Smithsonian National Museum of Natural History Geologic Time The Story of a Changing Earth
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The Ordovician
Eon Overview
Reef Ecosystems and the Paleozoic Fauna
New Animals as Biostratigraphic Tools
Ordovician Climate and Plate Tectonics
Ordovician trilobite
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Ordovician 488 - 444 mya
Defining Characteristics:
  • • diversification of marine invertebrate Paleozoic Fauna
  • • end-Ordovician extinction

  • • Map of the Ordovician World
Secondary Characteristics:
  • • deep-water faunas, especially graptolites
  • • appearance of conodonts
  • • development of diverse carbonate platforms
  • • first major Paleozoic ice age

Charles Lapworth in 1879 named the Ordovician for a set of rock strata that were particularly well exposed in Wales. He took the name from Ordovices, the Latin name of a Celtic tribe in the Welsh region during Roman times. By creating the Ordovician, Lapworth helped settled a dispute, because previously Adam Sedgwick had assigned these rock layers to his Cambrian, whereas Roderick Murchison called them Silurian. Lapworth recognized them as distinct from the (older) Cambrian rocks below and the (younger) Silurian rocks above. This process—determining the age of a rock by the fossils contained within it—is called biostratigraphy.

Since Lapworth’s time, Ordovician rocks have been identified in many locations outside of Britain. Some of the most important sites are in the United States, particularly the regions around Cincinnati, the Ozark Mountains, western Utah, and eastern Tennessee. The eastern regions of Canada, especially the Maritime Provinces, also include significant deposits of Ordovician rocks.

Ordovician rock layers were deposited between 443 and 490 million years ago. During this time period, marine life saw a major shift from the Cambrian Fauna, which appeared during the Cambrian Explosion, to the Paleozoic Fauna, which would dominate the marine world for the next 200 million years. Important new animal forms, such as graptolites and conodonts, first appeared during the Ordovician, along with the development of complex shallow-water reef ecosystems. The first land organisms, lichens and bryophytes, appeared. The end of the period was marked by a major ice age and the second-largest mass extinction of the Paleozoic Era.

Overview | Reef Ecosystems and the Paleozoic Fauna | New Animals as Biostratigraphic Tools
Ordovician Climate and Plate Tectonics

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