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The Mesozoic is divided into three time periods: the Triassic (245-208 Million Years Ago), the Jurassic (208-146 Million Years Ago), and the Cretaceous (146-65 Million Years Ago).

Mesozoic means "middle animals", and is the time during which the world fauna changed drastically from that which had been seen in the Paleozoic. Dinosaurs, which are perhaps the most popular organisms of the Mesozoic, evolved in the Triassic, but were not very diverse until the Jurassic. Except for birds, dinosaurs became extinct at the end of the Cretaceous. Some of the last dinosaurs to have lived are found in the late Cretaceous deposits of Montana in the United States.

The Mesozoic was also a time of great change in the terrestrial vegetation. The early Mesozoic was dominated by ferns, cycads, ginkgophytes, bennettitaleans, and other unusual plants. Modern gymnosperms, such as conifers, first appeared in their current recognizable forms in the early Triassic. By the middle of the Cretaceous, the earliest angiosperms had appeared and began to diversify, largely taking over from the other plant groups.

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In the Paleozoic Era, life flourished in the seas. After the Cambrian Period came the 45-million-year Ordovician Period, which is marked in the fossil record by an abundance of marine invertebrates. Perhaps the most famous of these invertebrates was the trilobite, an armored arthropod that scuttled around the seafloor for about 270 million years before going extinct.

After the Ordovician Period came the Silurian Period (443 million years ago to 416 million years ago), which saw the spread of jawless fish throughout the seas. Mollusks and corals also thrived in the oceans, but the big news was what was happening on land: the first undisputed evidence of terrestrial life.
Paleozoic evolution

Life continued its march in the late Paleozoic. The Carboniferous Period, which lasted from about 359 million years ago to 299 million years ago, answered the question, "Which came first — the chicken or the egg?" definitively. Long before birds evolved, tetrapods began laying eggs on land for the first time during this period, allowing them to break away from an amphibious lifestyle.

Trilobites were fading as fish became more diverse. The ancestors of conifers appeared, and dragonflies ruled the skies. Tetrapods were becoming more specialized, and two new groups of animals evolved. The first were marine reptiles, including lizards and snakes. The second were the archosaurs, which would give rise to crocodiles, dinosaurs and birds. Most creepily, this era is sometimes referred to as the "Age of the Cockroaches," because roaches' ancient ancestor (Archimylacris eggintoni) was found all across the globe during the Carboniferous.

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It is the longest era in Earth's existence and is considered a Supereon because it is divided further into several eons. The three eons are known as the Hadean

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The Precambrian period began with the formation of Earth around 4.5 billion years ago and ended with the first sign of complex life around 540 million years ago. Explore the events and facts of this period and learn about eons of the Precambrian including the Hadean, Archean, and Proterozoic eon.
All life-forms were long assumed to have originated in the Cambrian, and therefore all earlier rocks were grouped together into the Precambrian. Although many varied forms of life evolved and were preserved extensively as fossil remains in Cambrian sedimentary rocks, detailed mapping and examination of Precambrian rocks on most continents have revealed that additional primitive life-forms existed approximately 3.5 billion years ago. Nevertheless, the original terminology to distinguish Precambrian rocks from all younger rocks is still used for subdividing geologic time.

The Precambrian is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of the Phanerozoic Eon, which is named after Cambria, the Latinised name for Wales, where rocks from this age were first studied.

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The first multicelled animals appeared in the fossil record almost 600 million years ago. Known as the Ediacarans, these bizarre creatures bore little resemblance to modern life-forms. They grew on the seabed and lacked any obvious heads, mouths, or digestive organs. Fossils of the largest known among them, Dickinsonia, resemble a ribbed doormat. What happened to the mysterious Ediacarans isn't clear. They could be the ancestors of later animals, or they may have been completely erased by extinction.
The earliest multicelled animals that survived the Precambrian fall into three main categories. The simplest of these soft-bodied creatures were sponges. Lacking organs or a nervous system, they lived by drawing water through their bodies and filtering out food particles. The cnidarians, which included sea anemones, corals, and jellyfish, had sac-like bodies and a simple digestive system with a mouth but no anus. They caught food using tentacles armed with microscopic stinging cells. The third group, the annelids, or segmented flatworms, had fluid-filled body cavities and breathed through their skins.
It's thought the final stages of Precambrian time were marked by a prolonged global ice age. This may have led to widespread extinctions, mirroring the bleak endings to the geologic periods that followed.

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Although the Precambrian contains some seven-eighths of Earth's history, its fossil record is poor, with the majority of fossils being the stromatolites that are often heavily metamorphosed or deeply buried. However, preserved cells have been discovered at selective sites, such as the 2.0 billion yar old Gunflint Formation. The earliest life forms were prokaryotes (eubacteria or archaea) that evolved in the seas, possibly as early as 3.8 Ba. The first were possibly chemotrophs existing in an anoxic world and producing H2S or CO2, which were followed by photosynthetic cyanobacteria before the end of the Archaean some 2.5 billion years ago. When the Eukaryotes (single-celled organisms with a nucleus) evolved through Endosymbiosis is disputed, with claims as early as 3.4 billion years ago, but with less equivocal fossils dating from 1.8 to .8 billion yars ago. With the eukaryotes comes sexual reproduction, enabling genetic diversity and the concomitant ability to adapt to and survive environmental changes. Multi-celled, soft-bodied marine fossil organisms (the metazoans), the so-called Ediacara fauna, are found in strata dating between 590 to 700 million years ago. The first mineralized fossils appear after the Ediacaran, but before Cambrian begins at around 580 - 590 my; they comprise ambiguous parts, possibly denticles and plates and tubes of unknown affinity and putative calcareous algae. Many of the genes and the proteins they encode are found to be conserved across geologic time from the Precambrian, especially those involved in the most basic cellular functions.

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During the Paleozoic Era (541 to 252 million years ago) Fish diversified and marine organisms were very abundant during the Paleozoic. Common Paleozoic fossils include trilobites and cephalopods such as squid, as well as insects and ferns. The greatest mass extinction in Earth's history ended this era.

The Paleozoic Era is the oldest of the three Eras and dates from 540 Million to 248 Million Years Ago. During the Paleozoic Era multicelled living things acquired hard body parts, bones, vertebral columns, mandibles, and teeth. Common in the Paleozoic Era were trilobites, crinoids, brachiopods, fish, insects, amphibians, and early reptiles.

Arthropods, molluscs, fish, amphibians, synapsids, and diapsids all evolved during the Paleozoic. Life began in ocean but eventually transitioned onto land, and by the late Paleozoic, it was dominated by various forms of organisms.

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The Cenozoic era is also known as the Age of Mammals because the extinction of many groups of giant mammals, allowing smaller species to thrive and diversify because their predators no longer existed.

The Cenozoic Era (66 million years ago through today) is the "Age of Mammals." Birds and mammals rose in prominence after the extinction of giant reptiles. Common Cenozoic fossils include cat-like carnivores and early horses, as well as ice age fossils like wooly mammoths.

Tertiary sediments are widespread in Wyoming. The climate during the Tertiary supported growth of thick forests. As a result, plant fossils are common in rocks from this period. Much of Wyoming's economic coal deposits are found in Tertiary sediments.

Cenozoic Era, third of the major eras of Earth’s history, beginning about 66 million years ago and extending to the present. It was the interval of time during which the continents assumed their modern configuration and geographic positions and during which Earth’s flora and fauna evolved toward those of the present.

The term Cenozoic, originally spelled Kainozoic, was introduced by English geologist John Phillips in an 1840 Penny Cyclopaedia article to designate the most recent of the three major subdivisions of the Phanerozoic Eon. Derived from the Greek for recent life, it reflects the sequential development and diversification of life on Earth from the Paleozoic (ancient life) through the Mesozoic (middle life). Today, the Cenozoic is internationally accepted as the youngest of the three subdivisions of the fossiliferous part of Earth history.

Cenozoic Era: Stratigraphy. The Cenozoic is divided into two main sub-divisions: the Tertiary and the Quaternary. Most of the Cenozoic is the Tertiary, from 65 million years ago to 1.8 million years ago. The Quaternary includes only the last 1.8 million years.

Several of the world’s great mountain ranges were built during the Cenozoic. The main Alpine orogeny, which produced the Alps and Carpathians in southern Europe and the Atlas Mountains in northwestern Africa, began roughly between 37 million and 24 million years ago. The Himalayas were formed some time after the Indian Plate collided with the Eurasian Plate. These lofty mountains marked the culmination of the great uplift that occurred during the late Cenozoic when the Indian Plate drove many hundreds of kilometres into the underbelly of Asia. They are the product of the low-angle underthrusting of the northern edge of the Indian Plate under the southern edge of the Eurasian Plate.

From about five million years ago, the Rocky Mountains and adjoining areas were elevated by rapid uplift of the entire region without faulting. This upwarping sharply steepened stream gradients, enabling rivers to achieve greater erosional power. As a result, deep river valleys and canyons, such as the Grand Canyon of the Colorado River in northern Arizona, were cut into broad upwarps of sedimentary rock during late Cenozoic time.

On a global scale the Cenozoic witnessed the further dismemberment of the Northern Hemispheric supercontinent of Laurasia: Greenland and Scandinavia separated during the early Cenozoic about 55 million years ago, and the Norwegian-Greenland Sea emerged, linking the North Atlantic and Arctic oceans. The Atlantic continued to expand while the Pacific experienced a net reduction in size as a result of continued seafloor spreading. The equatorially situated east–west Tethyan seaway linking the Atlantic and Pacific oceans was modified significantly in the east during the middle Eocene—about 45 million years ago—by the junction of India with Eurasia, and it was severed into two parts by the confluence of Africa, Arabia, and Eurasia during the early Miocene approximately 18 million years ago. The western part of the Tethys evolved into the Mediterranean Sea not long after it had been cut off from the global ocean system about 6 million to 5 million years ago and had formed evaporite deposits which reach up to several kilometres in thickness in a land-locked basin that may have resembled Death Valley in present-day California. Antarctica remained centred on the South Pole throughout the Cenozoic, but the northern continents converged in a northward direction.

The global climate was much warmer during the early Cenozoic than it is today, and equatorial-to-polar thermal gradients were less than half of what they are at present. Cooling of Earth began about 50 million years ago and, with fluctuations of varying amounts, has continued inexorably to the present interglacial climatic period. It is to be noted that a unique feature of the Cenozoic was the development of glaciation on the Antarctic continent about 35 million years ago and in the Northern Hemisphere between 3 million and 2.5 million years ago. Glaciation left an extensive geologic record on the continents in the form of predominantly unconsolidated tills and glacial moraines, which in North America extend in a line as far south as Kansas, Illinois, Ohio, and Long Island, New York, and on the ocean floor in the form of ice-rafted detritus dropped from calving icebergs.

Cenozoic life was strikingly different from that of the Mesozoic. The great diversity that characterizes modern-day flora is attributed to the explosive expansion and adaptive radiation of the angiosperms (flowering plants) that began during the Late Cretaceous. As climatic differentiation increased over the course of the Cenozoic, flora became more and more provincial. Deciduous angiosperms, for instance, came to predominate in colder regions, whereas evergreen varieties prevailed in the subtropics and tropics.

Fauna also underwent dramatic changes during the Cenozoic. As was discussed in earlier sections, the end of the Cretaceous brought the eradication of dinosaurs on land and of large swimming reptiles (e.g., ichthyosaurs, mosasaurs, and plesiosaurs) in marine environments. Nektonic ammonites, squidlike belemnites, sessile reef-building mollusks known as rudistids, and most microscopic plankton also died out at this time. The Cenozoic witnessed a rapid diversification of life-forms in the ecological niches left vacant by this great terminal Cretaceous extinction (or K–T extinction). In particular, mammals, which had existed for more than 100 million years before the advent of the Cenozoic Era, experienced substantial evolutionary radiation. Marsupials developed a diverse array of adaptive types in Australia and South America free from the predations of carnivorous placental mammals. The placental mammals, which today make up more than 95 percent of known mammals, radiated at a rapid rate. Ungulates (or hoofed mammals) with clawed feet evolved during the Paleocene (66 million to about 55.8 million years ago). This epoch saw the development and proliferation of the earliest perissodactyls (odd-toed ungulates, such as horses, tapirs, rhinoceroses, and two extinct groups, the chalicotheres and titanotheres) and artiodactyls (even-toed ungulates, including pigs, peccaries, hippopotamuses, camels, llamas, chevrotains, deer, giraffes, sheep, goats, musk-oxen, antelopes, and cattle). During the later Cenozoic, perissodactyl diversity declined markedly, but artiodactyls continued to diversify. Elephants, which evolved in the late Eocene about 40 million years ago, spread throughout much of the world and underwent tremendous diversification at this time. Many placental forms of giant size, such as the sabre-toothed cat, giant ground sloths, and woolly mammoths, inhabited the forests and the plains in the Pliocene (5.3 million to 1.8 million years ago). It was also about this time that the first hominids appeared. Early modern humans, however, did not emerge until the Pleistocene.

Among marine life-forms, mollusks (primarily bivalves [pelecypods] and gastropods) became highly diversified, as did reef-building corals characteristic of the tropical belt. Planktonic foraminiferans (pseudopod-using unicellular organisms protected by a test or shell) underwent two major radiations—the first in the Paleocene and the second in the Miocene—punctuated by a long (15-million–20-million-year) mid-Cenozoic reduction in diversity related in all likelihood to global cooling.

Cenozoic life was affected significantly by a major extinction event that occurred between 10,000 and 8,000 years ago. This event, which involved the sudden disappearance of many mammals after the most recent Ice Age, has been attributed to either of two factors: climatic change following the melting of the most recent Pleistocene glaciers or overkill by Paleolithic hunters. The latter is regarded by many as the more likely cause, as the rapidly improved technology of Paleolithic humans permitted more efficient hunting.

The Cenozoic era.which began about 65million years ago and continue into the present is the third documented era in the history of earth .the current location of the continue and their modern day inhabitants including humans can be traced to this period

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Mesozoic Era, second of Earth's three major geologic eras of Phanerozoic time. Its name is derived from the Greek term for “middle life.” The Mesozoic Era began 252.2 million years ago, following the conclusion of the Paleozoic Era, and ended 66 million years ago, at the dawn of the Cenozoic Era.

Earth during the Mesozoic era was much warmer than today, and the planet had no polar ice caps. During the Mesozoic, or "Middle Life" era, life diversified rapidly and giant reptiles, dinosaurs and other monstrous beasts roamed the Earth.
The era began in the wake of the Permian–Triassic extinction event, the largest well-documented mass extinction in Earth's history, and ended with the Cretaceous–Paleogene extinction event, another mass extinction whose victims included the non-avian dinosaurs. The Mesozoic was a time of significant tectonic, climatic, and evolutionary activity. The era witnessed the gradual rifting of the supercontinent Pangaea into separate landmasses that would move into their current positions during the next era. The climate of the Mesozoic was varied, alternating between warming and cooling periods. Overall, however, the Earth was hotter than it is today. Dinosaurs first appeared in the Mid-Triassic, and became the dominant terrestrial vertebrates in the Late Triassic or Early Jurassic, occupying this position for about 150 or 135 million years until their demise at the end of the Cretaceous. Archaic birds appeared in the Jurassic, having evolved from a branch of theropod dinosaurs, then true toothless birds appeared in the Cretaceous. The first mammals also appeared during the Mesozoic, but would remain small—less than 15 kg (33 lb)—until the Cenozoic. The flowering plants appeared in the Early Cretaceous period and would rapidly diversify throughout the end of the era, replacing conifers and other gymnosperms as the dominant group of plants.
Life
Marine invertebrate life during the Mesozoic was dominated by the Mollusca, of which the ammonites were the most important. Other important invertebrate groups were the bivalves, brachiopods, crinoids, and corals.

The vertebrates of this era were dominated by reptiles, especially the dinosaurs. All the major groups were established by the Triassic, and this period marked the spread of these reptiles into almost every major habitat. The dinosaurs underwent extinction at the end of the Cretaceous. Mammals evolved from therapsid reptiles during the Triassic, and the first birds appeared during the Jurassic.

Mesozoic plants consisted of the ferns and the gymnosperm orders of cycads, ginkgos, and conifers. Angiosperms, which may have first appeared in the Triassic Period, became well established by the end of the Mesozoic.