Cambrian/ Ordovician:

  • Time period:marks the begininning of the Paleozoic Era. It was from 543 million years ago until 490 million years , it lasted about 53 million years.
  • The continentsduring this time period were still forming. They were mostly barren rocks. Most land masses were found in tropical and subtropical regions, and several were clustered into a large area called Gondwana.
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  • During this time period many lineages of animals appear in the fossil record for the first time. Nearly half of all extant animal phyla appear for the first time in the fossil record during the first half of the Cambrian period.
  • Inveetebrates, namely mollusks and arthropods dominated the oceans. Fish, the worls first true invertebrates continued to evolve.
  • The oceans became oxygenated. Although there was plentiful atmospheric oxygen by the beginning of the period, it wasn't until the Cambrian that there was a sufficient reduction in the number of oxygen-depleting bacteria to permit higher oxygen levels in the waters. This dissolved oxygen may have triggered the "Cambrian Explosion" — when most of the major groups of animals, especially those with hard shells, first appeared in the fossil record.
  • Changes in animals: They delevoped jaws toward the end of the period.
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  • Changes in plants: The first land plants started to evolve during the end of the period.
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  • Mass Extinction: The Ordovician came to a close in a series of extinction events that, taken together, comprise the second largest of the five major extinction events in Earth's history in terms of percentage of genera that went extinct.The most commonly accepted theory is that these events were triggered by the onset of most cold conditions in the late Katian, followed by an ice age, in the Hirnantian faunal stage, that ended the long, stable greenhouse conditions typical of the Ordovician. At the end of the second event, melting glaciers caused the sea level to rise and stabilise once more. The rebound of life's diversity with the permanent re-flooding of continental shelves at the onset of the Silurian saw increased biodiversity within the surviving Order.


  • Find out more about the Cambrian paleontology and geology of North America at the Paleontology Portal.