Middle Ordovician

The Middle Ordovician (from 472 to 461 million years ago) is the second subdivision of the Ordovician period. During this time warm shallow sea covered much of the land, and was home to a rich diversity of marine life. Traditionally the Middle Ordovician has been divided into the Llanvirn and Llandielo stages but however this division has been replaced by two newer stages known as Ordovivian III and the Darriwilian.

North America straddled the equator, while the south-polar supercontinent of Gondwana included not only South America, Africa, India, Antarctica and Australia, but Southern Europe as well. The trilobite-dominated Early Ordovician communities are replaced by generally more mixed ecosystems, in which brachiopods, bryozoa, molluscs and echinoderms all flourish, tabulate corals diversify and the first rugose corals appear; trilobites are no longer typically predominant. The planktonic graptolites remain diverse, with the Diplograptina making their appearance. Bioerosion becomes an important process, particularly in the thick calcitic shells of corals, bryozoans and brachiopods, and on the extensive carbonate hardgrounds which appear at this time.

The earliest known armoured agnathan ("ostracoderm") vertebrate, "Arandaspis", dates from the Middle Ordovician of Australia.

External links

* [http://zircon.geology.union.edu/Gildner/globe.html The World during the Middle and Late Ordovician] paleogeography
* [http://www.palaeos.com/Paleozoic/Ordovician/MidOrd.html The Middle Ordovician ]
* [http://www.stratigraphy.org/geowhen/stages/Middle_Ordovician.html GeoWhen Database - Middle Ordovician]