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Ordovician strata 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.In addition, blastoids, bryozoans, corals, crinoids, as well as many kinds of brachiopods, snails, clams, and cephalopods appeared for the first time in the geologic record in tropical Ordovician environments.
Though less famous than the Cambrian explosion, the Ordovician featured an adaptive radiation that was no less remarkable; marine faunal genera increased fourfold, resulting in 12% of all known Phanerozoic marine fauna. The trilobite, inarticulate brachiopod, archaeocyathid, and eocrinoid faunas of the Cambrian were succeeded by those which would dominate for the rest of the Paleozoic, such as articulate brachiopods, cephalopods, and crinoids; articulate brachiopods, in particular, largely replaced trilobites in shelf communities. Their success epitomizes the greatly increased diversity of carbonate shell-secreting organisms in the Ordovician compared to the Cambrian. In North America and Europe, the Ordovician was a time of shallow continental seas rich in life.Initially, only these two periods existed, until 1979 when Charles Lapworth defined the Ordovician period as a period of it’s own based on fossil fauna that were different from both the Cambrian and the Silurian Period.The Ordovician period was started by an extinction called the Cambrian-Ordovician extinction which lasted about 44.6 million years, and ended with a mass extinction event known as the Ordovician-Silurian extinction event that wiped out approximately 60% of all the marine genera. As a result, continents were flooded to an unprecedented level, with North America almost entirely underwater at times.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.