Earth's Ordovician Period

The Ordovician Period of Earth's history lasted from 488 million to 443 million years ago. These bookends are comprised of two periods of extinction, the latter of which wiped out around 60% of all life in the oceans.

Sea levels were higher in the Ordovician Period compared to the earlier Cambrian Period. The formation of a giant collection of continents to the south called Gondwana also created a number of shallow seas across this supercontinent. By the time this period ended, Gondwana had migrated to the south pole and frozen over.

The shallow continental seas of the Ordovician Period allowed for an abundance of life. Invertebrates such as trilobites and brachipods continued to thrive in these favorable conditions, forming complex food chains of increasingly larger organisms. The first corals appear during this period as well as other organisms that use calcium to produce their outer coverings. Although corals made their first steps in evolution during this period, autotrophic niches were still dominated by marine algae. Filter feeding organisms that fed on microflora and microfauna were becoming much more widespread during the Ordovician Period.

Cephalopods, which include present-day squid and octopi were one of the groups of organisms that began to flourish in the Ordovician. These tentacled predators were very efficient hunters and their success led to their increase in size and diversity. The largest cephalopod was also the largest animal ever to exist on Earth up to that point. Endocerids, which were squid-like creatures that had a very long, conical shell grew to lengths of up to 10 meters.

Although the first vertebrates appeared in the late Cambrian Period, they continued to evolve during the Ordovician Period and by the end of the eon, the first jawed fish had appeared in the oceans. In terms of plant life, the first lichens began to colonize the barren landscape of the dry continents. These lowly autotrophs would open a new chapter in Earth's evolution, in which the land became a vibrant new space for ecological diversity.

The Ordovician came to an end with a series of mass extinctions that ranks as the second largest mass extinction in Earth's history. Several families of organisms that had thrived in the Cambrian and Ordovician such as the trilobites went extinct during this series of catastrophic events. The migration of the continent of Gondwana can help account for the extinctions. As this giant land mass migrated to the poles and locked in water in the form of ice, sea levels subsequently dropped worldwide. Many shallow continental seas drained entirely as they ceased to be fed by nonexistent waterways. The climate in turn shifted to a much colder one as ocean currents and storm patterns changed. Entire niches of organisms and ecosystems would have been wiped out by such a dramatic shift in the earth's climate and geology.

Although the Ordovician Ice Age is the accepted cause for the mass extinction, many scientists are still baffled as to how it could have occurred. Greenhouse gases in the Earth's atmosphere were as high as twenty times their current levels, which would make freezing at the poles not a very likely phenomenon. Geothermic models have also frustrated efforts to further explain the Ice Age.