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542.0 to 488.3 million years ago
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The first eon of Earth’s history, from the first coalesence of the planet, about 4500 Mya, to about 542 Mya, is referred to as the Precambrian. From this hint, one might well suppose that the Cambrian comes next -- which it does, in a way. Actually, this is the biggest break point in all of geology. It marks the beginning of the , the , the Cambrian Period, the , and the (alternatively and previously known as the Manykaian Age or Nemakit-Daldynian Age). The Cambrian Period was named in 1835 by the geologist Adam Sedgwick, after the region of Cambria in North Wales, where rocks of this age were first found. The name "Cambria" is a version of Cumbria, a latinisation the Welsh Cymry (= countryman, compatriot against the invading Anglo-Saxons).
Long before it was a formal stratigraphic unit, the Cambrian was a concept about Earth history. It was understood to be the earliest period in which one could find the fossils of multi-celled animals (). Since then, metazoans and their fossilized traces have been found well before 542 Mya. In particular, ediacarans, a group of very strange and poorly understood creatures - but obviously metazoans - have been found in many parts of the world with ages pushing the 600 Mya mark.
Consequently, paleontologists now view the Cambrian as the period when the first appeared and, almost at the same time, the first metazoans with shells. The Bilateria include all living metazoans except and . Bilaterians have a head at one end of an elongate body which is bilaterally symmetrical (hence the name). Their embryos all develop a separate layer of embryonic cells, called mesoderm, between the gut or coelom and the outer wall of the animal. If this whole description suggests a worm, you’ve got the right idea. A flatworm is the most basal living form of bilaterian.
Unlike many other, somewhat arbitrary, geological markers, the base of the Cambrian Period is defined with reference to the underlying paleontological concept. Small worms rarely leave body fossils, but their burrows are frequently preserved. The burrows of bilaterian worms are fairly distinctive. Trace fossils are often given names as if they were organisms, and the earliest well-known bilaterian trace fossil is a type of fossilized burrow referred to as Treptichnus pedum. The base of the Cambrian is currently defined as the first occurrence of T. pedum at , near the town of Fortune, on the north coast of western Newfoundland, Canada.
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The has not yet finalized an international stratigraphical scheme for the Cambrian. In the meantime we use a mixture of the bits they have decided on and various informal names, mostly from the Siberian platform:
| Period | Epoch | Age | Informal Name | Base Date | Duration (Ma) |
|---|---|---|---|---|---|
| Ordovician | Early Ordovician | Tremadoc | 488 | 9 | |
| Cambrian | Furongian | Cambrian X | Dolgellian | ~492? | 4? |
| Cambrian IX | ~496? | ~4? | |||
| Paibian | Maentwrogian | 499 | ~3 | ||
| "Middle Cambrian" Cambrian Epoch 3 | Guzhangian | Menevian | ~503 | ~4 | |
| Drumian | Late Amgan | ~507 | ~4 | ||
| Cambrian V | Early Amgan | ~510? | ~3? | ||
| Cambrian Epoch 2 | Cambrian IV | Botomian (including Toyonian) | ~515? | ~5? | |
| Cambrian III | Atdabanian | ~521? | ~6? | ||
| Terreneuvian | Cambrian II | Tommotian | ~528? | ~7? | |
| Fortunian | Nemakit-Daldynian, or Manikayan, Manykajan, etc. | 542 | ~14 | ||
| Ediacaran | ~630 | ~88 |
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The Cambrian saw most continents located in the southern hemisphere at low paleolatitudes (near the equator). The of continued to assemble in some regions but fragmented into , , and various mostly submerged Asian blocks.
Laurentia stretched across the Cambrian equator, partly submerged by the , with a mostly submerged Baltica and approaching from the south-east.
Gondwana remained the largest supercontinent. Other continents included and (actually China, Thailand, Malaysia, and Indochina). Seas were for the most part shallow, especially along the edges of the continents.
Global (eustatic) transgressions occured in the and , as shallow seas repeatedly invaded the land, providing a perfect habitat for many types of marine invertebrates. These shallow epeiric seas covered much of the continents except for Gondwana, where there were highlands. Other highlands could be found in Eastern Siberia and Central Kazakhstan.
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The Cambrian climate was generally warm, wet and mild. As there were no continental landmasses located at the poles, ocean currents were able to circulate freely, hence there was no significant ice formation. As a result temperatures worldwide were mild. The Cambrian constituted a benign spell between two great ice ages - the and the Ice Age.
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At the beginning of the Cambrian, about 540 million years ago, life was entirely confined to the oceans. During the 53 million years that the Cambrian period lasted there was the sudden appearance and diversification of almost every major group () of animal life, as well as many types that later died out. with and appeared for the first time, including , , , and many other groups. This sudden evolutionary burst was so spectacular that it has been termed the "Cambrian explosion". There hasn't been anything like it on Earth before or since.
The most characteristic animals of the Cambrian period were the , a primitive form of remarkable for their highly developed (unusual in such an early organism).
The trilobites appear suddenly during the epoch (several groups are known, including large spiny types and small planktonic forms), reached their fullest development in the and the following () period, and gradually declined after that and became by the end of the era.
Other very important groups of Cambrian animals were the , (represented by a large number of different ), and most interesting of all the soft-bodied , which were burrowing worm-like creatures, which seem to have been the most important and predominant carnivores of the time.
The earliest (marine snails) also appeared in this period, as did the (during the ) and other now extinct lineages of molluscs. Molluscs however were still relatively rare; they did not become an important element of the marine fauna until the period.
The first ( ancestors) occurred as did the first (shelled ).
Greatest of all the Cambrian beasties were as the . Averaging 45 to 60 cm, with exceptional specimens reaching 1 or even 2 metres, these animals dwarfed even the largest trilobites. They were armed with twin grasping organs and a wicked mouth with a ring of teeth in shape rather like a pineapple slice. Many trilobite exoskeletons have been found with large bites taken out of them, the result of an encounter with Anomalocaris. Like the shark of today, Anomalocaris, was perfectly adapted to its environment, and a single species existed without change for some 30 million years or more. Scientists do not agree what group of modern creatures Anomalocaris was most closely related too. Suggestions most often include , but the group of worm-like creatures is another possibility. One should be wary of pigeonholing extinct organisms with living types on the basis of superficial similarity. Like many Cambrian creatures, Anomalocaris is best put in a phylum of its own.
is one of a number of the strange armoured "coat of mail" animals that inhabited the early Cambrian oceans. In addition to the calcareous scales it possessed two mollusc-like limpet-shaped shells. Later forms such as the Middle Cambrian lost the shells and developed long spines instead. It is not certain that these strange creatures even are molluscs, although a structure has been found in Wiwaxia. Jan Bergström suggests that they represent a group of late surviving "", the ancestors of all higher (coelomate body plan) animals.
Many Cambrian creatures however did not fit into modern categories of organisms; they seemed to have been representatives of unknown phyla or phyla which weren't successful in the long run.
The enigmatic sponge-like cup-shaped were very common during the early Cambrian, forming great reefs, but died out almost completely by the middle of the period, leaving no descendants.
of the Cambrian period included only (seaweeds). There were no known land plants, the land was still bare of any life other than .
The first possible tracks on land, presumably made by arthropods date back to the Cambrian period (c 530 MYA).
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Cambrian life is known from several spectacular , most famously the in British Columbia, Canada, but also including in China and in Greenland.
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- , at the University of California Museum of Paleontology. best on the web
- - good essay, from from
- - Dr Pamela Gore - some useful study notes, complete with links and a few photos of several types of . An excellent introduction, if you don't mind the Geology 102 format. Includes material on the famous Burgess Shale site.
- - - explains how multicelluar animals suddenly appeared during the Cambrian, after billions of years of nothing but micro-organisms
- MSN Encarta - - Short entry, summarizes the life forms and geologic activity that mark this time period.
| order prescription viagra 542-488 | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| 542-521 | 521-510 | 510-499 | 499-488 | ||||||
| 542-528 | 528-521 | 521-515 | 515-510 | Stage 5 510-506 | 506-503 | 503-499 | 499-496 | Stage 9 496-492 | Stage 10 492-488 |
| | |
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page uploaded on Kheper Site on 28 May 1998, page uploaded on Palaeos Site 11 April 2002, last modified 26 October 2002
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