Ordovician

The Ordovician period started at an apparently minor extinction event some time 490 million years ago and lasted for about 50-80 million years. The Ordovician period follows the Cambrian period. The Ordovician was named after the Welsh tribe of the Ordovices. It ended with a major extinction event 443.5 million years ago that wiped out 60% of marine genera. It was the third-largest of the five major extinction events in Earths history in terms of percentage of genera that went extinct and second largest overall in the overall loss of life. The immediate cause of extinction appears to have been the continental drift of a significant landmass into the south polar region, causing a global temperature drop, glaciation, and consequent lowering of the sea level, which destroyed species' habitats around the continental shelves.

The Ordovician Period is the second period of the Paleozoic Era and is broken into subdivisions:

• Lower (Tremadocian and Arenig)
• Middle (Llanvirn - subdivided into Abereiddian and Llandeilian, and Llandeilo)
and
• Late (Caradoc and Ashgill)

More About Ordovician

This important period saw the origin and rapid evolution of many new types of invertebrate animals which replaced their Cambrian predecessors. Primitive plants move onto land which until then were totally barren. During the early Ordovician period the first vertebrate fish have been found. Graptolites and corals also flourished. Sea levels were very high during this period. The Early Ordovician climate is thought to have been quite warm, at least in the tropics. Ordovician rocks contained abundant life and contain major oil and gas reservoirs in some regions. In North America and Europe shallow continental seas that were rich in life.

Trilobites (an extinct class of Arthropods) and Brachiopods (one of the major animal phyla) in particular were rich and diverse. The first Bryozoa (tiny creatures with a ring of tentacles surrounding the mouth) appear in the Ordovician as do the first coral reefs. Solitary corals date back to the Cambrian at least. It was long thought that the first true vertebrates (fish appeared in the Ordovician, but recent discoveries in China reveal that they probably originated in the Early Cambrian period. Now, extinct marine animals called graptolites thrived in the oceans.

The southern continents were collected into a single continent called Gondwana during the Ordovician. Gondwana started the period in equatorial latitudes and drifted toward the South Pole during this period. As with North America and Europe, Gondwana was largely covered with shallow seas during the Ordovician. By the end of the period, Gondwana had neared or approached the pole and was largely glaciated - a glacier is a large, long-lasting flow of ice that is formed on land and moves in response to gravity. The Ordovician-Silurian Extinction Events may have been caused by an ice age that occurred at the end of the Ordovician period as the end of the Late Ordovician was one of the coldest times in the last 600 million years of earth history.