485 Million B.C.T. - The Ordovician Period Began

Around 485 million years ago, the Ordovician Period began.

The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 485.4 ± 1.9 to 443.4 ± 1.5 million years ago. It follows the Cambrian Period and is followed by the Silurian Period. The Ordovician, named after the Celtic tribe of the Ordovices, was defined by Charles Lapworth in 1879 to resolve a dispute between followers of Adam Sedgwick and Roderick Murchison, who were placing the same rock beds in northern Wales into the Cambrian and Silurian periods respectively. Lapworth, recognizing that the fossil fauna in the disputed strata were different from those of either the Cambrian or the Silurian periods, realized that they should be placed in a period of their own. While recognition of the distinct Ordovician Period was slow in the United Kingdom, other areas of the world accepted it quickly. It received international sanction in 1960, when it was adopted as an official period of the Paleozoic Era by the International Geological Congress.

Life continued to flourish during the Ordovician as it did in the Cambrian, although the end of the period was marked by a significant mass extinction. Invertebrates, namely mollusks and arthropods, dominated the oceans. Fish, the world's first true vertebrates, continued to evolve, and those with jaws may have first appeared late in the period. Life had yet to diversify on land.

The Ordovician Period started at a major extinction event called the Cambrian–Ordovician extinction events some time about 485.4 ± 1.9 million years ago, and lasted for about 44.6 million years. It ended with the Ordovician–Silurian extinction event, about 443.4 ± 1.5 Mya (ICS, 2004) that wiped out 60% of marine genera.

The dates given are recent radiometric dates and vary slightly from those used in other sources. This second period of the Paleozoic era created abundant fossils and in some regions, major petroleum and gas reservoirs.

The boundary chosen for the beginning both of the Ordovician Period and the Tremadocian stage is highly useful. Since it correlates well with the occurrence of widespread graptolite, conodont, and trilobite species, the base of the Tremadocian allows scientists not only to relate these species to each other, but to species that occur with them in other areas as well. This makes it easier to place many more species in time relative to the beginning of the Ordovician Period.