In the Ordovician Verkhoyansk geosyncline, there was a vast marine basin with an archipelago of islands, where terrigenous deposits accumulated. The maximum transgression falls on the Middle Ordovician. In the Cordillera and Andean geosynclines, marine conditions also prevailed with the accumulation of terrigenous rocks. This is the early geosynclinal stage of development of this territory.

Minerals

In the Ordovician, the productive horizons of the US Midcontinent (Kansas and Oklahoma) are known, which provide a third of the annual oil production. A large oil field was discovered in the Algerian Sahara in the Cambrian and Ordovician. There are signs of oil on the Siberian platform. Sedimentary uranium is known in the shales of the Lower Ordovician in Sweden. The middle Ordovician includes oil shales - kukersites of the Baltic states (Estonia) and the Leningrad region. In the Ordovician, deposits of oolitic iron ores are traced on Newfoundland Island in Canada, as well as in Argentina and a number of Western European countries. Ordovician magmatism is associated with copper and cobalt deposits in Norway, polymetals of the Salair Ridge and gold in Kazakhstan. The tremadok (obol horizon) of the Baltic states includes actively developed phosphorite deposits.

History of the development of geosynclinal belts

Grampian geosynclinal region. Grampian geosyncline. A section of the Silurian of Wales, the stratotype locality where the Silurian system was identified, can be seen in Scheme III, col. incl.

The Silurian rests on the Ordovician with a structural unconformity caused by the Taconian orogeny. At the base of the Llandovery lie conglomerates and sandstones, above which are replaced by a sandy-clayey stratum with shell rocks; Pentamerides are numerous (the thickness of Llandovery reaches 1.5 km). Wenlock is lithologically diverse: in some areas there are calcareous-argillaceous rocks and limestones with remains of brachiopods and corals (300-400 m), in others there is a thick layer of sandstones and siltstones (thickness -1.2 km). The Ludlov deposits are predominantly carbonate: limestone, calcareous shales, calcareous siltstones. There are numerous stromatoporates, corals, brachiopods (thickness - 0.5 km). There are fossil banks with Conchidium knighti. In the upper part of the stage, there is a layer of the so-called bone-bearing breccia, which consists of parts and fragments of the bone cover of armored fish.

The described section of three tiers refers to "shell" formations - shallow-water deposits of considerable thickness containing the indicated fauna.

Another type of section of the same stages is also known - in the form of a thin stratum of graptolitic shales. Clay material in this case was deposited in the deep sea areas. The third type of incision is mixed. It contains rocks of the first and second types.

The uppermost part of the Silurian section in England is distinguished as the Downton Stage (thickness -0.6-0.9 km). These are red and variegated sandy-argillaceous rocks with interlayers of red marls. They contain shells of ostracods and ichthyofauna. Downton is gradually replaced by the lower red-colored Devonian. All this is overlapped with structural unconformity by Middle Devonian conglomerates.

In Wales, the total thickness of the Silurian is 3 km. The deposits are folded and metamorphosed. Caledonian folding manifested itself repeatedly and was accompanied by magmatism.

In the Scandinavian part of the Grampian geosyncline, thick clastic strata accumulated, at first typically marine, and towards the end of the Silurian - continental.

Ural-Mongolian geosynclinal belt

The Ural-Tien Shan geosynclinal region extends from Novaya Zemlya to the southern Tien Shan.

Ural geosyncline. Silurian deposits are widely developed in the Urals. On the western slope of the Urals, carbonate and terrigenous sediments (up to 2 km) accumulated quietly under miogeosynclinal conditions. On the eastern slope, in the eugeosyncline, lavas and tuffs, siliceous shales and limestones accumulate (thickness - 5 km). In the Silurian in the Urals, the main geotectonic structures were laid, which later turned into the existing anticlinoria and synclinoria. The Silurian of the Urals of the western and eastern slopes contains the same fauna, which indicates a single geosynclinal Ural basin in the Silurian. On the territory of the western slope of the Urals and on Novaya Zemlya, miogeosynclinal conditions prevailed, so carbonate and carbonate-argillaceous deposits (500-1500 m) with a diverse complex of organic remains accumulated here. Shallow coastal sand-pebble rocks are known on the western margin Northern Urals(Polyudov ridge). In the west of the central part of the Urals, on Pai-Khoi and in places on Novaya Zemlya, black clayey graptolite shales are exposed.

Caledonian folding, in contrast to other geosynclines of the Ural-Mongolian belt, is not typical for the Urals; it did not cause structural unconformities, but the ultrabasic and basic intrusions of the central zone are considered Caledonian.

Silurian deposits are widespread in the Kazakh part of the Ural-Mongolian belt. They are represented by typical geosynclinal formations of considerable thickness with the remains of a rich fauna. Horizons of brachiopod and coral limestones are characteristic.

In the context of Mt. Chingiztau Silur is represented only by the lower section. Silurian deposits (up to 2.5 km) accumulated in eugeosynclinal marine conditions with strong volcanism. Actively manifested Caledonian folding. The most pronounced is the last - Late Caledonian - phase of folding, which led to the retreat of the sea from the territory of the Chingiztau Ridge, to the completion of the first, actually geosynclinal, stage of its development. The gently dipping Lower and Middle Devonian effusive rocks and felsic tuffs crowning the section accumulated already in terrestrial conditions. They are usually identified as volcanogenic molasses of the orogenic stage of development. The repeated intrusion of large granitoid intrusions is associated with folding.

Altai-Sayan folded region. Silurian deposits are known in the same place as the Ordovician, but in the west limestones and terrigenous rocks with rich fauna predominate, in the east (Western Sayan, Tuva) the role of coarse clastic rocks with depleted fauna increases. The thickness of the Silurian deposits in the west is 4.5 km, in the east - up to 7.5 km.

In the Silurian section of Western Tuva (see Scheme III, color incl.), the Silurian deposits (Chergak Group) overlie the Ordovician. They have a large thickness (2.5-3 km), consist of sandy-argillaceous rocks with interlayers, packs and lenses of limestones. The highest carbonate content is confined to the middle part of the section. The fauna is rich and varied. These are stromatoporates, tabulates, heliolithids, rugoses, crinoids, bryozoans, brachiopods, trilobites. Many local (endemic) forms. Evidently, in the Silurian there existed a shallow marine basin with small reefs, coral and crinoid thickets, and banks of brachiopods. The endemism of the fauna speaks of the difficult communication with other seas. By the end of the Silurian, the basin gradually shrank, became shallow, its salinity changed, and only euryhaline organisms survived in it.

In the Ordovician, Silurian and early Devonian in Western Tuva, a single huge (10 km) transgressive-regressive Tuva complex was formed with marine deposits in the middle part and red-colored continental rocks in the sole and roof. The deposits of the Tuva complex are folded and intruded by small basic and felsic intrusions. The upper part of the section under consideration is composed of thick Lower Devonian terrestrial effusive rocks and red clastic rocks of the Middle Devonian. These are continental deposits of intermountain depressions formed during the regression caused by the Caledonian orogeny. In the section of Western Tuva, three structural stages sharply differing from each other are clearly distinguished: the first is the Lower Cambrian; the second - Ordovician, Silurian, lower Devonian; the third is the upper part of the Lower Devonian and the Middle Devonian. The floors record different stages of geological development: the first is eugeosynclinal, the third is orogenic, and the second is intermediate (transitional). At the second stage, the subsidence developed on the already consolidated basement, the regime was reminiscent of miogeosynclinal. Ore deposits of iron and copper are associated with acid intrusions.

Thus, the Caledonian epoch of tectogenesis covered the regions of northwestern Kazakhstan, partly the Altai Mountains, the northern Tien Shan and the eastern part of the Altai-Sayan folded region - the Western Sayan and Tuva, where the Caledonides arose.

Mediterranean geosynclinal belt

In the European part of this belt, conditions are preserved that are close to those previously described in the Ordovician. This is still the insular land of the Franco-Czech massif (Moldanub block) and marine conditions to the north and south of it (Prague synclinorium, see scheme III, color incl.). In northern Europe, sandstones, black shales, bituminous limestones (0.5 km thick) accumulate, siliceous shales appear, due to manifestations of underwater volcanic activity. In southern Europe, between the Franco-Bohemian Massif and the Atlas Mountains in Africa, the Silurian is represented by monotonous facies: black shales with graptolites, which are replaced by limestones at the top of the section.

In the Asian geosynclinal region, the Silurian is known in Turkey, the Caucasus, in the mountain structures of Iran, Afghanistan, and the Pamirs. Here, under eugeosynclinal conditions, thick strata of terrigenous rocks and volcanics of basic and felsic composition, or small terrigenous-carbonate facies in miogeosynclinal zones (Zagros Himalayas, etc.) accumulated.

Minerals

Rock salt deposits, commercial oil and gas deposits are known on the North American (Canadian) and Siberian platforms. In the Silurian, deposits of oolitic iron ores Clinton (USA) and a number of small ones in Africa were formed. Gold deposits associated with the Caledonian felsic intrusions Northern Kazakhstan, Kuznetsk Alatau and Mountain Shoria.

In late Caledonian intrusions in the Scandinavian mountains, iron, copper, and chromite were found: Nickel, platinum, asbestos, and jasper are known in the Urals. Deposits of rare metals in the Appalachians and Eastern Siberia are associated with pegmatites. Silurian limestones are a building material and a good ceramic raw material.

2.5.

2.6.

2.7.

Fauna of the Permian period

2.8.

2.9.

2.10.

Animal world of the Cretaceous period

2.11.

Fauna of the Paleogene period

2.12.

Fauna of the Neogene period

2.13.

2.3. Fauna of the Ordovician period

Ordovician period (490 - 443 million years ago)

Ordovician - Ordovician, second period Paleozoic era geological history of the earth. The Ordovician period follows from the Cambrian and overlaps with the Silurian period. Start Ordovician system radiological methods determined 490-500 million years ago, and the duration was approximately 60 million years.

Rice. 2.3.1. Seabed of the Ordovician period. The marine fauna was characterized by such a wealth of forms that the Ordovician period seems to us the most important era in the entire history of the Earth. It was in the Ordovician that the main types were formed. marine organisms. Life in the Ordovician seas (Figure 2.3.1) was even more diverse than in the Cambrian seas.

In the Ordovician period, the first fish appeared, but most of the inhabitants of the sea remained small - few of them grew to a length of more than 4-5 cm. The formation of hard covers in many animals meant that they acquired the ability to rise above bottom sediments and feed in food-rich waters above the seabed.

During the Ordovician period, more and more animals appeared, extracting food from sea ​​water. Some groups of invertebrates flourished during this period, other groups only began to develop more magnificently. In general, the evolution of the organic world at that time advanced so much that vertebrates appeared in the later Ordovician. From echinoderms(3, 4 Fig. 2.3.1) sea ​​bubbles(MITROCYSTELLA, DENDROCYSTITES, ARISTO-CYSTITES, ECHINOSPHARITES and others) reached the peak of their development at that time. For the first time, representatives of another class of echinoderms appeared in large numbers - sea ​​lilies CRINOIDEA - 2, fig. 2 .3 .1), probably descended from more ancient sea bladders. If crinoids were not widespread in the Cambrian and there were no such beautiful forms, as in the later seas, in the Ordovician seas they were one of the best decorations. Their body, covered with plates that formed regular corollas, was attached to the bottom with a long movable stem, which consisted of a large number annular segments. Around the mouth opening was a crown of movable, sometimes branching arms - rays. With long flexible rays covered with a sticky substance, sea lilies caught food particles from the water. Some species of such rays had up to 200. Sea lilies, like their stemless relatives - starfish, have successfully survived to this day. Sea lilies often formed beautiful underwater thickets. And if we imagine that flocks of transparent bell-shaped or hat-shaped jellyfish with ribbon-like tentacles floated above the cup-shaped bodies of sea lilies, which strongly resembled buds or flowers and swayed on long stems, then we can say with confidence that at that time the beginning of exist on our Earth what we call beauty. Brachiopods(Fig. 2.3.2) in the Ordovician, a number of new families, genera and species were formed, and at the beginning of this period, forms with calcareous shells and with a lock (СLITAMBONITES, PORAMBONITES, ORTHIS and others) already prevailed. The most common owners of shells were oyster-like brachiopods, reaching a size of 2 - 3 cm.

gastropods and lamellar molluscs were represented by a significant number of genera and species.

Rice. 2.3.4. Ammonite structure. In the seas of the Ordovician, the first significant development of four-gilled cephalopods took place, a characteristic feature of which is the presence of a multi-chambered shell; these are all primitive nautiloid rice. 2.3.3 (NAUTILOIDEA), the oldest forms of which we already see in the Cambrian seas (VOLBORTHELLA) and the last endangered genus of which, the boat (NAUTILUS), still lives in the amount of four species at considerable depths in the Indian Ocean. The shells of the Ordovician nautiloids, unlike the horn-shaped shells of modern nautilus species, were straight or conical; the animal itself was placed in the last, living chamber, the remaining chambers, separated from each other by partitions, were filled with air or gas, due to which the entire shell was a hydrostatic apparatus. In each partition there was a hole with a tube-like drawn edge. Through these holes, starting in the initial chamber of the shell, passed a special cord-like process of the animal's body, the so-called siphon. The purpose of the siphon has not yet been precisely established; apparently, it served to firmly connect the animal with the shell and made it possible to control it. These cephalopods (ENDOCERAS, ORTHOCERAS, etc.) were predators that roamed the Ordovician seas. The greatest height of development was reached in the Ordovician seas and trilobites, which had a very different size and shape of the body (ASAPHUS, ILLENUS, CYCLOPYGE with hypertrophied eyes, CRYPTOLITHUS, with a wide horseshoe-shaped border along the edge of the head shield, DALMANITINA, SELENOPELTIS, with large spines on the head shield and trunk segments).

A completely new group of animals appeared in the Ordovician seas graptolites(Fig. 2.3.5). They evolved very quickly and, leading a mainly planktonic lifestyle, were very widespread. Graptolites formed bushy or ribbon-like colonies, which in one group (DENDROIDEA) were widely attached to floating algae (less often they were attached to the seabed), and in the second group (GRAPTOLOIDEA) they swam directly at the sea surface with the help of special swim bladders, or with a long thread attached to algae. Each individual of these small animals was placed in a tubular cell made of flexible chitin.

Graptolites reproduced by budding and thus created colonies. Previously, graptolites belonged to the intestinal cavities, but at the present time, based on the research of the Polish paleontologist R. Kozlowski, they are classified as wing-branchs (PTEROBRANCHIA), which, together with enteropneathers (ENTEROPNEUSTA), form in many respects a highly organized group of invertebrates, the so-called hemichordas. Graptolites completely died out by the end of the Paleozoic, but in the modern fauna there are animals that are their distant relatives. These include, for example, RHABDOPLEURA NORMANNI, living in the North Sea.

Colonies of older graptolites were bushy. In the process of their evolution, the number of branches was gradually reduced to two. These branches moved aside or formed a fork; at a later time, they began to bend upwards in the direction of the thread until the latter turned out to be included between them. Thus, the so-called two-row types of graptolites arose. Later (in the Silurian), one row of cells disappeared and single-row graptolites appeared. At this stage of development, graptoloid graptolites became extinct. Only bushy and funky forms of dendroid graptolites existed until the Carboniferous. Of the Ordovician graptolites, the following are important: DICHOGRAPTUS - with eight branches, TETRAGRAPTUS - with four branches, DIDYMORGRAPTUS - with two fork-shaped branches, DICELLOGRAPTUS - with two branches bent upwards, PHYLLOGRAPTUS - with four mutually growing branches, two-row DIPLOGRAPTUS and others.

Rice. 2.3.6. Bryozoans. At that time, another strange group of colonial animals appeared, helping stromatoporoids and corals build reefs. These were bryozoans(BRYOSO.E), inhabiting the seas in an amazing variety until now fig. 2.3.6. Some of the bryozoans formed beautiful, fine meshed bushes with regular cells, which the old Czech flagstones aptly called "lace".

An important event in the Ordovician seas was also the appearance corals(ANTHOZOA) belonging to three different groups. The first of these were four-ray corals (TETRACORALLA), also characteristic of all subsequent seas of the Paleozoic, in which they played the same role with later, six-ray corals (HEXACORALLA), which originated from them, replaced them in the seas, starting from the beginning of the Mesozoic and live up to the present. These corals differ from each other mainly in that the number of septa and tentacles in four-ray corals is a multiple of four, while in six-ray corals it is a multiple of six. The corals were solitary or formed colonies. The second group of corals, the so-called tabulates (TABULATA), always created colonies of the most diverse form, in which each polyp built a hard calcareous skeleton, separated by numerous transverse partitions - bottoms (TABULA). last group The corals of the Ordovician seas were the so-called heliolithids, which also formed colonies of various shapes, sometimes reaching a size of several meters.

Rice. 2.3.7. Arandaspis prionotolepis (from the Arandaspidae group) fig. wikipedia Fragmentary remains of a jawless fish have been found in sandstones near Harding, Colorado. The age of these layers was about 450 million years. Other interesting remains of vertebrates have been recovered from the same rocks, including the scales of a gnathostomy, a shark-like predator equipped with jaws. The oldest well-preserved fossils were sacabambaspida found in Bolivia and arandaspida from Australia (Figure 2.3.7).

Fossils show that the jawless Ordovician period is very different from the few jawless species that exist today - lampreys and hagfish. Their bodies and heads were covered with tough, leathery plates made of a bone-like substance. Only scaly tails had the flexibility necessary for swimming. Having neither jaws nor teeth, they were forced to eat small foods that were found in large quantities - for example, planktonic microorganisms.

Rice. 2.3.8. Conodonts Some of the earliest animals to have teeth were conodonts ( fig.2.3.8) appeared at the end of the Cambrian. The group of conodonts combines fossil skeletal elements that belonged to different types animals - protoconodonts , paraconodonts and euconodonts . The animals themselves are now also called conodont bearers(Conodontophora). They were eel-like creatures, oral apparatus which consisted of 15 or, more rarely, 19 elements and was radically different from the jaws of modern animals. The shape of the elements is tooth-shaped, comb-shaped, leaf-shaped; composition - calcium phosphate. Among the conodont bearers were both very tiny (about 1 cm long) and gigantic (for example, Promissum, whose length reached 40 cm). Currently, paleontologists agree that conodont bearers are characterized by the presence of big eyes, fins with fin rays, chords and powerful transverse muscles.

Rice. 2.3.9. sea ​​world Ordovician. According to the researchers, the "teeth" of some conodonts were like filtering apparatus, with the help of which plankton was filtered out of the water and sent to the throat. Other teeth, based on their structure, in their opinion, were intended to "grab and tear the flesh." The lateral position of the eyes of conodonts, however, makes it unlikely that they are predatory. The surviving musculature impressions suggest that some conodonts (Promisuma, anyway) were skilled swimmers, but incapable, however, of rapid throws.

The amazingly diverse marine life of the Ordovician (Fig. 2.3.9) - scientists count 600 different families of sea inhabitants - did not last long. The climate on the planet became colder and drier, and at the end of the period it turned into a global glaciation, which caused the extinction of many species. The polar ice caps absorbed more and more ocean water, the sea level dropped by 330 m. The shallow seas of the continental shelf turned into arid plains, and the creatures that lived in these seas died, especially those who could not migrate anywhere from the seabed.

Fauna of the Ordovician period

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A.S.Antonenko

The Ordovician period, or Ordovician (485 - 444 million years ago) is one of the least known geological periods in the history of the Earth. He did not witness the same surge 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. Ordovician - the second period (542-252 million years ago), which was preceded by the Cambrian, and then it was replaced by periods.

Climate and geography

For most of the Ordovician period, global climatic 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 moved the Earth's continents to strange places; for example, most of the landmass that later became Australia and Antarctica was in the northern hemisphere! These early continents were of biological importance: their coastlines provided protected habitats for shallow marine life.

Sea life

Invertebrates

During this period, the Great Ordovician Radiation occurred, an event of significant biodiversity (biodiversification) second only to the Cambrian Explosion in importance to the early history of life on Earth.

Within about 25 million years, the number of marine organisms around the world increased significantly, new species, trilobites, brachiopods and (early starfish) appeared. One theory is that the formation and migration of new continents contributed to the conservation of biodiversity along their shallow coastlines, although climatic conditions likely also affected.

On the other side of the evolutionary coin, the end of the Ordovician period marked the first great in the history of life on Earth (or, shall we say, the first for which scientists have sufficient fossil evidence). The change in global temperatures, accompanied by a sharp drop in sea level, destroyed a huge number of species, although in general it recovered quite quickly by the beginning of the next Silurian period.

Vertebrates

Pretty much everything you need to know about life during the Ordovician period is in arandaspice and astraspice. These were two genera of the first jawless, lightly armored prehistoric fish, ranging in size from 12 to 14 cm in length and vaguely resembling giant tadpoles. The bone plates of the Arandaspis and its ilk later evolved 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 develop teeth.

Vegetable world

As in the previous Cambrian, evidence of terrestrial plant life in the Ordovician is quite elusive. If land plants existed, they consisted of microscopic green algae floating on or below the surface of the water. However, it was only after the Silurian period that the first land plants appeared that have solid fossil evidence.

The Ordovician period began about 485 million years ago and continued until about 440 million years ago. This period was identified by Charles Lapworth in 1879 and named after a Celtic tribe called the Ordovicians. Charles Lapworth identified this period because two of his colleagues disputed in which strata certain rocks in north Wales lay. The followers of Adam Sedgwick believed they belonged to the Cambrian period, and the followers of Roderick Murchison believed they belonged to the Silurian period. However, Lapworth believed that these strata belonged to a separate period. However, this period was not recognized by mainstream science until 1960. This year it was officially recognized by the International Geological Congress.

During the Ordovician period, life continued to evolve and become more complex. Organisms formed communities that became more complex and food chains became more intricate, far surpassing those of the Cambrian period. There was an explosion of life in the Ordovician period, although it did not attract as much attention from the scientific world as the Cumbrian explosion. During this time, the number of marine species has quadrupled, and trilobites have become extremely diverse. It was during this time that the first reef-building coral arose.

Mollusks were another group of marine animals that flourished during this time. A number of different molluscs figured prominently, including bivalves, navitolid cephalopods, and gastropods. During this time, the first jawed fish appeared, and the first star fish. Scientists also believe that the first land plants appeared at this time.

Trilobites dominated the oceans during this period, this ecosystem was gradually replaced by a more mixed one. An ecosystem in which a variety of different organisms thrived.

Organisms that include molluscs, bryozoans, brachiopods, and echinoderms. However, during this time, the triobites continued to evolve. They acquired attributes that made them more successful in their environment. Adaptations that include chitinous head shields or spines on the body to protect themselves from predators.

This period came to an end with a series of extinctions of many animal species that mark the boundary between the Ordovician and Silurian periods. It ended approximately 447 - 444 million years ago. During this time, about half of all genera of the fauna will disappear, and many groups have been significantly reduced, including trilobites, brachiopods and bryozoans.

Mainland Lawrence in the Ordovician period broke up into four large and a number of smaller islands. In place of the Russian mainland, two large islands were formed, separated by a narrow strait. Almost half of the territory of the Siberian and Chinese continents was flooded with shallow seas. 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 North Asia. The Northern Tien Shan, Altai, the Australian Cordillera, and the West Siberian ranges begin to form. In the sea basins that existed on the territory of the Urals, Chukotka and the Cordillera, thousands of volcanoes were active, which produced thick deposits of volcanic rocks.

organic world

Algae almost did not change during this period. The marine fauna was characterized by such a wealth of forms that the Ordovician period seems to us the most important era in the entire history of the Earth. It was in the Ordovician that the main types of marine organisms formed.

Compared with 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. Due to the migration of fauna from west to east and adaptation to new conditions in the Ordovician seas, 77 new genera of trilobites appear.

All the most important groups of animals that lived in the seas at a later time were found in the Ordovician deposits. Loose green sandstones near Leningrad contain many foraminifera nuclei. Radiolarians are 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 fish appeared - the first vertebrates. Planktonic radiolarians and foraminifers lived in the water column of the oceans and seas; graptolites flourished. Numerous and diverse trilobites, brachiopods, echinoderms, bryozoans, sponges, laminabranchs, gastropods, and cephalopods lived at the bottom of shallow seas, in coastal zones and on shallows. Corals and other coelenterates lived in warm-water seas.

At the end of the Ordovician, 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 formed from the plates surrounding the mouth opening. One of the new groups - the so-called placoderms (lamellar-skinned fish) - included the largest marine fish of that period, including the ferocious predators of dunkleostei, 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 strongly that the fish could bite and crush the prey with both jaws.