Eras. The Cretaceous period lasted for 79 million years, starting at 145 million years ago and ending at 66 million years ago. In order not to get confused in the eras and periods of the Earth's history, use the geochronological scale, which is located.
“Chalk” received its name due to the rich chalk deposits that are found in the geological strata of this period. It’s worth knowing that the very chalk you use to write in school are fossilized invertebrate marine organisms that lived tens of millions of years ago.
Chalk is divided into two sections - and. The beginning and end of the period differ radically. If the lower and part of the upper chalk are active development life, the emergence of new species, the kingdom and diversity of dinosaurs, then the end of the Upper Cretaceous is a real tragedy for the animal kingdom of that period. A catastrophe on a planetary scale occurred in the Upper Cretaceous, as a result of which all dinosaurs, as well as many species of plants and animals, perished.
During the Cretaceous period, the continental breakup continued. There is no mention left of the former supercontinent Pangea. The continents moved further and further away from each other. Many scientists believe that due to the divergence of continents, the expansion of the Atlantic Ocean, changes in air currents in the atmosphere and ocean currents, the Earth began to cool during the first period of the Cretaceous period. However, at the end of the Cretaceous period, temperatures began to rise. Judging by some hypotheses, the reason for the rise in temperature was the increase in the area of the world's oceans.
Animals of the Cretaceous period
The Cretaceous period is an active development of life of almost all species. The first appeared in the Cretaceous period flowering plants. This led to an increase in the diversity of insects that began to pollinate flowers. The seas were inhabited by such large predators as ichthyosaurs, plesiosaurs, and mososaurs.
Marine animals sometimes reached colossal sizes, for example, ichthyosaurs grew up to 24 meters in length, plesiosaurs - up to 20 meters, mososaurs - up to 14 meters. It is worth noting that they were still not as huge as, for example, the modern Blue whale, reaching a length of 33 meters, however, the Blue Whale is a peaceful creature that feeds on plankton, but predators that reached 20 meters posed a real threat to their victims in the seas.
Giant animals, dinosaurs, existed on land. A large species diversity is observed already in the period, and in the Cretaceous their diversity became even greater. Some dinosaurs were over 10 meters tall and over 20 meters long. These sizes are a record for terrestrial animals.
In addition to large lizards, this period is also noticeable by a wide variety of flying animals. If in our time air environment Only birds mastered it, then in the Cretaceous period there were flying lizards (pterosaurs), lizard-tailed birds and ordinary birds (fan-tailed birds). The largest flying creature in those days was a representative of the pterosaur order Quetzalcoatlus, whose wingspan reached from 12 to 15 meters.
During the same period, the first snakes appeared. Reptiles without legs or limbs, snakes, are considered the youngest group of reptiles. Also, this species of animal, along with some others, was able to survive the upcoming catastrophe and survive to this day.
The Cretaceous also saw a diversity of mammals. If in the Jurassic period there were only small species of warm-blooded mammals, then in the Cretaceous period ungulates, insectivores, predators, as well as the very first primates appeared, which, as everyone knows, became the ancestors of modern people.
Cretaceous-Paleogene extinction
The Cretaceous period and the entire Mesozoic era ended with the mass extinction of animals. The causes of the Cretaceous-Paleogene catastrophe have not yet been reliably elucidated. The most likely cause was the fall of a large asteroid or even several asteroids. In addition, there are also versions such as: increased volcanic activity, changes in weather conditions, excess oxygen in the atmosphere, mass epidemic, excessive development of flowering plants and many others. One way or another, as a result of the mass extinction, all the dinosaurs that developed over many tens of millions of years disappeared. In layers after the Cretaceous-Paleogene extinction, archaeologists no longer find the remains of any dinosaurs, which suggests that none of the dinosaurs could survive. In addition, many aquatic reptiles, flying pterosaurs, ammonites, and brachiopods became extinct. In total, 16% of marine animal families and 18% of land vertebrate families died. Many small reptiles, birds, and warm-blooded animals survived. After the global extinction of animals, mammals began to reign on Earth.
Dinosaurs of the Cretaceous period
Velociraptor
Gainosaurus
Ichthyosaurs
Carnotaurus
Quetzalcoatlus
Majungasaurus
Mosasaurus
Parasaurolophus
Plesiosaurs
Pteranodon
Styracosaurus
Tarbosaurus
Tyrannosaurus
Torosaurus
Triceratops
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Cretaceous period
In the lower reaches of the Volga, in Ukraine near Kharkov and in other places, there are thick layers of white writing chalk.
Look at a grain of chalk under a microscope. You will see that half of it consists of tiny shells covered with holes and their fragments. Shell-dwelling foraminifera (“hole makers”) lived in the sea that covered these places 70–80 million years ago. And they inhabited the sea in such innumerable numbers that from the myriads of their shells, over time, the main sedimentary rock of this period was formed - chalk.
What do Cretaceous fossils tell us?
New species of ray-finned and bony fish appeared in the Cretaceous seas. Ammonites and belemnites lived in the same abundance as in the Jurassic period. But at the end of the period they began to die out.
A mosasaurus appeared in the seas.
Its snake-like body with two pairs of fins and a crocodile head reached a length of 13–15 meters. The fossilized remains of this sea lizard were found near the Meuse River, in Western Europe. Latin name this river is Moza. Mosasaurus is the "lizard from the Mosa River".
Like the ichthyosaur, this reptile hunted for fish.
Snakes first appeared during the Cretaceous period. Their flexible scaly bodies slid through the thickets with a slight rustle. Huge turtles basked in the sun on the sandbanks.
Dinosaurs were still the rulers of the land. New giants appeared among them. We see them in a painting hanging in one of the halls of the museum. It depicts the sea coast of the North Atlantic continent during the Cretaceous.
Cretaceous period. A tyrannosaurus tears apart a herbivorous lizard.
...It's getting evening. The sand, the edge of the grove, light clouds floating in the darkening sky - everything is illuminated by sunset fire.
A dying trachodont lies on the sand, stretching out long neck, half-opening his duck-shaped mouth. The last trembling runs through his 10-meter torso, pressed to the ground by the monster standing on him. This is a tyrannosaurus - a “killer lizard”. Indestructible power emanates from his 14-meter, massive, rock-like body. The fury of the fight still sparkles in the eyes. Huge half-meter claws dug into the body of the prey.
In the distance, at the edge of the forest, a herbivorous Triceratops (“three-horned lizard”), the size of a large elephant, froze in indecision. True, the predator is busy with its prey and the Triceratops itself is well armed: it has three huge, forward-pointing horns on its head and its neck - the most vulnerable place - is protected by a bone collar. But still, it’s better to quickly get away from the dangerous predator...
Triceratops.
The flying pteranodon (“winged toothless lizard”) flies over the clearing on huge leathery wings reaching 8 meters in span. These tailless flying lizards are already becoming extinct. Soon the last flying dragons will disappear and they will be replaced by various breeds of birds.
Pteranodon.
We see one of these ancient birds in the picture. This is a toothy ichthyornis, already reminiscent of modern birds in its structure.
Ichthyornis.
The reign of lizards on Earth lasts for tens of millions of years. There seems to be no force that could resist their power, crush their powerful bodies. Dinosaurs felt equally at home on the sandy shallows of the seashore, in the swamps, and in the thickets of forests. But their bodies had one significant drawback: they were cold-blooded animals that could only live in warm climates. The cooling that occurred on Earth at the end of the Cretaceous period played a fatal role in the life of lizards.
In the Cretaceous period, a new mountain-building cycle began, the so-called Alpine orogeny. The lights of volcanoes sparkled on the coasts of the oceans, where new mountain ranges grew. The rising chains of mountains deprived the land of the beneficial influence of the sea winds.
Warm and humid climate, so favorable for reptiles with cold blood, became increasingly cooler.
The cooling of the climate had a detrimental effect on the lizards. After all, reptiles, like fish and amphibians, do not have a constant body temperature. It depends on the ambient temperature. At low temperatures, reptiles become lethargic and fall into torpor.
The movement of the seas also played a significant role in the beginning of the extinction of lizards.
At the end of the Cretaceous period, the internal forces of the Earth raised the land in many places, forcing the sea to retreat.
The drainage of the swampy lowlands that stretched along the sea coasts sharply worsened the living conditions of the lizards. The sea retreated hundreds and thousands of kilometers, and wetlands began to dry out quickly.
The giant herbivorous lizards who lived here lost shelter and food. With difficulty moving their heavy bodies across the parched ground, exhausted by hunger, they easily became prey predatory dinosaurs. Their mass death, in turn, led to the rapid extinction of the predators that fed on them.
By the beginning of the new, Cenozoic era, dinosaurs were no longer on Earth. But life did not stop in its development, it manifested itself in new, more perfect forms, and took new paths in its development.
Once again, a qualitative restructuring of the entire organic world of the Earth has begun in nature. Mammals became the winners in the struggle for existence.
While the giant reptiles flourished, these small animals, reminiscent of modern marsupial rats, shrews and hedgehogs, were few and far between and led an inconspicuous lifestyle. But now their time has come - the time of warm-blooded animals.
The changed living conditions immediately revealed the enormous advantages of mammals over cold-blooded reptiles.
The mole, fox, bear and other mammals have a constant body temperature: an average of plus 39 degrees, and is maintained at the same level by a number of devices. The lungs of mammals have a large volume and a large respiratory surface. Therefore, oxygen entering the lungs during breathing is immediately absorbed by the blood. Oxygenated blood moves quickly through the vessels, ensuring vigorous metabolic processes and generating large amounts of heat in the body. Hairline and the subcutaneous layer of fat protect the animal from excessive heat loss during the cold season.
The brain of mammals is more developed than that of lizards; teeth serve not only for capturing food, but also for chewing it. They give birth to live young and feed them with milk, caring for and protecting their offspring.
At the end of the Cretaceous period, a variety of mammals began to rapidly spread across the Earth.
An equally profound qualitative restructuring took place in flora.
Angiosperms, or flowering plants, the first forms of which appeared in the Jurassic period, developed rapidly and everywhere.
In angiosperms, the seeds are located inside the fruit, and the reproductive organ is the flower.
Among flowering plants there is greatest number species and there is astonishing species diversity. Angiosperms have extraordinary endurance and adaptability to environmental conditions. They penetrate into deserts, where not a drop of rain falls for months, grow on soils saturated with salt, and inhabit cold tundras and the shores of northern seas, where frosts in winter reach 50 degrees. By the end of the period, forests of palm trees, magnolias, laurels, plane trees, oaks, and maples gradually covered the territory of the continents. Forests interspersed with grassy meadows.
The plains and mountains were full of flowers. Insects appeared in abundance. For the first time in its long history Live nature dressed in a bright, flowery outfit.
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Starting 145 million years ago, it lasted about 79 million years and ended 66 million years ago. Its name comes from the abundance of writing chalk, formed from dying invertebrate organisms, in its sediments. The Cretaceous period is significant for the second largest (after the Permian) worldwide extinction of species.
In 2016, the International Union of Geological Sciences adopted the following division of the Cretaceous:
Periods, geography and climate of the Cretaceous
During the Cretaceous period, the division of Laurasia into the North American continent and the Euro-Asian continent continued. Gondwanaland finally split into the South American continent, the African, Indian segments, Antarctica and Australia. Throughout the Cretaceous, these gigantic land areas diverged more and more from each other, the southern and northern parts of the Atlantic Ocean were no longer connected by narrow straits, but acquired an integral oceanic structure. But despite this, a significant part of Europe, the Middle East, the Caucasus and northern Africa were still under water until the very end of the Cretaceous period.
The climate of the Cretaceous period became noticeably colder compared to the previous Jurassic period. At first, its average temperature across the planet dropped by 5 degrees, which led to the formation of polar ice caps, but after some time the climate warmed again, and in general the entire planet was relatively warm, winter temperatures even in the coldest zones of the globe fluctuated on average within +4°C. By the end of the period, the greenhouse effect caused by side factors led to an even greater and sharp increase in temperature.
Sedimentation
The Cretaceous period is characterized by the maximum flysch accumulation in geosyncal areas in the entire history of the Earth. As a result of violent magmatism caused by the splitting of continental regions, siliceous and split-dibasic formations were formed, and granitoid emissions were extensive and colossal. In general, the accumulation of trigenic and volcanogenic strata was widespread during the Cretaceous period. Such rift zones appeared in Africa and Brazil. IN sea depths Huge layers of writing chalk accumulate.
Animals of the Cretaceous period
The most significant marine invertebrates during the Cretaceous period were cephalopods. In the Upper Cretaceous, the role of external-shells (ammonoids) slightly decreased, but internal-shells (belemnites) were fundamental until the very end of the period. Closer to the middle, some ammonoids, for example, such as ammotoceras, reached 2 meters in size.
Molluscs such as pelecypods (bivalves) and gastropods (gastropods) also developed widely. Most bivalves would become completely extinct by the end of the Cretaceous. Incorrect ones have also developed sea urchins along with large foraminifera.
The insects of the Cretaceous period also felt great, having adapted to existing flowering plants. Due to biological changes in vegetation, they also progressed in their development. All kinds of worms also felt great. The first lobsters and other predatory crustaceans such as crabs and shrimp appeared in coastal seas and oceanic areas.
Dinosaurs
The vertebrates of the Cretaceous period - dinosaurs - stood out because among them, as in the Jurassic period, reptiles reigned supreme (Fig. 1). Among them were crawling creatures, walking on four limbs, moving only on two hind limbs, waterfowl and, of course, flying Hymenoptera. The richness of their diversity and forms was truly amazing. This entire numerous army of reptiles constantly devoured both huge masses of green space and themselves, meanwhile increasing in numbers until, in an incomprehensible way, in the upper Maastrichtian stage of the Cretaceous period it became almost completely and universally extinct.
Rice. 1 – Dinosaurs of the Cretaceous period
The first snakes appeared (Fig. 2). Some grew to truly gigantic size and hunted mainly in aquatic environment, in coastal or river basins. It was not difficult for some of them to wrap around and crush or strangle a gaping one and a half meter raptor.
Rice. 2 – Cretaceous snake
The variety of flying dinosaurs was also great. The real giant was the pteradon, whose wingspan averaged 8 meters. These giant reptiles hunted mainly over the sea, easily diving in air currents and every now and then snatching fish and other representatives of marine fauna from the water.
Birds also developed widely, the first varieties of which appeared in the Jurassic period. In the Cretaceous period, highly organized and specialized formations appeared among them.
And in the depths of the sea, fish with a hard-boned skeleton developed further. The ray-finned offspring of the Triassic and Jurassic multiplied unusually, a huge number of new varieties appeared both among the inhabitants of freshwater and inland basins, and among salty marine and oceanic species (Fig. 3).
Rice. 3 – Marine animals of the Cretaceous period
Despite the undivided dominance of reptiles, mammals still advanced in their evolutionary development in the Cretaceous period. Having appeared on the threshold of the Mesozoic, these beast-like animals (synapsids) slowly but surely waited in the wings throughout the entire era, increasingly adapting to the difficult life in the background. Synapsids often settled in cold areas of continents, where predatory but heat-loving reptiles were rare guests. Those who were forced to live among reptiles in hot areas went out to hunt mainly at night. All this greatly contributed to their adaptation to difficult conditions, which determined the mammals’ survival in the difficult conditions of the asteroid winter that struck the Earth at the end of the Cretaceous.
All synapsids were divided into three main varieties - dicynodonts, cynodonts and allotherians. Dicyodonts and cynodonts almost completely went extinct during the Cretaceous period, and allodonts developed into mammals. During the Late Jurassic and subsequent Cretaceous period, they clearly divided into three branches - oviparous, marsupial and placental. Oviparous animals, unable to withstand competition with marsupials and placentals, soon also disappeared; today marsupials survive only in Australia, and it is from placentals that all subsequent species of modern mammals developed. Placentals at that time were divided into Laurasiatherians and Gondwanatherians. It was Gondwanotheria that were the ancestors of modern rodents and primates.
From the marsupial branch, possum-like animals evolved, and from the oviparous branch, only platypuses remain today. The ancestor of primates is considered to be the ancient mammal Purgatorius.
Basically, mammals of the Cretaceous period (Fig. 4) weighed no more than half a kilogram and rarely exceeded the size of a modern rat. Of course, there were also such rare specimens as meter-long and fourteen-kilogram repenomamas, but they were too few in number.
Rice. 4 – Mammals of the Cretaceous period
For the most part, reptiles owe their extinction to these small creatures, which, having multiplied unusually towards the end of the Cretaceous period, fed mainly on insects, but did not disdain the eggs of reptiles.
Despite the fact that the first flowering plants began to appear long before the Cretaceous period, it was at this time that the formation of flowering vegetation entered a real boom stage. It is no coincidence that half of all currently existing known plants are flowering plants. And this is connected with this.
By spreading spores in the wind, primitive plants took a great risk. And not in vain, since the bulk of the disputes never achieved their intended goal. And many plants of that era had not yet acquired at least some varieties of spore spraying mechanisms. Their spores were forced to fall to the ground, right in the same place where the plants themselves grew. It is clear that with such reproduction it was not possible to achieve a more or less reliable result. Hence the vital need to develop new, more effective techniques pollen spread. And insects came to the aid of plants.
A kind of union began to develop and become stronger between the flower groups. While insects carried pollen from plants, plants produced nectar for them so that they would work more intensively on pollination. In the process of evolution, it turned out that many insects simply could no longer do without flowering plants, since their entire lives and body biology were inextricably linked and aimed at a life associated with such plants. And the plants, with the help of their insect helpers, began to multiply many times faster, and soon dense vegetation spread even to those areas of land where it had never existed before. This type of partnership between plants and insects continues to this day.
Rice. 5 – Plants of the Cretaceous period
Underwater plants of the Cretaceous period were in many ways similar to plants of previous periods of the Mesozoic. The only difference was that microscopic algae such as nanoplankton (for example, golden coccolithophores) and diatoms multiplied unusually. It is nanoplankton and small foramnifera that are responsible for the formation of such thick layers of writing chalk.
Towards completion Mesozoic era The flora of the land has undergone a number of significant changes. From the middle of the Cretaceous period, the first angiosperms began to appear, which towards the end of the Cretaceous already constituted the overwhelming majority among terrestrial plants. The first varieties of plants with foliage of increased succulence began to appear. This most applies to places where the climate has become more arid and hot.
Cretaceous extinction
Occurring at the boundary of the Mesozoic and Cenozoic, or more precisely, in the Maastrichtian final stage of the Upper Division, the Cretaceous extinction of species was the second largest after the Permian. Overnight, coccolithophores ceased to exist, and there were no Cretaceous planktonic foramonifers, ammonites, belemnites, or coral-like bivalves—rudists. Dinosaurs and many other species of reptiles disappeared from the face of the earth. Many species of birds and insects, both above-water and underwater world. In particular, the total number of all kinds of rliolarians decreased by 50%, 75% of all brachiopods, from 30 to 75% of bivalves and gastropods died out, sea lilies and hedgehogs. Only 25% of the total shark population remains. More than 100 different families of marine invertebrates have become extinct. In general, the damage suffered by flora and fauna was truly enormous.
What caused such a mass extinction of species during the Cretaceous period is still not known. Scientists' opinions on this matter are divided. Opinions have also been expressed that powerful cosmic radiation generated as a result of a supernova explosion reached the Earth. Some people talk about a strong greenhouse effect associated with extremely intensified volcanic activity. But the majority support the version that is based on the fall of a giant asteroid to the earth (Fig. 6). This version is confirmed by the presence of iridium inclusions in the strata of this era, which is constantly found in places where meteorites fall.
Rice. 6 – Asteroid impact
It is alleged that an asteroid with a size of 10 to 15 km, entering the earth’s atmosphere at great speed, split into several segments, which collided earth's surface. The explosive energy, amounting to approximately 10 30 ergs, raised a lot of pollutants from the earth's crust, which for a long time blocked plants and animals from accessing sunlight. Thus, as a result of the unique “asteroid winter” that was created, most terrestrial animals became extinct. Apparently, this did not have such an impact on the plant world because the atmosphere cleared in a relatively short period of time. And if the plant seeds were able to safely survive this catastrophe in the soil and soon simply sprouted as if nothing had happened, then animal world The Cretaceous period could not endure this global catastrophe with such ease. And as a result, only the most adapted and more tenacious species survived, such as, for example, mammals.
Minerals of the Cretaceous period
The Cretaceous period is unusually prolific in different kinds minerals, most of which arose as a result of intrusive magmatism and volcanism, which accompanied the worldwide division of Pangea into smaller components. About 20% of coal deposits were accumulated during this time. The largest coal basins of this time are Lensky and Zyryansky, as well as a number of North American coal basins.
Also associated with the Cretaceous period are most Russian, French and Spanish bauxite deposits, Western Siberian oil and gas fields, and oil and gas fields of Kuwait and Canada. Vast oolitic deposits were discovered on the territory of Western Siberia. iron ores. There are also numerous phosphate deposits in the territories of Russia, Morocco, and Syria. Extensive salt deposits have been found on the territory of Turkmenistan and in some North American regions. In the northeast of Russia, in the territory North America deposits of tin, lead and gold were discovered. The famous Indian and South African diamond deposits also belong to the Cretaceous period.
Writer's chalk has been found almost everywhere in Cretaceous sediments.
During the transition to the Middle Cretaceous period, the most important changes occurred in the plant world - the first flowering plants appeared. At the same time, the evolution of huge herbivorous dinosaurs continued.
The first flowering plant, Archaefmctus ("ancient fruit"), is known from rocks of the Lower Cretaceous. Its fossils were found in the Chinese province of Liaodun (in honor of which it received its name - Archaefruclus liaoningensis) 400 km north of Beijing, in an area that 140 million years ago was covered with swampy forest. The fruits of Arcbaefructus bore little resemblance to the fruits of modern plants, they looked more like a pair of leaves wrapped around seeds, however, the presence of a shell surrounding the seeds is the main characteristic of a flowering (angiosperm) plant. Determining the age of the rocks containing these fossils causes some difficulties. While some paleontologists believe that they are no more than 120 million years old, others estimate their age at 140 million years. In any case, Archaefruclus is the oldest known flowering plant.
Among the discoveries of plant fossils from the Late Cretaceous period, especially in regions located in high latitudes with a temperate climate, flowering plants account for 50 to 80%.
A fossilized magnolia leaf found in Upper Cretaceous rocks in Saxony, Germany. Reconstruction of the plant shows that it was very similar to Magnolia (Magnolia grandiflora), a favorite of gardeners.
The increase in the number of flowering species was accompanied by a decrease in the diversity of cycads and ferns, while the proportion of coniferous plant species in the local flora was relatively constant. However, in terms of biomass produced, the main components of terrestrial plant ecosystems at this time remained conifers, ferns and cycads.
Co-evolution?
In the 1970-80s. theories appeared in which the flourishing of angiosperms was associated with an increase in the number of herbivorous dinosaurs. It was stated that "flowering plants were spread by dinosaurs." The idea was that damaged flowering plants nowadays recover faster and more efficiently than gymnosperms (conifers and ferns). In the Cretaceous, the role of modern large cattle, whose grazing sometimes almost completely destroys the vegetation cover, was played by large herbivorous dinosaurs that consumed huge amounts of plant food. Under such conditions, the increased resistance of flowering plants to damage gave them great advantages over gymnosperms.However, recent studies in England have shown that the basic assumptions of these theories are unfounded. First, the distribution of angiosperms did not coincide in time with the peak numbers of herbivorous dinosaurs feeding on low-growing plants, and second, the geographical distribution of these tank-like and bulldozer-like animals did not coincide with the zones of origin and species diversity of flowering plants. Moreover, these theories assumed the dominant position of angiosperms in the plant world of the beginning of the Late Cretaceous period, which is also not true.
The Triceratops depicted in the picture fed on young shoots of plants and most likely led a gregarious lifestyle. Its terrifying horns and bone collar covering its neck provided reliable protection from any predator. These animals reached 7 m in length.
A large diversity of species of a group of plants does not automatically mean its important role in the flora of a given region. For example, the orchid family is now unusually diverse. But in any region where orchids grow, they are found as individual plants and constitute a negligibly small part of the biomass of the local ecosystem. Therefore, it is unlikely that during the Cretaceous period any species of herbivorous dinosaurs, let alone their entire community, fed only on a variety of but rarely found angiosperms.
Social insects
The oldest fossilized remains of termites and ants date back to the late Cretaceous period. The appearance of these insects should have significantly influenced the development of both flora and fauna. This is an important and interesting point in evolution, since it is believed that the body structure of some fossil animals, including small dinosaurs, allowed them to tear apart termite mounds in search of food. But, firstly, some of these animals existed before the appearance of social insects. And, secondly, the fossilized remains of the first social insects do not indicate their life in large communities immediately after their emergence. They became a significant source of food for large animals only after they began to create huge colonies. Nowadays, large animals such as anteaters, aardvarks and aardwolves feed on them.The emergence of flowering plants undoubtedly accelerated the evolution and complicated the organization of communities of such social insects as bees, although finding out the details of the evolution of these small and fragile creatures is a rather difficult task.
Beginning of separation
By the beginning of the Cretaceous period, fossil remains of tetrapods (which include all vertebrates except fish) begin to demonstrate increasing differences between the fauna of the North and Southern hemispheres, although limited exchange of land animals between them still continued. The major change in the fauna of the Northern Hemisphere of this era was a decrease in the number and number of species of giant herbivorous sauropods that fed on the leaves and shoots of tall plants.Along with these herbivorous giants, in the Early Cretaceous period the number of stegosaurs also decreased significantly, which, judging by their structure, were also herbivores and fed on shoots and leaves growing at low and medium altitudes. The slow decline in their numbers was accompanied by the spread of another type of large herbivorous dinosaurs - four-legged ankylosaurs covered with a strong shell, reaching 6 m in length and weighing, it is estimated, up to 3 tons.
Although they, like stegosaurs, occupied the ecological niche of "herbivores that feed on short plants", their wide, massive skulls were radically different from the long, low, small-toothed skulls of stegosaurs. The heads of ankylosaurs were almost completely covered (even the eyelids) by a shell. But, despite the complex structure of the skull, the teeth of the ankylosaur differed little from the teeth of the stegosaurus. The peculiarities of their abrasion made it possible to establish how ankylosaurs crushed food, and to conclude that, most likely, they ate roots, tubers and the core of plants. Differences in feeding habits explain why these two species of herbivorous dinosaurs, occupying almost the same ecological niche, could coexist for a long time. It is also possible that they ate different types of plants.
The gentle giants of the iguanodon settle down for the night. They reached 9 m in length and up to 5 m in height. Next to them was a group of small reptiles called Hypsilophodon. Speed and dexterity helped the “little ones” (their size did not exceed 70 cm) survive.
North and South
Giant sauropods continued to dominate the south during this time, and the dominant herbivorous ornithopods in the Northern Hemisphere, such as hadrosaurs ("duck-billed dinosaurs"), were quite rare here.A feature of the Cretaceous period was the very rapid spread of herbivorous dinosaurs from the suborder of orthithopods in the Northern Hemisphere: hadrosaurs, iguanodons (Iguanodon) and tenontosaurs (Tenontosaurus). They reached much larger sizes at this time than their Jurassic predecessors (eg Camptosaurus), and probably therefore foraged at higher levels.
Tyrannosaurus rex catching prey. The largest of all terrestrial predators that ever existed, it reached 13 m in length and rose 5 m above the ground. It probably used its disproportionately short forelimbs to rise from a lying position. Remains of a Tyrannosaurus rex discovered in the USA. Similar creatures They also lived in Canada and China.
In these ornithopods, there is a clear evolutionary tendency towards a more complex mechanism for chewing food. Their teeth interlocked with their bite, allowing for efficient grinding of hard plant foods. The peculiarities of the connection of the cranial bones of iguanodons allowed the upper jaw to move slightly forward under the pressure of the teeth of the lower jaw. Unlike mammals (such as the camel), reptiles could not chew because they did not have jaw muscles that move the lower jaw laterally. However, the described structural features of ornithopods allowed them to grind food quite well with a longitudinal displacement of the jaws, which probably became one of the main reasons for their wide distribution throughout the Cretaceous period.
Other advanced herbivorous dinosaurs (not belonging to the suborder Ornithopod) appeared in the Late Cretaceous, and in many respects their jaws were even more developed than those of Iguanodons. These were the so-called. horned dinosaurs, or ceratopsians. The first ceratopsians were apparently the bipedal psittacosaurs from the Early Cretaceous of Mongolia and the massive, pig-like protoceratopsians from slightly later rocks. These were massive animals with short limbs and a protective collar around the neck formed by overgrown skull bones (such a collar was absent in psittacosaurs).
Closely related to them were the pachycephalosaurs (“thick-skull lizards”) with massive and durable skulls. IN mating season males used their heads as weapons in fights with rivals. Their descendants, for example, the huge Triceratops, are typical dinosaurs last days prosperity of these huge reptiles.
During the Late Cretaceous period, there was an established and extremely diverse community of herbivorous dinosaurs of all shapes and sizes, which served as prey for the numerous predators of the era. Among the latter there were those who could hunt the largest herbivores.
Animals such as Trodden weighed no more than a modern dog, while the mass of the largest of the carnivorous dinosaurs, the giant tyrannosaurus (Tyrannosaurus rex), according to most scientists, reached 7 tons (according to other estimates, 4 tons). The diversity of dinosaurs' feeding habits and the way they obtained food in this era is amazing. In the late Cretaceous period, at the last stage of dinosaur development, their most progressive forms arose.
million years ago
as of April 2016.
Geology
During the Cretaceous period, the continental breakup continued. Laurasia and Gondwana were falling apart. South America and Africa were moving away from each other, and Atlantic Ocean became wider and wider. Africa, India and Australia also began to diverge in different directions, and giant islands eventually formed south of the equator.
Climate
70 million years ago the Earth was cooling. Ice caps formed at the poles. The winters became harsher. The temperature dropped below +4 degrees in some places. For the dinosaurs of the Cretaceous period, this difference was sharp and very noticeable. Such temperature fluctuations were caused by the breakup of Pangea, and then Gondwana and Laurasia. Sea levels have risen and fallen. The jet streams in the atmosphere have changed, causing ocean currents to change.
At the end of the Cretaceous period, temperatures began to rise sharply. There is a hypothesis that the oceans were the cause of these changes: instead of absorbing heat, they may have reflected it back into the atmosphere. Thus, they caused the greenhouse effect.
Vegetation
Angiosperms - flowering plants - appeared in the Cretaceous period. This led to an increase in the diversity of insects, which became flower pollinators. As temperatures rose during the late Cretaceous period, plants with richer foliage evolved.
Animal world
Among the land animals, a variety of large reptiles reigned. This was the heyday of giant lizards - many dinosaurs reached 5-8 meters in height and 20 meters in length. Winged reptiles - pterodactyls - occupied almost all the niches of aerial predators, although real birds had already appeared. Thus, flying lizards, lizard-tailed birds such as Archeopteryx, and true fan-tailed birds existed in parallel.
There were no mammals in the seas, and the niche of large predators was occupied by reptiles - ichthyosaurs, plesiosaurs, mosasaurs, sometimes reaching 20 meters in length.
The diversity of marine invertebrates was very large. As in the Jurassic, ammonites and belemnites, brachiopods, bivalves and sea urchins were very common. Among bivalves, a major role in marine ecosystems was played by rudists that appeared at the end of the Jurassic - mollusks similar to solitary corals, in which one valve looked like a cup, and the second covered it like a kind of lid.
By the end of the Cretaceous period, many heteromorphic ones appeared among ammonites. Heteromorphs arose earlier, in the Triassic, but the end of the Cretaceous became the time of their mass appearance. The shells of heteromorphs were not similar to the classic spiral-twisted shells of monomorphic ammonites. These could be spirals with a hook at the end, various balls, knots, unfolded spirals. Paleontologists have not yet come to a common explanation of the reasons for the emergence of such forms and their way of life.
Orthoceras were still found in the seas - relics of the long-past Paleozoic era. Small shells of these straight-shelled cephalopods are found in the Caucasus.
Cretaceous disaster
At the end of the Cretaceous period, the most famous and very large extinction of many groups of plants and animals occurred. Many gymnosperms, aquatic reptiles, pterosaurs, and all dinosaurs became extinct (but birds survived). Ammonites, many brachiopods, and almost all belemnites disappeared. In the surviving groups, 30-50% of species became extinct. The causes of the Cretaceous disaster are not fully understood.
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Notes
Literature
- Iordansky N. N. Development of life on earth. - M.: Education, 1981.
- Koronovsky N.V., Khain V.E., Yasamanov N.A. Historical geology: Textbook. - M.: Academy, 2006.
- Ushakov S.A., Yasamanov N.A. Continental drift and climates of the Earth. - M.: Mysl, 1984.
- Yasamanov N.A. Ancient climates of the Earth. - L.: Gidrometeoizdat, 1985.
- Yasamanov N.A. Popular paleogeography. - M.: Mysl, 1985.
Links
- - A site with information about the state of research in the field of Cretaceous stratigraphy and paleogeography in Russia. Library of scientific publications related to the study of Cretaceous deposits.
| ← | P A l e O h O th |
Mesozoic (252.2-66.0 million years ago) | TO A th n O h O th |
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| ← | Triassic (252,2-201,3) |
Jurassic period (201,3-145,0) |
Cretaceous period (145,0-66,0) |
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Excerpt characterizing the Cretaceous period
“And I dare to report: a good deed, your Excellency.”“How easy he thinks it is,” thought Pierre. “He doesn’t know how scary it is, how dangerous it is.” Too early or too late... Scary!
- How would you like to order? Would you like to go tomorrow? – Savelich asked.
- No; I'll put it off a little. I'll tell you then. “Excuse me for the trouble,” said Pierre and, looking at Savelich’s smile, he thought: “How strange, however, that he does not know that now there is no Petersburg and that first of all it is necessary for this to be decided. However, he probably knows, but he’s only pretending. Talk to him? What does he think? - thought Pierre. “No, someday later.”
At breakfast, Pierre told the princess that he had been to Princess Marya yesterday and found there - can you imagine who? - Natalie Rostov.
The princess pretended that she did not see anything more extraordinary in this news than in the fact that Pierre had seen Anna Semyonovna.
- Do you know her? – asked Pierre.
“I saw the princess,” she answered. “I heard that they were marrying her to young Rostov.” This would be very good for the Rostovs; They say they are completely ruined.
- No, do you know Rostov?
“I only heard about this story then.” Very sorry.
“No, she doesn’t understand or is pretending,” thought Pierre. “It’s better not to tell her either.”
The princess also prepared provisions for Pierre's journey.
“How kind they all are,” thought Pierre, “that now, when they probably couldn’t be more interested in this, they are doing all this. And everything for me; That’s what’s amazing.”
On the same day, the police chief came to Pierre with a proposal to send a trustee to the Faceted Chamber to receive the things that were now being distributed to the owners.
“This one too,” thought Pierre, looking into the police chief’s face, “what a nice, handsome officer and how kind!” Now he deals with such trifles. They also say that he is not honest and takes advantage of him. What nonsense! But why shouldn’t he use it? That's how he was raised. And everyone does it. And such a pleasant, kind face, and smiles, looking at me.”
Pierre went to dinner with Princess Marya.
Driving through the streets between the burned-out houses, he was amazed at the beauty of these ruins. The chimneys of houses and fallen walls, picturesquely reminiscent of the Rhine and the Colosseum, stretched, hiding each other, along the burnt blocks. The cab drivers and riders we met, the carpenters who cut the log houses, the traders and shopkeepers, all with cheerful, beaming faces, looked at Pierre and said as if: “Ah, here he is! Let's see what comes out of this."
Upon entering the house of Princess Marya, Pierre was filled with doubt as to the justice of the fact that he was here yesterday, saw Natasha and spoke with her. “Maybe I made it up. Maybe I’ll walk in and not see anyone.” But before he had time to enter the room, in his entire being, after the instant deprivation of his freedom, he felt her presence. She was wearing the same black dress with soft folds and the same hairstyle as yesterday, but she was completely different. If she had been like this yesterday when he entered the room, he could not have failed to recognize her for a moment.
She was the same as he had known her almost as a child and then as the bride of Prince Andrei. A cheerful, questioning gleam shone in her eyes; there was a gentle and strangely playful expression on her face.
Pierre had dinner and would have sat there all evening; but Princess Marya was going to the all-night vigil, and Pierre left with them.
The next day Pierre arrived early, had dinner and sat there all evening. Despite the fact that Princess Marya and Natasha were obviously pleased with the guest; despite the fact that the whole interest of Pierre’s life was now concentrated in this house, by the evening they had talked everything over, and the conversation constantly moved from one insignificant subject to another and was often interrupted. Pierre stayed up so late that evening that Princess Marya and Natasha looked at each other, obviously waiting to see if he would leave soon. Pierre saw this and could not leave. He felt heavy and awkward, but he kept sitting because he couldn’t get up and leave.
Princess Marya, not foreseeing an end to this, was the first to get up and, complaining of a migraine, began to say goodbye.
– So you’re going to St. Petersburg tomorrow? – said oka.
“No, I’m not going,” Pierre said hastily, with surprise and as if offended. - No, to St. Petersburg? Tomorrow; I just don't say goodbye. “I’ll come for the commissions,” he said, standing in front of Princess Marya, blushing and not leaving.
Natasha gave him her hand and left. Princess Marya, on the contrary, instead of leaving, sank into a chair and looked sternly and carefully at Pierre with her radiant, deep gaze. The fatigue she had obviously shown before was now completely gone. She took a deep, long breath, as if preparing for a long conversation.
All of Pierre's embarrassment and awkwardness, when Natasha was removed, instantly disappeared and was replaced by excited animation. He quickly moved the chair very close to Princess Marya.
“Yes, that’s what I wanted to tell you,” he said, answering her glance as if in words. - Princess, help me. What should I do? Can I hope? Princess, my friend, listen to me. I know everything. I know I'm not worthy of her; I know it's impossible to talk about it now. But I want to be her brother. No, I don't want to... I can't...
He stopped and rubbed his face and eyes with his hands.
“Well, here,” he continued, apparently making an effort on himself to speak coherently. “I don’t know since when I love her.” But I have loved only her, only one, all my life and love her so much that I cannot imagine life without her. Now I don’t dare ask her hand; but the thought that maybe she could be mine and that I would miss this opportunity... opportunity... is terrible. Tell me, can I have hope? Tell me what should I do? “Dear princess,” he said, after being silent for a while and touching her hand, since she did not answer.
“I’m thinking about what you told me,” answered Princess Marya. - I'll tell you what. You’re right, what should I tell her about love now... - The princess stopped. She wanted to say: it is now impossible to talk to her about love; but she stopped because for the third day she saw from Natasha’s sudden change that not only would Natasha not be offended if Pierre expressed his love to her, but that this was all she wanted.