Cryogenian

The Cryogenian (from Greek cryos "cold" and genesis "birth") is a geologic period that lasted from. The Cryogenian forms the second geologic period of the Neoproterozoic Era, preceded by the Tonian period and followed by the Ediacaran. The Sturtian and Marinoan glaciations, which are the greatest ice ages known to have occurred on Earth and may have covered the entire planet, occurred during this period. These so-called 'snowball earth' events are the subject of much scientific controversy. The main debate involves whether these glaciations were truly global or merely localised events.

The period has not received the international ratification that all geological time periods undergo (the most recent being the Ediacaran Period, which was ratified in 2004). The period is defined only on the ages of the rocks and not on any observable and documented global event. This is problematic as estimates of rock ages are variable and are subject to laboratory error. For instance, the Cambrian Period is marked not by rock younger than a given age ( million years), but by the appearance of the world-wide Trichophycus pedum diagnostic trace fossil assemblage. This means that rocks can be recognised as Cambrian when examined in the field and do not require extensive testing to be performed in a lab to find a date. As yet, there is no consensus on what global event is a suitable candidate to mark the start of the Cryogenian period, and its base is only loosely set to 850 Ma.

Climate
The name of the geologic period refers to the very cold global climate of the Cryogenian: characteristic glacial deposits indicate that Earth suffered the most severe ice ages in its history during this period. Glaciers extended and contracted in a series of rhythmic pulses, possibly reaching as far as the equator. It is generally considered to be divisible into at least two major worldwide glaciations. The Sturtian glaciation persisted from 750 million years ago to 700 Ma, and the Marinoan/Varanger glaciation terminated at circa 635 Ma. The deposits of glacial tillite also occur in places that were at low latitudes during the Cryogenian, a phenomenon which led to the hypothesis of deeply-frozen planetary oceans called "Snowball Earth".

Paleogeography
During the Cryogenian, the supercontinent Rodinia broke up, and the supercontinent Pannotia began to form.

Cryogenian biota and fossils
Fossils of testate amoeba (or Arcellinida) first appear during the Cryogenian period. During the Cryogenian period, the oldest known fossils of sponges make an appearance.