[awr-duh-vish-uh
n] 
Geology| 1. | 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. |
| 2. | the Ordovician Period or System. |
| Or·do·vi·cian (ôr'də-vĭsh'ən) adj. Of or belonging to the geologic time, system of rocks, or sedimentary deposits of the second period of the Paleozoic Era, characterized by the appearance of primitive fishes. See Table at geologic time. n. The Ordovician Period. [From Latin Ordovicēs, an ancient Celtic tribe of Wales, from Celtic Ordovices; see weik-3 in Indo-European roots.] |
| 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. |