The Neoproterozoic is the third of three subdivisions of the Proterozoic Eon (occurring from 1 billion years ago to 542 million years ago). This terminal era of the Proterozoic is itself divided into three sub-periods called the Tonian, Cryogenian, and Ediacaran Periods. The most severe glaciation known in the geologic record occurred during the Cryogenian Period, when ice sheets reached the equator and formed a possible “Snowball Earth.” And the earliest fossils of multi-cellular life can be found in the Ediacaran Period, including the earliest animals.
The supercontinent Rodinia, which had been assembled during the late Mesoproterozoic Eon, had straddled the equator at the onset of the Neoproterozoic. The Tonian Period is marked by rifting that ultimately broke Rodinia up into a number of individual land masses. Possibly a consequence of the low-altitude position of most continental land masses, several large-scale glacial events occurred during the Neoproterozoic’s Cryogenian Period.
A few early fossils found in the Ediacaran Period appear to be possible ancestors of modern animals. Although most fall into ambiguous groups of frond-like animals, discoids, medusoids, small calcareous tubes, and armored animals of unknown provenance. These animal groups were commonly known as Vendian biota until the formal naming of the Period, and are now known as Ediacaran biota. Most were soft bodied and relationships to modern forms of animal life are obscure at best. While many experts relate these early organisms to modern animals, others only acknowledge a few possibilities and feel the rest are representatives of unknown animal types.
The terminal period of the Neoproterozoic, referred to as the Ediacaran by North Americans and Australians, has been typically called the Vendian Period by Russians and the Sinian Period by Chinese. However, in 2004, the International Union of Geological Sciences ratified the Ediacaran age to be a geological age of the Neoproterozoic, ranging from 635 to 542 million years ago. The Ediacaran boundaries are the only Precambrian boundaries defined by biologic Global Boundary Stratotype Section and Points, rather than absolute Global Standard Stratigraphic Ages.
Image Caption: The geological clock: a projection of Earth’s 4,5 Ga history on a clock (“MA” = a million years (Megayear) ago; “GA” = a billion years (Gigayear) ago). Credit: Woudloper/Wikipedia