What is Cryogenian?
{{nosources|date=April 2017}} The '''Cryogenian''' is a [[geology|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 age]]s 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 CO<sub>2</sub> from the [[atmosphere]] by [[photosynthesis]]ing [[bacteria]] and [[eukaryote]]s in the previous [[Great Oxygenation Event]]. Much else happened: the supercontinent [[Rodinia]] broke up and another, [[Pannotia]], began to form. Fossil [[steroid]]s suggest the presence of early [[sponge]]s, and amoeboid cells with tests (coverings) appear in the [[fossil]] record. {{Geologic History}} {{sci-stub}} [[Category:Neoproterozoic]]