The Cambrian period started with life explosion, and probably ended with extinction. However, life found its way back in the Ordovician period. It also includes the life explosion, this time a bit smaller. The end of the period also brings the mass extinction, but this time it is enormous and very dramatic.
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The Great Ordovician Bio-Diversification Event
Only 40 million years after the Cambrian explosion another life revolution happened. The triggers of the event might have been the changes in geography, that led to creating new environments. Another study suggests that it was the dropping of Earth’s temperature or the volcanic activity. The exact solution remains unknown, though we know more about results. In a way, the event continues the work of the Cambrian explosion.
First, more phytoplankton developed, leading to creating new ecological niches. That led to filling new phyla with lower-level taxa, increasing the biodiversity of ocean life like never before. Moreover, organisms began to be more complex and started to spread on Earth. As a result of that, they started to adapt to different conditions, which led to even more new kinds of organisms. The gastropods, today known as snails and slugs, appeared, so did mollusks.
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First Vertebrates
The first thing about vertebrates is that they have bones or cartilage, that surrounds the spinal cord. First vertebrates appeared on Earth in Ordovician, today we classify them as jawless fish. That’s why Ordovician and Silurian periods are often called ‘Age of fishes’. Moreover, first jaw fish also showed up in Ordovician, about 10 million years later. This is so important because all other taxons of vertebrates evolved from fishes. However, new pieces of evidence, found in China, suggest that organisms with the spinal cord evolved during the Cambrian explosion. The argument remains unsolved for now. Despite the fact, that vertebrates can adapt to more changes than invertebrates, during the Ordovician primitive species, like sponges, dominated the sea.
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Changes on Earth During the Ordovician Period
As we said, in the previous period Gondwana started falling apart. It probably continued that through most of the period. It also started to move north-east. That led to dividing Earth’s waters into three major oceans. The temperature was quite stable for most times, however, it started to change in the end. The activity of volcanoes led to more greenhouse gases in the air. As a result, the temperature started to rise.
Sea levels dropped, since Gondwana started to move back south, and uniting with remaining land. Moreover, it caused glaciation, which led to dropping sea levels. Those changes contributed to the extinction event at the end of the period. Besides, a hypothesis suggests a meteor shower in the middle of the period. Scientists do not associate it with the extinction event, moreover, it could cause more biodiversity.
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The Ordovician-Silurian Extinction
The end of the period is marked with a big extinction event. It might have been triggered by the changes on Earth we discussed. Another solution suggests it was caused by a gamma-ray burst, that destroyed the ozone layer. Whatever was the cause, or whatever causes summed up to cause the mass extinction, it wiped out 85% of existing species. That impressive percentage makes it the second-biggest mass extinction event in Earth’s history.
Almost 50% of complex marine families were eliminated, including brachiopods and, again like in Cambrian, trilobites. The event ended the Ordovician radiation and lasted until the beginning of Silurian when the ice probably melted and sea level could rise again. Until then, the remaining 15% of species, mostly one-cellular ones, lived under the layers of ice. The reason life even survived, is that liquid water is denser than ice.
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Ordovician Period Evidence
If we look at the Ordovician fossils, we can see some familiar shapes. It is due to Ordovician radiation when a lot of species that survived until today appeared. We can observe first fishes, starfishes, and mollusks. Trilobites from that period also often appear. However, they developed since Cambrian, for example, some had defense structures they could use against predators. Of course some organisms, like graptolites, did not survive the mass extinction event and now their fossils seem alien to us.
On land, fungus spores from around 460 million years ago were found in Wisconsin, US, proving that they emerged on Earth. Another evidence includes trace fossils of algae, which ruled the oceans. Additionally, some recent works question the current view on the extinction event. Studies suggest that it was a single episode, that lasted just several hundred thousand years, and was associated only with dropping sea levels.
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Previous in series: The Cambrian Explosion – Life Revolution
Author’s Message
I hope this article helped you understand the importance of the events of the Ordovician period. In the next article, we will continue digging into Paleozoic, moving to the Silurian. If you enjoyed this article, please share it with family and friends. In case you have any questions, please contact me. You can find an email in the author’s section of the page.
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