The cambrian period

موضوع: The cambrian period 2007-09-25, 2:10 pm

The Cambrian Period. 543 to 490 Million Years Ago

[The Cambrian Period marks an important point in the history of life on earth; it is the time when most of the major groups of animals first appear in the fossil record. This event is sometimes called the "Cambrian Explosion", because of the relatively short time over which this diversity of forms appears. It was once thought that the Cambrian rocks contained the first and oldest fossil animals

the Cambrian world was concentrated in the southern hemisphere. The largest landmass (lower right) was Godwin (a collection of today's southern continents). The second largest continent, Laurentia is just left of center in the map below, and includes most of North America (the southeastern US can be seen wedged between Africa and South America as part of Gondwana). Between Gondwana and Laurentia lie Siberia (just south of the equator) and Baltica (Scandinavia, eastern Europe, and European Russia). The rest of Europe and much of what is today Asia lay in fragments along the north coast of Godwin[/color]

[The Cambrian world was bracketed between two ice ages, one during the late Late Proterozoicand the other during the Ordovician. During these ice ages, the decrease in global temperature led to mass extinctions. Cooler conditions eliminated many warm water species, and glaciation lowered global sea level. However, during the Cambrian there was no significant ice formation. None of the continents were located at the poles, and so land temperatures remained mild. In fact, global climate was probably warmer and more unifrom than it is today. With the beginning of the Cambrian at the retreat of Proterozoic ice, the sea level rose significantly. Lowland areas such as Baltica were flooded and much of the world was covered by epeiric seas. This event opened up new habitats where marine invertebrates, such as the trilobites, radiated and flourished

Plants had not yet evolved, and the terrestrial world was therefore devoid of vegetation and inhospitable to life as we know it. Photosynthesis and primary production were the monopoly of bacteria and algal protists that populated the world's shallow seas.

[size=12]Also during the Cambrian, oxygen first mixed into the world's oceans in significant quantity. Although there was plentiful atmospheric oxygen by the opening of the Cambrian, only in the Cambrian did the numbers of oxygen-depleting bacteria reduce in numbers sufficiently to permit the high levels we know today. This made dissolved oxygen available to the diversity of animals, and may have triggered the "Cambrian Explosion". This was when most of the major groups of animals, especially those with hard shells, first appear in the fossil record.

The Cambrian Mass Extinction

Fast Facts

the Cambrian period ranges from 543-510 million years ago

-the most important animal group of the Cambrian were the trilobites
-four mass extinctions occurred during the course of the Cambrian
Geological Setting

[During the Cambrian period the world was largely covered by epeiric seas, and existing organisms were entirely marine. At the beginning of the period, only small skeletonized sponges and molluscs were present, but by about the middle of the Cambrian, diversification of the shelly fauna occurred. The most important phyla present in Cambrian communities included trilobites ]archaeocyathids brachiopods molluscs, and echinoderms

Species Affected

[size=12]At least four major extinctions occurred during the Cambrian. The first extinction occurred at the Early Cambrian epoch boundary. During this event, the oldest group of trilobites, the olnellids, perished as well as the primary reef-building organisms, the archaeocyathids. The remaining three extinctions were irregularly distributed around the Late Cambrian epoch boundary, and as a whole, severly affected trilobites brachiopods
Archaeocyathids

The first reef-buiders dominant from the Middle to Late Cambrian

Geologic Range: Early to Late Cambrian

Speculated Causes For the Cambrian Extinction

The two most accepted current hypotheses for the Cambrian extinction are:

[*]Glaciation in the early Ordovician
[*]Cooling and depletion of oxygen in marine water

Glacial Cooling Hypothesis The advancement of the theory of glaciation as the predetermining agent for the Cambrian extinctions has been developed by James F.Miller of Southwest Missouri State University. Through research undertaken by Miller, evidence of early Ordovician sediment of glacial origin has been uncovered in South America. Miller suggests in his hypothesis that this evidence of continental glaciation at the Cambrian-Ordovician boundary is responsible for a decrease in global climatic conditions. Such a decline in temperature is implied by Miller to destroy Cambrian fauna which are intolerant of cooler conditions, producing a mass extinction of mostly warm water species. He also suggests that a significant continental glaciation would bring large amounts of ocean water onto the land in the form of frozen glacial ice. This trapping of ocean water inevitably results in the decrease of sea-level and the withdrawal of shallow seas. Miller implicates that this reduction in sea-level would produce reduced habitat for marine species as continental shelves are obliterated. Ecological competition would consequently ensue, perhaps acting as a driving agent for extinction.

Oxygen Depletion Hypothesis

The development of a hypothesis invoking the cooling and depletion of water in marine waters as a causative agent for the Cambrian extinctions has been advanced by several geologists, primarily Allison Palmer and Michael Taylor of the U.S. Geological Survey and James Stilt of the University of Missouri. The cooling and oxygen depletion would occur when cool waters from deep zones of the ocean spread up onto the continent, eliminating all organisms not able to tolerate cool conditions. The cooling would also result in stratification of the water column. Thus, species would ultimately perish due to their inability to tolerate dramatic shifts in such limiting factors as temperature and oxygen availibility. Further research is required to more fully test the validity of the above outlined Cabrian extinction hypotheses

موضوع: رد: The cambrian period 2007-09-26, 9:49 pm

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موضوع: رد: The cambrian period 2007-09-27, 6:26 pm

thanx geology man 4 ur cambrian topic..

really complete topic as ur refinery topic also
i enjoy it so much especially that part of oxygen depletion
that it was my first time to read about it
really thanx geology man
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