The Ordovician* lasted about 45 million years and saw the transition from very primitive to relatively modern life-forms in the seas. The “Ordovician radiation” which followed the late Cambrian extinctions, lead to a tripling of marine diversity, the greatest increase in the history of life, and giving the highest levels of diversity seen during the Paleozoic Era. As a result, all of the common invertebrate fossil groups and a few vertebrates were present by the end of this period. Starfish, brittle stars, crinoids, and echinoids, all of which have living representatives, first appeared in the Ordovician. In the evolutionary history of animal life this radiation was second only to the “Cambrian explosion” in importance. The new Paleozoic fauna created by the “Ordovician radiation” dominated the seas for the next 230 million years. Pandemic species of planktonic graptolites and conodontes appear in the fossil record during this Period. Their world-wide distribution and evolution during the Ordovician make them key species for correlating fossil deposits. Unlike in the Cambrian, most animal evolution in the Ordovician involved refining existing body plans rather than developing new ones. Bryozoans, the last animal phyla to appear in the fossil record, have the only new body plan, and they may have evolved in the Cambrian, but only became mineralized, and thus left fossils, in the Ordovician.

The Ordovician saw the replacement of the Cambrian marine fauna composed largely of deposit feeders, such as trilobites, with the Paleozoic fauna dominated by filter feeders arranged in tiers. Tiered suspension feeder communities attached to the sea floor dominate the new Paleozoic ecosystems: brachiopods filter bottom waters, corals and branched bryozoans filter water just above, and crinoids filter water at the highest level. The Paleozoic fauna was also characterized by a move away from the interface of the ocean and ocean floor—animals both learned to bore deeper into the substrate and to invade the pelagic (in the water column ,not on shore or the bottom) regions of the oceans. Grazing gastropods increased in importance while efficient predators like sea stars and cephalopods evolved. Remnant Cambrian fauna, such as deposit feeding worms and trilobites, continued with diminishing importance throughout the Paleozoic.

Tectonically, the Ordovician saw a rapid reorganization of plates around the Iapetus Ocean. Today’s southern continents were fused into the supercontinent of Gondwana, while others remained independent. Plate movement resulted in a migration of Gondwana from an equatorial position toward the South pole, as a result the South Pole moved from North Africa to central Africa. The movement of Gondwana to the South Pole resulted in global cooling and the most severe ice age of the Phanerozoic Eon. Sea levels rose through much of the Ordovician to the highest sea levels of the Paleozoic before receding. The period ended with a strong lowering of sea levels (a strong negative eustatic event) as high latitude glaciation occurred.

There were significant fluctuations in climate with prolonged periods of warm “hothouse” conditions with periodic cold, “icehouse” intervals and turnovers of the oceans. High levels of CO₂ early in the Period lead to very high temperatures—oceans may have reached 113°F (45°C). These high temperatures may have delayed animal diversification in the early Ordovician. Later, ocean temperatures decreased gradually to those seen in modern equatorial waters. The Ordovician ended with the second greatest extinction event of the Phanerozoic Eon. Two massive reductions, a million years apart, created this event—resulting in the extinction of more than 60% of invertebrates. This event is associated with sea level changes and glacier formation. Only the great Permian extinction event was more devastating to Earth’s biosphere.

* The term Ordovician was proposed by Lapworth in 1879 for strata which had been included in Cambrian or Silurian systems. It is named for the Ordovices, a tribe of northern Wales.