(45 million years ago)
Beginning 488.3 million years ago and ending 443.7 million years ago
Tectonics and paleoclimate
From the Lower to Middle Ordovician, the Earth experienced a milder climate. The weather was warm and the atmosphere contained a lot of moisture. Though, when Gondwana settled on the South Pole during the Upper Ordovician, massive glaciers formed, causing shallow seas to drain and sea levels to drop. This caused mass extinctions that characterize the end of the Ordovician in which 60% of all marine invertebrate genera went extinct. The layers of rock that were the Ordovician are characterized by numerous and diverse trilobites and conodonts (phosphatic fossils with a tooth-like appearance) found in sequences of shale, limestone, dolostone, and sandstone. Also, blastoids, corals, and crinoids, as well as many kinds of brachiopods, snails, and clams, appeared for the first time in the geologic record in tropical Ordovician environments. Remains of ostracoderms (jawless, armored fish) from Ordovician rocks comprise some of the oldest vertebrate fossils. Shallow seas covering much of Gondwana became breeding grounds for new forms of trilobites after global patterns of life changed. Many species of graptolites went extinct, but the first planktonic graptolites appeared. In the late Lower Ordovician, the diversity of conodonts decreased in the North Atlantic Realm, but new lineages appeared in other regions. Seven major conodont lineages went extinct, but were replaced by nine new lineages that resulted from a major evolutionary radiation.
The Ordovician was named by the British geologist Charles Lapworth in 1879. He took the name from an ancient Celtic tribe, the Ordovices, renowned for its resistance to Roman domination. For decades, the epochs and series of the Ordovician each had a type location in Britain, where their characteristic faunas could be found, but in recent years, the stratigraphy of the Ordovician has been completely reworked. Graptolites, extinct planktonic organisms, have been and still are used to correlate Ordovician strata.