The Ordovician Period started at a major extinction event called the Cambrian–Ordovician extinction event about 485.4 Mya (million years ago), and lasted for about 42 million years.
It ended with the Ordovician–Silurian extinction event, about 443.8 Mya (ICS, 2004) that wiped out 60% of marine genera.
The boundary chosen for the beginning of both the Ordovician Period and the Tremadocian stage is highly significant.
Since it correlates well with the occurrence of widespread graptolite, conodont, and trilobite species, the base (start) of the Tremadocian allows scientists to relate these species not only to each other, but also to species that occur with them in other areas.
This makes it easier to place many more species in time relative to the beginning of the Ordovician Period.
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.