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The Cryogenian (from , meaning "cold" and γένεσις, romanized: , meaning "birth") is a geologic period that lasted from . It is the second of the three periods of the Neoproterozoic era, preceded by the Tonian and followed by the Ediacaran.
The Cryogenian was a time of drastic . After the long environmental stability/stagnation during the Boring Billion, the Sturtian glaciation began at the beginning of Cryogenian, freezing the entire planet in a state of severe icehouse Earth known as a snowball Earth. After 70 million years it ended, but was quickly followed by another global ice age, the Marinoan glaciation. There is controversy over whether these glaciations indeed covered the entire planet, or whether a band of open sea survived near the equator (i.e. "slushball Earth"), but the extreme climates with massive expanse of blocking off sunlight would nevertheless have significantly hindered primary production in the and caused major and biosphere turnovers.
This could cause ambiguity because estimates of rock age are subject to variable interpretation and laboratory error. For instance, the time scale of the Cambrian Period is not reckoned by rock younger than a given age ( million years), but by the appearance of the worldwide Treptichnus pedum diagnostic trace fossil assemblages, which can be recognized in the field without extensive lab testing.
Currently, there is no consensus on what global event is a suitable candidate to mark the start of the Cryogenian Period, but a ice age would be a likely candidate.
extended and contracted in a series of rhythmic pulses, possibly reaching as far as the equator.
The Cryogenian is generally considered to be divisible into at least two major worldwide glaciations. The Sturtian glaciation persisted from 720 to 660 million years ago, and the Marinoan glaciation which ended approximately 635 Ma, 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".Hoffman, P.F. 2001. Snowball Earth theory Between the Sturtian and Marinoan glaciations was a so-called "Cryogenian interglacial period" marked by relatively warm climate and Anoxic event, along with marine transgression.
Eyles and Young state, "Most Neoproterozoic glacial deposits accumulated as glacially influenced marine strata along rifted continental margins or interiors." Worldwide deposition of dolomite might have reduced atmospheric carbon dioxide. The break up along the margins of Laurentia at about 750 Myr occurs at about the same time as the deposition of the Rapitan Group in North America, contemporaneously with the Sturtian in Australia. A similar period of rifting at about 650 Ma occurred with the deposition of the Ice Brook Formation in North America, contemporaneously with the Marinoan in Australia. The Sturtian and Marinoan are local divisions within the Adelaide Rift Complex.
Fossils of testate amoeba (or Arcellinida) first appear during the Cryogenian Period. Since 2009, some researchers have argued that during the Cryogenian Period, potentially the oldest known fossils of sponges, and therefore animals, were formed. However, it is unclear whether these fossils actually belong to sponges, though the authors do not rule out the possibility of such fossils to represent proto-sponges or complex microbial precursors to sponge-grade organisms. 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, , , , and testate amoeba.
The end of the period also saw the origin of plankton, which would feed on unicellular algae and , ending the dominance of the oceans. Fossil fats reveal how complex life kicked off after Snowball Earth phase The unicellular algae (Archaeplastida) went through a big bang of diversification, and their population went up by a factor of a hundred to a thousand. We Finally Know Which Groundbreaking Period in Earth's History Gave Rise to The First Animals The algae that terraformed Earth
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