720–635 million years ago
The Cryogenian (pronounced //, 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 main debate contests whether these glaciations covered the entire planet (the so-called 'Snowball Earth') or if a band of open sea survived near the equator (termed 'slushball Earth').
The Cryogenian period was ratified in 1990 by the International Commission on Stratigraphy. In contrast to most other time periods the beginning of the Cryogenian is not linked to a globally observable and documented event. Instead the base of the period is defined by a fixed rock age, that was set at 850 million years until 2015, when it was changed to 720 million years.
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 (541.0 ± 1.0 million years), but by the appearance of the worldwide Treptichnus pedum diagnostic trace fossil assemblage. This means that rocks can be recognized as Cambrian when examined in the field and do not require extensive testing to be performed in a lab to find a date.
Currently, there is no consensus on what global event is a suitable candidate to mark the start of the Cryogenian Period, but a global glaciation would be a likely candidate.
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 (Sturtian and Marinoan).
The Cryogenian is generally considered to be divisible into at least two major worldwide glaciations. The Sturtian glaciation persisted from 750 to 700 million years ago, and the Marinoan glaciation which ended approximately 635 M.Y.A., at the end of the Cryogenian. 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".
Before the start of the Cryogenian, around 750 M.Y.A., the cratons that made up the supercontinent Rodinia started to rift apart. The superocean Mirovia began to close while the superocean Panthalassa began to form. The cratons (possibly) later assembled into another supercontinent called Pannotia, in the Ediacaran.
Cryogenian biota and fossils
|This section requires expansion. (January 2011)|
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. The issue of whether or not biology was impacted by this event has not been settled, for example Porter (2000) suggests that new groups of life evolved during this period, including the red algae and green algae, stramenopiles, ciliates, dinoflagellates, and testate amoeba, which have already been mentioned above.
In popular culture
- BBC Horizon Snowball Earth (2001) 
- The Time Travellers Guide To Australia (Australia, 2012).
- BBC/CBC/NHK "Miracle Planet : Snowball Earth" (2010s) episode 
- Arnaud, E.; Halverson, G. P.; Shields-Zhou, G. (30 November 2011). "Chapter 1 The geological record of Neoproterozoic ice ages". Geological Society, London, Memoirs 36 (1): 1–16. doi:10.1144/M36.1.
- Brain, C. K., Prave, A. R., Hoffmann, K. H., Fallik, A. E., Herd D. A., Sturrock, C., Young, I., Condon, D. J., Allison, S. G. (2012). "The first animals: ca. 760-million-year-old sponge-like fossils from Namibia". S. Afr. J. Sci. 108 (8): 1–8. doi:10.4102/sajs.v108i1/2.658.
- Macdonald, F. A.; Schmitz, M. D.; Crowley, J. L.; Roots, C. F.; Jones, D. S.; Maloof, A. C.; Strauss, J. V.; Cohen, P. A.; Johnston, D. T.; Schrag, D. P. (4 March 2010). "Calibrating the Cryogenian". Science 327 (5970): 1241–1243. doi:10.1126/science.1183325. PMID 20203045. (Duration and magnitude are enigmatic)
- "Discovery of possible earliest animal life pushes back fossil record". phys.org. Retrieved 7 December 2012.
- These events were formerly considered together as the Varanger glaciations, from their first detection in Norway's Varanger Peninsula.
- Plumb, Kenneth A. (1991). "New Precambrian time scale" (PDF). Episode. 2 14: 134–140. Retrieved 7 September 2013.
- "GSSP Table - Precambrian". Geologic Timescale Foundation. Retrieved 7 September 2013.
- "Chart". International Commission on Stratigraphy. Retrieved 2015-07-13.
- Dave Lawrence (2003). "Microfossil lineages support sloshy snowball Earth". Geotimes.
- 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; Grosjean, Emmanuelle; Stalvies, Charlotte; Fike, David A.; Grotzinger, John P.; Bradley, Alexander S.; Kelly, Amy E.; Bhatia, Maya; Meredith, William et al. (2009). "Fossil steroids record the appearance of Demospongiae during the Cryogenian period" (PDF). Nature 457 (7230): 718–721. Bibcode:2009Natur.457..718L. 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; Simons, Frederik J. (17 August 2010). "Possible animal-body fossils in pre-Marinoan limestones from South Australia". Nature Geoscience 3 (9): 653–659. Bibcode:2010NatGe...3..653M. doi:10.1038/ngeo934.
- "Discovery of possible earliest animal life pushes back fossil record". 2010-08-17.
- "Cryogenian Period". GeoWhen Database. Archived from the original on December 2, 2005. 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.
- Brain, C. K.; Prave, A. R.; Hoffmann, K. H.; Fallick, A. E.; Herd, D. A. Sturrock, C.; Young, I.; Condon, D. J. Allison, S. G. (2012). "The first animals: ca. 760-million-year-old sponge-like fossils from Namibia". South African Journal of Science S. Afr. J. Sci. 108: 1–8. doi:10.4102/sajs.v108i1/2.658.
- The Cryogenian Period
|Preceded by Archean Eon||Proterozoic Eon||Followed by Phanerozoic Eon|
|Paleoproterozoic Era||Mesoproterozoic Era||Neoproterozoic Era|