850–630 million years ago
The Cryogenian (from Greek cryos "cold" and genesis "birth") is a geologic period that lasted from . It 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 start of 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 (542 million years), but by the appearance of the worldwide Treptichnus 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.
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 glaciation which ended approximately 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".
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 (and therefore animals) make an appearance.   
- ^ These events were formerly considered together as the Varanger glaciations, from their first detection in Norway's Varanger Peninsula.
- ^ Dave Lawrence (2003). "Microfossil lineages support sloshy snowball Earth". Geotimes. http://www.agiweb.org/geotimes/apr03/WebExtra041803.html.
- ^ Shields, G. A. (2008). "Palaeoclimate: Marinoan meltdown". Nature Geoscience 1 (6): 351–353. doi:10.1038/ngeo214.
- ^ Hoffman, P.F. 2001. Snowball Earth theory
- ^ Porter, S.A., and Knoll, A.H. (2000). "Testate amoeba in the Neoproterozoic Era: evidence from vase-shaped microfossils in the Chuar Group, Grand Canyon". Paleobiology 26 (3): 360–385.. doi:10.1666/0094-8373(2000)026<0360:TAITNE>2.0.CO;2. ISSN 0094-8373.
- ^ Love et al.; Grosjean, Emmanuelle; Stalvies, Charlotte; Fike, David A.; Grotzinger, John P.; Bradley, Alexander S.; Kelly, Amy E.; Bhatia, Maya et al. (2009). "Fossil steroids record the appearance of Demospongiae during the Cryogenian period ". Nature 457 (7230): 718–721. doi:10.1038/nature07673. PMID 19194449.
- ^ Maloof, Adam C.; Rose, Catherine V.; Beach, Robert; Samuels, Bradley M.; Calmet, Claire C.; Erwin, Douglas H.; Poirier, Gerald R.; Yao, Nan et al. (17 August 2010). "Possible animal-body fossils in pre-Marinoan limestones from South Australia". Nature Geoscience 3: 653–659. doi:10.1038/ngeo934. http://www.nature.com/ngeo/journal/v3/n9/abs/ngeo934.html.
- ^ "Discovery of possible earliest animal life pushes back fossil record". 2010-08-17. http://www.physorg.com/news201273825.html.
- "Cryogenian Period". GeoWhen Database. Archived from the original on December 2, 2005. http://web.archive.org/web/20051202005002/http://www.stratigraphy.org/geowhen/stages/Cryogenian.html. Retrieved January 5, 2006.
- James G. Ogg (2004). "Status on Divisions of the International Geologic Time Scale". Lethaia 37 (2): 183–199. doi:10.1080/00241160410006492.
Preceded by Archean Eon 2.5 Ga - Proterozoic Eon - 542 Ma Followed by Phanerozoic Eon 2.6 Ga - Paleoproterozoic Era - 1.6 Ga 1.6 Ga - Mesoproterozoic Era - 1.0 Ga 1.0 Ga - Neoproterozoic Era - 542 Ma Siderian Rhyacian Orosirian Statherian Calymmian Ectasian Stenian Tonian Cryogenian Ediacaran Ice ages Quaternary glaciationCurrent glaciationPleistocene glaciation Pre-Quaternary glaciations
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