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The Ordovician period is the second geological period in the Paleozoic Era, which makes up the last 542 million years of life on Earth, the period during which complex multicellular organisms appear in the fossil record. The Ordovician period extends from roughly 490 to 440 million years ago. It is preceded by the Cambrian period and followed by the Silurian period. In this period, Earth's biosphere built on evolutionary successes from the Cambrian period. The number of marine fauna genera increased fourfold, and near the end of the Ordovician period, the intensity and diversity of bioeroding organisms exploded, leading to fewer well-preserved fossils than before.
During the Ordovician period, sea life dominated. There were simple plants and fungi on the land, but otherwise not much. The southern continents were aggregated into a supercontinent called Gondwana, which started off near the equator and eventually moved to cover the South Pole. The sea floor was relatively warm and shallow, making possible the growth of numerous marine organisms. Eurypterids, huge water scorpions whose closest living relative is the horseshoe crab, were among the most fearsome predators on Earth at the time. The largest eurypterids species, Pterygotus, was the largest arthopod that ever lived, up to 2 m in length. Arthopods in general were the largest organisms during the Ordovician.
Trilobites were present in large numbers, although their diversity was severely curtailed by an extinction event separating the Ordovician period and the Cambrian period which came before it. This extinction wiped out about half of all marine fauna genera, including the first apex predator, Anomalocaris, the "strange shrimp." Ammonites were numerous in the oceans, while reef-building organisms were well-established.
Early vertebrates existed during the Ordovician peroid, but were not numerous. They originally evolved during the Cambrian period. These included simple fish, most prominently the jawless (agnathan) fish, some similar in appearance to the tubelike lampreys, others more similar in appearance to modern fish. Fish began to evolve armor during this period.