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The Cryogenian is a geological period, from ~720 million years ago (mya) to 635 mya. It lies in the middle Proterozoic eon before the Ediacaran period.

The period is important to both Earth science and biology because it contains the two longest and most severe ice ages of all time:

  • The Sturtian ice age 717 mya to 660 mya
  • The Marinoan ice age 650 mya to 635 mya

These two snowball earth events were previously put together as the Varangian glaciation. They covered much, possibly all, the Earth with ice. The root cause of the temperature drop may have been the removal of CO2 from the atmosphere by photosynthesising bacteria and eukaryotes in the previous Great Oxygenation Event. Much else happened: the supercontinent Rodinia broke up and another, Pannotia, began to form. Fossil steroids suggest the presence of early sponges, and amoeboid cells with tests (coverings) appear in the fossil record.