[awr-duh-vish-uhn] Geology.
noting or pertaining to a geologic period of the Paleozoic Era, from 500 million to 425 million years ago, notable for the advent of fish. See table under geologic time.
the Ordovician Period or System.

1879; after the Ordovices (plural) (< Latin) an ancient British tribe in N Wales, where rocks characterizing the period were found; see -ian

post-Ordovician, adjective
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World English Dictionary
Ordovician (ˌɔːdəʊˈvɪʃɪən)
1.  of, denoting, or formed in the second period of the Palaeozoic era, between the Cambrian and Silurian periods, which lasted for 45 000 000 years during which marine invertebrates flourished
2.  the Ordovician the Ordovician period or rock system
[C19: from Latin Ordovices ancient Celtic tribe in N Wales]

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Word Origin & History

geological period following the Cambrian and preceding the Silurian, 1879, coined by Eng. geologist Charles Lapworth (1842-1920) from L. Ordovices, name of an ancient British tribe in North Wales. The period so called because rocks from it first were studied extensively in the region around Bala in North
Wales. The tribe's name is Celtic, lit. "those who fight with hammers," from Celt. base *ordo "hammer" + PIE *wik- "to fight, conquer" (cf. victor).
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Science Dictionary
Ordovician   (ôr'də-vĭsh'ən)  Pronunciation Key 
The second period of the Paleozoic Era, from about 505 to 438 million years ago. During this time most of the Earth's landmasses were gathered in the supercontinent Gondwanaland, located in the Southern Hemisphere. Much of this continent was submerged under shallow seas, and marine invertebrates, including trilobites, brachiopods, graptolites, and conodonts were widespread. The first primitive fishes appeared; some evidence suggests the first land plants may also have appeared at this time. By the end of the Ordovician massive glaciers formed on Gondwanaland, causing sea levels to drop and approximately 60 percent of all known marine invertebrates to become extinct. See Chart at geologic time.
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