The Ordovician was the second of the six Paleozoic periods lasting about 47 million years, that began around 490 million years ago (488.3 million years go to be more precise) and ended around 443 million years ago (443.7 million years ago).

During the Ordovician most organisms that have left us fossil records were living the shallow seas, and there was very little life on the land surface. The seas were full of brachiopods, crinoids, trilobites, and conodonts. Scottish fossils of conodonts soft tissue show that these creatures had some resemblance to lampreys. Toward the end of the Ordovician there was a mass extinction event.

At the start of the Ordovician era (or at the of the Cambrian) there is some evidence for widespread glaciation and a dramatic drop in oxygen concentrations in the atmosphere. Perhaps a severe ice age caused the mass extinction at the end of the Cambrian and the start of the Ordovician Period. For most of the Ordovician Period, however, temperatures were warm, and carbon dioxide (CO2) concentrations in the atmosphere were incredibly high (eight to ten times more CO2 in the atmosphere than now), so that the greenhouse effect would have kept the world very warm. However, at the end of the Ordovician during the Himantian era of the Ordovician Period, there is considerable evidence of a long and severe ice age (of perhaps half a million years duration or longer), and that is a candidate for causing the mass extinction at the end of the Ordovician. The ice age at the end of the Ordovician (and perhaps at its start) would be a result of lower temperatures that would in themselves cause extinctions, but as polar ice caps formed over the south pole, where Gondwana was situated, sea levels would have dropped, and the shallow seas of continental shelves would have been exposed above water, greatly reducing habitat for species living in shallow seas.

During the Ordovician Period life began to move on to land. The first pioneers of life moving out of the seas were mainly bacteria, and they probably got out of the water more than 600 million years ago, perhaps even billions of years ago, although fossil records of microbes on land are exceedingly rare, and therefore scientists do not write much about speculative microbial life on land in the hundreds of millions of years before the Ordovician Period. By 440 million years ago (actually in the early Silurian, but implying ancestries that would have begun in the late Ordovician) some fungi such as Tortotubus were established in terrestrial soils. Fossilized fungi spores dated back to about 450 millions years ago also demonstrate that fungi were on land in the late Ordovician. Some of the earliest algae to live on land probably needed to have evolved a symbiotic relationship with fungi in order to survive out of the water. In fact, one theory about how an ice age could develop late in the Ordovician despite the high CO2 concentrations suggests that the earliest lichens, mosses, and liverworts, which probably evolved in the late Ordovician around 450 million years ago, weathered silicate rocks such as granite, and this would have contributed to a rapid reduction of CO2 and possibly a plunge in global temperatures.

The two cephalopods above (Actinoceras bigsbyi) and the crinoids behind them (Cupulocrinus plattesvillensis is orange and Glyptocrinus charitoni is purple) lived in the shallow seas over Illinois in the middle Ordovician, about 470 million years ago. These specimens can be found in the Illinois State Museum in Springfield, IL.

Back to the Biological Timeline: terrestrial animals, trilobite , prokaryote , oxygen catastrophe , Metazoa , eukaryote .

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This work by Eric & Chun-Chih Hadley-Ives is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.