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The Cryogenian period is a geologic period from 850 million to 630 million years ago. It occurred before the Ediacaran Period and after the Tonian Period, and is part of the much longer Proterozoic era, meaning "era of primitive life." In Greek, Cryogenian means "ice origin." Although ice did not really originate with the Cryogenian period, it was everywhere, and glaciers may have extended from pole to pole. A human transported to that time might have been able to ski all the way around the Earth. The Cryogenian period is one of the few geologic periods in the last billion years to be named after a substance or concept (in this case, cold) rather than a modern area where fossils from the period are found (for instance, the Jurassic is named after the Jura Mountains).
During the Cryogenian period there were at least two major ice ages, and perhaps as many as four. Glacial deposits in the Cryogenian strata at equatorial paleolatitiudes (specifically, the Congo and Kalahari cratons) has led many scientists to consider the possibility of a "Snowball Earth" — a planet so cold the oceans froze solid. This has led to a great amount of controversy in the scientific community. Many scientists doubt the geophysical viability of a completely frozen ocean. Simulations have been run, but such calculations push the limits of available computing power, and radical simplifying assumptions are sometimes necessary. One study ignores the existence of the continents, for instance. Some scientists take a middle ground and argue for a "Slushball Earth" scenario, where the oceans are covered in large amounts of sea ice, but not frozen all the way to the bottom.
The two confirmed major glaciations in the Cryogenian period were the Sturtian glaciation (760 million years ago to 700 mya) and the Marinoan/Varanger glaciation (710/650 to 635 Ma). During these glaciations, the average global temperature would have dropped at least 20 °C, from 22 °C (71 °F) to about 2 °C (37 °F), and possibly much lower, to less than -30 °C (-22 °F). The temperature at the poles may have been so low that carbon dioxide might have frozen into dry ice, which has a freezing point of -78.5 °C (-109.3 °F). Dry ice is the main constituent of Mars' polar ice caps.
Life existed long before the Cryogenian period and obviously survived through it. Plants, animals, and fungi all existed, albeit almost exclusively in unicellular form. There is some evidence for small burrows under microbial mats, perhaps made by simple multicellular organisms. Small organic fossils called acritarchs have been found in large quantities and diversity prior to the Cryogenian period, but both crashed during the period. Acritarchs have been isolated from Cryogenian sediments, they just aren't very numerous or diverse.