Geological Time

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Stratigraphy / Precambrian Time
The Phanerozoic Eon
References

Scientists divide the Earth into a number of periods - the "Geological time-scale", according to the rock types and sort of fossils found in each one. These divisions are pretty arbitrary, like any man-made divisions, but they at least can serve as useful labels. So the Paleozoic, the era of "ancient life" is characterized by fossils of invertebrates, primitive tetrapods, etc; the Mesozoic or era of "middle life", by fossils of dinosaurs etc, and the Cenozoic or era of "recent life" by mammals and modern plants and invertebrates.

These eras are divided into periods, the system of which was established by the end of the last century. The periods are in turn divided into epochs, and the epochs are divided into ages called ages. (more on these subdivisions)

Scientists know these periods lasted for millions of years, because they can date them with a fair degree of accuracy according to the amount of radioactivity that occurs in the rocks.

The Geological time-scale can also be used to define the major stages in the history of life on Earth. Often each era ends with a major extinction, which eliminates the dominant life forms of the time and paves the way for newcomers

Hadean Eon / This era begins with the formation of the Solar System and Earth, outgassing of first atmosphere and oceans, bombardment by left-over planetessimals and debris. The name says it all; a hellish period lasting some 760 million years, when the Earth was subject to frequent bombardment by comets, asteroids, and other planetary debris. At one point, early in this era, the moon was formed when a Mars-sized body struck the original Earth, pulverizing both. Yet incredibly, some evidence suggests that the first primitive life may have emerged even at this early stage. This eon was an era characterized by extensive volcanism and formation of the first crust. By the end of the Hadean, the Earth had an atmosphere (unbreathable to most organisms today), and oceans filled with water.
Archean Eon / Lasting more than twice as long as the Phanerozoic eon, the Archean was a time when diverse microbial life flourished in the primordial oceans, and the continental shields developed from volcanic activity. The reducing (anaerobic) atmosphere enabled archea (anaerobic microbes) to develop, and plate tectonics followed a regime of continental drift different to that of the Proterozoic and later. During this era, one type of organism, the Cyanobacteria (blue-green algae) produced oxygen as a metabolic by-product; the eventual build-up of this highly reactive gas was to eventually prove fatal to many life-forms, and converted the atmosphere from.
Proterozoic Eon / The Proterozoic, which lasted even longer than the Archean Era, saw the atmosphere changes from reducing to oxygenated, driving the original anaerobic inhabitants of the Earth into a few restricted anoxic refuges and enabling the rise of aerobic life (both prokaryote and the more complex eukaryotic cell, which requires the high octane boost that oxygen enables.) Stromatolites (colonial cyanobacteria), which had appeared during the Archean, were common. The modern regime of continental drift began, and saw the formation of supercontinent of Rodinia, and several extensive ice ages. Late in the Proterozoic a runaway icehouse effect meant that the preceding warm conditions were replaced by a "Snowball Earth" with ice several kilometers deep covering the globe. Warming conditions saw the short-lived Edicarian biota and finally the appearance of first metazoa.
Paleozoic Era / Early in the 300 million year history of the Paleozoic, atmospheric oxygen reached its present levels, generating the ozone shield that screens out ultraviolet radiation and allows complex life to live in the shallows and finally on land. This era witnessed the age of invertebrates, of fish, of tetrapods, and (during the Permian) reptiles. From the Silurian on, life emerged from the sea to colonize the land, and in the later Paleozoic pteridophyte and later gymnospermous plants flourished. The generally mild to tropical conditions with their warm shallow seas were interspersed with Ordovician and Permo-Carboniferous ice ages. Towards the end of the Paleozoic the continents clustered into the supercontinent of Pangea, and increasingly aridity meant the end of the great Carboniferous swamps and their unique flora and fauna. The Paleozoic was brought to an end by the end Permian mass-extinction, perhaps the most severe extinction the planet has seen.
Mesozoic Era / Lasting little more than half the duration of the Paleozoic, this was a spectacular time. The generalized archosaurian reptiles of the Triassic gave way to the dinosaurs, a terrestrial megafauna the like of which the Earth has not seen before or since. While dinosaurs dominated the land, diverse sea-reptiles ruled the oceans, and invertebrates, especially ammonites, were extremely diverse. Pterosaurs and later birds took to the sky. Mammals however remained small and insignificant. Climatic conditions remained warm and tropical worldwide. The supercontinent of Pangea broke up into Laurasia and Gondwana, with different dinosaurian faunas evolving on each. During this era modern forms of corals, insects, new fishes and finally flowering plants evolved. At the end of the Cretaceous period the dinosaurs and many other animals abruptly died out, quite likely the result of an asteroid impact and associated extensive volcanism (acid rain)
Cenozoic Era / With the extinction of the dinosaurs and the end of the Mesozoic, the mammals swiftly inherit the Earth. Archaic mammals co-existed with birds and modern reptiles and invertebrates. The current continents emerged, and the initial tropical conditions were replaced by a colder drier climate, possibly caused by the Himalayan uplift. The appearance of grass meant the rise of grazing mammals, and the cooler drier world allowed modern mammalian groups to evolve, along with other lineages now extinct and a few archaic hold-overs. Among the newcomers were the anthropoid apes that culminated in the australopithecine hominids of Africa. Decreasing temperatures and a polar landmass of Antarctica resulted in a new Ice Age. Most recently, in the blink of an eye geologically speaking, this era saw the rise of Man (Homo erectus, Neanderthal and Cro Magnon) and use of stone tools and fire, the extinction of Megafauna, and civilization and human activities that have transformed the globe, but at a cost of great environmental destruction.

The Age of Ancient Life

Of the three main eras that make up the Phanerozoic, the Paleozoic is the longest and most diverse, spanning the period from very early multicellular life that only inhabited the oceans to quite advanced tetrapods* and reptiles and extensive forests on land.

Early Paleozoic: Age of Invertebrates
Coelomate radiation (Cambrian explosion) - origin of major groups of organisms; nervous system, behavior patterns and simple consciousness (the nascent Noosphere); continents drift apart.

Middle Paleozoic: Age of Fish
Tropical conditions. Extinction of many "experimental" animal groups; diversification of surviving invertebrate groups, rise of vertebrates (fish). Life moves on land (rhyniophytes, lycopods, uniramious arthropods, proto-amphibians)

Late Paleozoic: Age of Tetrapods* and Reptiles
Ice age. Coal forests of giant lycopods, calamites, pteridophytes and ferns cover the tropical landmasses. Southern landmass of Gondwanaland buried under glaciers; continents drift together.Reptiles conquer the land.
More on the Paleozoic

The Age of Middle Life

The Mesozoic has been called the "age of reptiles", but "age of dinosaurs" would be more appropriate. There is still controversy over whether dinosaurs really were stupid sluggish ectotherms ("reptiles") or active high-metabolism (endotherm) creatures more like birds. Even if we define them as "reptiles" the age of reptiles as such begins in the Permian period of the Paleozoic era anyway.

Tropical (Greenhouse) Conditions. Pangaea continues during the early Triassic; then landmasses begin to drift apart. Shallow oceans cover much of the continents, breaking the land into large islands.Mammals remain small, possibly nocturnal. Most modern groups of organisms appear. Vertebrate animals (mammals, birds, theropod dinosaurs) develop larger brains then their earlier reptilian ancestors.

The Age of Recent Life

Last of all, the Cenozoic - also spelt "Cainozoic" - is the age of mammals. During this period, following the extinction of the dinosaurs, mammals evolved from small shrew-like types into all the diverse types around today, as well as many different prehistoric forms.

The modern world. Land masses take their present shape. "Intelligence race" as herbivores develop larger brains and carnivores do likewise. The climate, originally tropical, becomes increasingly more seasonal as ice age conditions develop, possibly triggered by the rise of the Himalayan mountain uplift.
More on the Cenozoic

Triassic Period
The Triassic Period represents a period of time 250 – 200 million years ago.
During this period the world was very different, consisting of one large super-continent known as Pangaea. England lay near the equator where hot desert conditions prevailed. Mountains in the West were eroded by huge rivers that flowed north, depositing pebbles and sands across southern England that later solidified to form the Aylesbeare Mudstones, Otter Sandstone and Budleigh Salterton Pebble Beds. The desert lakes that formed around the rivers were subject to intense evaporation, creating a concentrate of minerals and salts that remain in the cliffs today as gypsum deposits.
Triassic life largely consisted of survivors from a mass extinction at the end of the previous Period, the Permian. These first dinosaurs evolved and went on to dominate life during the following Jurassic and Cretaceous Periods that together with the Triassic, form the Mesozoic Era. One of the most numerous of creatures were Beaked Lizards, the Rynchosaurs, which grazed on the vegetation growing alongside the rivers.
By the end of the Triassic Period many early living groups of four legged animals had developed, including creatures such as frogs, turtles and crocodiles.
On land some reptiles evolved to become warm blooded, creating the first mammals. /

The Jurassic Period
The Jurassic Period represents a period of time 200 – 140 million years ago.
During this period the super-continent of Pangaea started to break up to form separate, smaller continents. England gradually drifted North with the Atlantic Ocean widening as the Americas drifted away from Europe. The earth was warm and as there was very little ice at the polar caps the sea levels were high and the desert landscape of southern England was submerged beneath a tropical sea.
The sea levels fluctuated in a series of cycles during the Jurassic Period. Clays were deposited on the deepest ocean floors, followed by Sandstones and finally shallow water Limestones. The sea was shallowest during the Mid-Jurassic, creating and environment of islands surrounded by shallow shoals, similar to the Caribbean today. The sea levels rose again before becoming shallow enough in the Late Jurassic for trees to grow on exposed areas of land. These tropical forests grew in swampy conditions and their remains can be seen in the layer of Jurassic Sediment, Dirt Bed at Lulworth and Portland.
Jurassic life was dominated by the sea, with ammonites, belemnites and fish being hunted by predatory air breathing marine reptiles such as Ichthyosaurs and Plesiosaurs. Above the waves soared flying reptiles. Dimorphodon hunted by using the sharp teeth on the bottom part of its beak to scoop up prey from the sea. /

The Cretaceous Period
The Cretaceous Period represents a period of time 140 – 65 million years ago.
During this period the gulf between America and Europe continued to grow as the continents drift further apart, opening up the expanse of the Atlantic Ocean.
The Early Cretaceous Period marked a time when shallow lagoons and slat flats similar to the sabkhas found in the Gulf of Arabia today.
Later lush swamps developed, providing a hunting ground for dinosaurs, but all that changed in the Mid-Cretaceous when tectonic plate movements under south-west England tilted the rocks to the east, and a vast sea developed over the area. The water was clear and warm allowing plankton to flourish. Over millions of years their tiny skeletons sank to the ocean floor and solidified to form Chalk.
Cretaceous life within southern England is mainly represented by marine creatures such as fish, sea urchins, crocodile and turtle. Although the seas also contained air breathing marine reptiles like the Plesiosaur, and Pterosaurs flew overhead in the skies, remains of these are rarely found within the World Heritage Site. But dinosaur footprints from the Early Cretaceous have been found, showing where they crossed shallow lagoons between flooded sections of land. /

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