The Ordovician Period – Life Makes a Comeback

490 to 440 Million Years Ago

 

After the Cambrian Extinction, life rebounded during the Ordovician as the oxygen levels began to rise and the seas receded from the continents for a time. Life began to thrive on the land and in the sea. While the number of phyla in the Ordovician was few compared to the Cambrian, there were families in abundance: 150 families existed during the Cambrian compared to the 400 in the Ordovician. This means that while there was not the huge variety of body plans as those found in the Cambrian, there were greater numbers and varieties within each body structure: more types of trilobites, more types of brachiopods, etc.

It was at this time that the first primitive land plants appeared, arising from the green algae living in shallow seas and along the shorelines. These first plants were simple and low-growing like our modern mosses and liverworts, using spores to colonize the riparian landscape.

Rising sea levels produced the highest ever sea levels recorded and extinguished a great many species on land and in the shallow seas.  Soon, the planet began to cool, producing extensive glaciation and leading to one of the greatest mass extinctions, second only to the Permian. 

Oxygen, Carbon Dioxide and the Atmosphere

• Oxygen levels increased after dropping low at the end of the Cambrian. This increase enabled life to thrive and promoted the creation of new body plans to adapt to rising oxygen levels.
  

Geography – Continents and Oceans

• The seas receded from their high points during the Cambrian; but by the Middle of the Ordovician, they once again flooded the continents and reached the highest point ever recorded.
  
• The supercontinent Gondwana drifted slowly southward towards the South Pole: having begun its journey in tropic latitudes, it gradually accumulated glaciers as it arrived in cooler regions.
  

Climate

• The climate was warm and humid but later developed into a glacial climate leading to severe extinctions at the end of the period.
  

Plant Life

• First primitive plants appeared on land.  Their impact cannot be overstated since they changed the landscape, soil, air and promoted life on land by providing shelter and food resources.
  

Animal Life

• The life forms that appeared in the Ordovician were more like modern forms than during the Cambrian.
  
• Diversity thrived on the family and species level.
  
• First true corals emerged – some scientists feel that this is the most significant event of the Ordovician period.
  
• Animals developed calcareous skeletons (shells made of calcium carbonate).
  
  • The benefits of these calcium skeletons include protection from UV rays in shallow waters, protection against predators, and protection against desiccation in shallow tidal waters.
    
• Trilobites developed many strange forms as they struggled against new types of predators.
  
• The food chain became more complicated as new varieties of life occurred.
  
• New life forms includes true corals, nautiloids, starfish, brachiopods, etc.
  
• Conodonts (small, eel-like creatures) became common and their fossils serve as markers in the rock layers greatly aiding paleontologists in their efforts to determine the different ages of the rocks.
  

Extinction Events

• The Ordovician Extinction is one of the Five Great Mass Extinctions.
  
• It is second only to the Permian extinction which means that the Ordovician was even more devastating than the extinction event that destroyed the dinosaurs.
  
• The extinction occurred in two waves possibly related to changing sea levels and glaciations.