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. The Ordovician Period started at a major extinction event called the Cambrian–Ordovician extinction events some time about 488.3 ± 1.7 Mya (million years ago), and lasted for about 44.6 million years.
During this radiation the number of marine biological families increased greatly, and these families included species that would dominate the marine ecosystems of the area for the next 215 million years.
Although the radiation spanned tens of millions of years, major changes in many species occurred during a geologically short time span within the radiation and, furthermore, appear to have occurred worldwide, suggesting that external events were major factors in the radiation.
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.
A major mountain-building episode was the Taconic orogeny that was well under way in Cambrian times.
In the beginning of the Late Ordovician, from 460 to 450 Ma, volcanoes along the margin of the Iapetus Ocean spewed massive amounts of carbon dioxide into the atmosphere, turning the planet into a hothouse.
During the Ordovician, the southern continents were collected into a single continent called Gondwana.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.The small continent Avalonia separated from Gondwana and began to head north towards Baltica and Laurentia.The Rheic Ocean between Gondwana and Avalonia was formed as a result.Carbonate hardgrounds were thus very common, along with calcitic ooids, calcitic cements, and invertebrate faunas with dominantly calcitic skeletons.Unlike Cambrian times, when calcite production was dominated by microbial and non-biological processes, animals (and macroalgae) became a dominant source of calcareous material in Ordovician deposits.Gondwana had by that time neared or approached the pole and was largely glaciated.The Ordovician was a time of calcite sea geochemistry in which low-magnesium calcite was the primary inorganic marine precipitate of calcium carbonate.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.