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Metazoan invertebrates are still the dominant form of life on Earth.Corals, crinoids, and clams evolve, as well as the first early vertebrates—primitive fish with bony armor plates.Organisms found in the Ordovician period were exclusively aquatic organisms.A small exception to this may have been extremely primitive non-vascular plants near the waterline of coasts. Even the fish that existed near the end of the Ordovician period were extremely primitive, and would be unrecognizable today.More recently, tetrahedral spores that are similar to those of primitive land plants have been found, suggesting that plants invaded the land at this time.From the Lower to Middle Ordovician, the Earth experienced a milder climate — the weather was warm and the atmosphere contained a lot of moisture.

These seas deposited widespread blankets of sediment that preserved the extraordinarily abundant proposed the Ordovician System (named for an ancient Celtic tribe of northern Wales called the Ordovices) in 1879 to define the disputed overlapping interval.

Trilobites and brachiopods in particular were rich and diverse.

The first bryozoa appeared in the Ordovician as did the first coral reefs.

Lapworth’s proposal was resisted in Britain into the 1890s and, despite subsequent widespread international usage, was not officially adopted there until 1960.

The Ordovician is divided into three epochs: Early Ordovician (485.4 million to 470 million years ago), Middle Ordovician (470 million to 458.4 million years ago), and Late Ordovician (458.4 million to 443.8 million years ago). Although the positions of these continents are frequently updated with new evidence, current understanding of their position is based on paleomagnetic evidence, fossil markers, and climatically sensitive sediments, such as —made up of most of present-day North America, Greenland, and part of Scotland—straddled the Equator and was rotated approximately 45° clockwise from its present orientation.

Plate Tectonics The current placement of our continents is vastly different from what the continents looked like in the Orodovician period.