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Ordovician

Pronunciation: /ˌɔːdəˈvɪʃɪən/

Definition of Ordovician

adjective

Geology

relating to or denoting the second period of the Palaeozoic era, between the Cambrian and Silurian periods.
(as noun the Ordovician) the Ordovician period or the system of rocks deposited during it.

The Ordovician lasted from about 510 to 439 million years ago. It saw the diversification of many invertebrate groups and the appearance of the first vertebrates (jawless fish)