It reaches its climax in the Ordovician sea, and then begins to decline, as more powerful animals come upon the scene.
The Ordovician sea stretched from Appalachia across the Mississippi valley.
The series is now placed at the top of the Ordovician System, above the Llandeilo beds.
During the ages of the Ordovician, life made great advances.
Unlike their Cambrian ancestors, many of the Ordovician trilobites could roll themselves into balls at the approach of danger.
They were more than likely the undisputed masters of the Ordovician seas.
Ordovician strata contain representatives of all the main types of Echinoderms in well-fossilized forms.
The characters of the Ordovician trilobites have already been noticed.
The oldest fossils found belong either to the Ordovician or Silurian systems.
They also rest unconformably upon the Ordovician rocks in this area.
geological period following the Cambrian and preceding the Silurian, 1879, coined by English geologist Charles Lapworth (1842-1920) from Latin 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, literally "those who fight with hammers," from Celtic base *ordo "hammer" + PIE *wik- "to fight, conquer" (see victor).
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.