The Cambrian is famous for the incoming of hard-shelled fossils for the first time, and marks the start of the Phanerozoic. The system was named from the Cambrian Mountains in Wales by Adam Sedgwick in 1835 where he had found the oldest rocks with the distinctive trilobites preserved. Since then, Cambrian rocks have been recognised in many places around the world, and the base of the Cambrian is now formally defined in Newfoundland, Canada, and has been dated at 541 Ma. It is now known that other smaller shelly fossils preceded the trilobites, and are found in sediments older than those preserved in Wales, and trilobites are first known only from rocks which are dated from some time into the Early Cambrian at about 535 Ma.
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The Ordovician is noted for the very large amounts of tectonic activity and volcanism that occurred in many regions; and also because of the oceanic distances separating many of the major continents which caused the distinctive faunal provinces that are found in the marine benthos of the continental shelves. There was also exceptional biological radiation, the Great Ordovician Biodiversification Event.
Because Adam Sedgwick and Roderick Murchison had disagreed so vehemently as to where the top of Sedgwick’s Cambrian and the base of Murchison’s Silurian should be taken, it was not until after the deaths of both men that Charles Lapworth suggested in 1879 that the rocks of overlapping ages previously in contention should be named the Ordovician System, a compromise that eventually became universally accepted. Because the originally disputed rocks are largely in Wales, Lapworth named his system after an old Welsh native tribe, the Ordovices. The global stratotype of the Cambrian-Ordovician boundary is now formally defined at the shelf edge of the Laurentian Craton in the Cow Head Peninsula of western Newfoundland, Canada, and is estimated at 487 Ma.
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The Silurian has the shortest length of any of the Palaeozoic systems, only some 24 million years; however, within it occurred one of the most important events in the Phanerozoic, the collision of the substantial continents of Laurentia with the combined Avalonia-Baltica in the Caledonide Orogeny.
Roderick Murchison in 1835 named the Silurian System after the Silures, an ancient Welsh Celtic tribe, but that early definition was imprecise on the limits of the system, and thus it was left to Lapworth in 1879 to define its lower boundary immediately above his new Ordovician system and at the base of the Llandovery Series, also in Wales. That base is now recognised formally as at the base of the Akidograptus ascensus graptolite Biozone at Dob’s Linn, Scotland, and is dated at 443 Ma.
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Although fossil vertebrates are now known to occur sporadically in earlier rocks, the Devonian is the oldest system in which they became common, particularly the fish which have been known for over two centuries from the Old Red Sandstone of northern Europe and North America. The period is also notable for the first substantial invasion of the land by more advanced organisms, both plants, including trees, and also animals. Adam Sedgwick and Roderick Murchison in 1837 together named the Devonian System after the south-western English county of Devonshire, where, in contrast to the continental deposits of the Old Red Sandstone seen in most of Britain, marine rocks of the same age occur. However, the original concept of its definition and extent has been much modified since then, and the base of the system is now formally defined at the base of the Monograptus uniformis graptolite Biozone, which is at the base of the Lochkovian Stage at Klonk in the Czech Republic, and has been dated at 419 Ma.
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It was during the Carboniferous that the majority of previously-independent continents merged to form the only Phanerozoic supercontinent, Pangea. ‘Carboniferous’ means ‘coal-bearing’ and the name was coined by William Conybeare and William Phillips in the 1820s for such rocks in northern England: coal which formed the major source of power for the Industrial Revolution of the eighteenth century. It was the one of the first two geological systems to receive a name which is still in current use (the other is the Cretaceous, which also reflects a common rock type, rather than a geographical area like most of the others). However, the concept of the system has subsequently been much changed and enlarged, and its base is now defined at the base of the Tournaisian by the start of the Siphonodella waubaunsensis conodont Biozone in the Montagne Noire, France, which is dated at 359 Ma.
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The Permian, which lasted for a substantial 47 my (from 299 to 252 Ma), is most notable for being the time when the supercontinent of Pangea, more than three-quarters of the Earth’s total land area, was at its maximum extent. Close to the end of the era, at 251 Ma, there was an immense outpouring of basalts in Russia, a Large Igneous Province known as the Siberian Traps, which even today covers over 40% of the area of political Siberia. The combination of that LIP and the climatic crises which it undoubtedly caused, together with the unusually meagre total combined lengths of the world’s coastlines, were the chief triggers for the largest ever biotic extinction event of the Phanerozoic. That Permo-Triassic Extinction Event affected the whole globe.
The Briton Roderick Murchison, the German Alexander von Keyserling, and the Frenchman Edouard de Verneuil together named this system in the 1840s following extensive field work near Perm, a city near the northern Ural Mountains of Russia, where there is a distinctive and largely complete sequence of marine rocks. The base of the Permian System (Asselian Stage) is now formally defined at the base of the Streptognathus sulcata conodont Biozone in northern Kazakhstan, which is dated at 299 Ma.
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Recovery of the biota was initially slow after the great Permo-Triassic boundary extinctions, but then gathered pace quickly, with many different groups of animals and plants becoming newly important. At the end of the Triassic there was intruded one of the largest-known Large Igneous Provinces, the Central Atlantic Magmatic Province (CAMP), which assisted the opening of the central part of the Atlantic Ocean, thus splitting the main area of the supercontinent into northern and southern Pangea, with the latter often termed Gondwana again, as in the Palaeozoic. CAMP was also the prime cause of yet another biotic mass extinction.
Frederich von Alberti coined the name Triassic (or Trias), meaning ‘threefold’, in the 1830s to recognize the major group of rocks which has three divisions in Germany, then termed the Bunter Sandstone, Muschelkalk, and Keuper Marl: names which were used by Adam Sedgwick soon afterwards for rocks in Britain. The base of the Trias is now defined at the base of the Hindeodus parvus conodont Biozone (Induan Stage) in Zhejiang, China, which is dated at 252 Ma.
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The Jurassic is famous (not least through the film Jurassic Park) for the many exceptionally large and diverse dinosaur reptiles on the land, and ichthyosaurs and other swimming reptiles in the seas, as well as for the emergence of the earliest bird. During the Early Jurassic, Pangea break-up saw the opening of the Central Atlantic Ocean, and subsequently the opening of the Somali and Mozambique Basins which divided the former Gondwana into two major blocks. However, despite the Central Atlantic Ocean opening, all of the former northern Pangea from western North America to Siberia (termed Laurasia) remained united throughout the Jurassic. The name Jurassic (or Jurassique as it was originally known) was coined by the Frenchman Alexander Brongniart in the 1820s to mark the rocks in the Jura Mountains at the border between France and Switzerland. Its base is now defined at the base of the Psiloceras planorbis ammonite Biozone, the earliest zone of the Hettangian Stage, in Somerset, England, which is dated at 201 Ma.
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The Cretaceous saw the highest sea levels in the Phanerozoic, with the result that much of the continents were submerged under shallow seas. As the long Cretaceous epoch progressed, much of the general location and outlines of continents and oceans steadily became more recognisable to a modern geographer. In particular, a much higher proportion of old Cretaceous ocean floor is still preserved today when compared with the preceding periods. ‘Cretaceous’ means ‘chalk-bearing’ and the name was coined in the 1820s by the Frenchman d’Omalius d’Halloy to denote the distinctive fine white limestones (the Chalk) which principally outcrop in southern England, Belgium, and France, including the White Cliffs of Dover, England. The base of the Cretaceous is not yet formally defined, but is often taken at the base of the Berriasella jacobi ammonite Biozone (Berriasian Stage) in France, which is dated at 145 Ma.
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‘The Cretaceous-Tertiary boundary event is one of the best-known milestones in geology: in addition to the many extinctions at that time, the global temperature dropped sharply and remained low for most of the Paleocene. Modern mountain belts, including the Himalaya, Alps, Cordillera, and Andes, were all initiated or much enhanced during the period. The Paleogene is divided into the Paleocene, Eocene, and Oligocene, which together form the first half of the Tertiary Era of earlier authors, and which are grouped together within a single chapter here. The Paleocene was originally thought of as the earlier part of the Eocene, and was not identified as a separate time period until the 1870s, whilst Eocene (meaning ‘Dawn of the Present’ from the Greek Eos ‘dawn’ and kainos ‘recent’) and Oligocene had both been previously coined by the famous British geologist Charles Lyell in the 1830s. The base of the Paleocene is defined by an iridium anomaly at El Kef, Tunisia, which reflects ejecta derived from the Caribbean Chixculub meteorite impact at 65 Ma.
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This period, which takes us up to the present day, has been dominated by very varied tectonic events, including the mountain building in the Alps, Himalaya, and the western Americas (Cordillera and Andes), all of which have not yet come to an end. The climate was dramatically changed by the Plio-Pleistocene glaciation, which also appears to be continuing today. The Neogene is divided into the Miocene, and Pliocene, both names coined by the famous British geologist Charles Lyell in the 1830s. The succeeding Quaternary is divided into the Pleistocene, a name also coined by Lyell, and the Holocene for approximately the most recent 100,000 years.
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"'Earth History and Palaeogeography" by Trond H. Torsvik and L. Robin M. Cocks and published by Cambridge University Press, © Trond H. Torsvik and L. Robin M. Cocks 2016