Ordovician period dating

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Glaciation locks up water from the ocean, and the interglacials free it, causing sea levels repeatedly to drop and rise.

The extinctions occurred approximately 444-447 million years ago and mark the boundary between the Ordovician and the following Silurian Period.More recently, however, fossils of other fish-like creatures, the 530-million-year-old Early Cambrian fossil dubbed Haikouella and then the 515-million-year-old middle Cambrian animal Pikaia have been promoted as the world's earliest chordate (Heeren 2000).The very first jawed fish appeared in the Late Ordovician epoch and now-extinct worm-shaped marine animals called graptolites thrived in the oceans.After Charles Lapworth first defined the Ordovician period in 1879 in the United Kingdom, other areas of the world accepted it quickly, while its acceptance came last to the United Kingdom.The Ordovician period received international sanction in 1906, when it was adopted as an official period of the Paleozoic era by the International Geological Congress.

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