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 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.
- 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.
- The climate was warm and humid but later developed into a glacial climate
leading to severe extinctions at the end of the period.
- 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.
- 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
- The food chain became more complicated as new varieties of life occurred.
- New life forms includes true corals, nautiloids, starfish,
- 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.
- 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