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The Ediacaran Period (named after the Ediacara Hills of South Australia, where fossils from this period have been found) is a geologic period ranging between about 635 to 542 million years ago. The Ediacaran period features the first macroscopic multicellular fossils, dated as far back as 610 million years ago (Twitya formation), although the most diverse communities are centered around 575 - 542 million years ago. Putative embryo fossils have been dated to the dawn of the Ediacaran, 632.5 million years ago.
The Ediacaran Period began immediately after the most severe planetary glaciation (Ice Age) of the last billion years — the Marinoan glaciation. This Ice Age, extending from 745 - 635 million years ago, was so extreme that the period prior to the Ediacaran is named the Cryogenian Period, after the Greek "cryo" meaning "cold." Some scientists believe that the world's oceans underwent almost-total freezing episodes during this period, leading to a "Snowball Earth" scenario. Deposits from glaciers have been found at equatorial latitudes. Life might have survived in refugia such as around deep-sea hydrothermal vents. The division between the Ediacaran and Cryogenian is the time when the Ice Age finally ended.
Although life in microbe form has existed since at least 2.7 billion years ago, and microbes with nuclei (eukaryotes) have existed since 1.2 billion years ago, the Ediacaran represents the first uncontroversial fossils of multicellular life. This family of life, the "Ediacaran fauna," consists of a variety of plant-like animals of unknown affinity with a quilted appearance, with the form of fronds, discs, bags, and "inflated mattresses." Alongside the Ediacaran fauna lived some precursors of modern phyla, such as the trilobite-like Spriggina, which are not generally considered "Ediacaran fauna" but rather "non-Ediacaran organisms that lived during the Ediacaran Period." It's confusing, but the term "Ediacaran" is also used to refer to a distinct grouping of ancient organisms as well as the geologic period itself.
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