Neoproterozoic

Neoproterozoic/Infobox The Neoproterozoic is the Geological Era from 1000 million years before the present to 542 million years before the present. The Neoproterozoic covers perhaps the most interesting time in the geological record, during which the Earth was hit by the most severe glaciations known (during which ice sheets reached the Equator), and in the latter parts of the Era (the Ediacaran Period) the earliest fossils of multicelled life are found, including the earliest animals. The Neoproterozoic is the final part of Proterozoic or Pre-Cambrian time.

The idea of the Neoproterozoic Era came on the scene relatively recently -- after about 1960. Nineteenth century paleontologists set the start of multicelled life at the first appearance of hard-shelled animals called trilobites and archeocyathids. This set the beginning of the Cambrian period. In the early twentieth century, paleontologists started finding fossils of multicellular animals that predated the Cambrian boundary. A complex fauna was found in South West Africa in the 1920s but was misdated. Another was found in South Australia in the 1940s but was not thoroughly examined until the late 1950s. Other possible early fossils were found in Russia, England, Canada, and elsewhere (see Vendian Biota. Some were determined to be pseudofossils, but others were revealed to be members of rather complex biotas that are still poorly understood. At least 20 regions worldwide yielded metazoan fossils prior to the classical Cambrian boundary.

A few of the early animals appear possibly to be ancestors of modern animals. Most fall into ambiguous groups of frond-like animals(?); discoids that might be holdfasts for stalked animals(?) ("medusoids"); mattress-like forms; small calcaerous tubes; and armored animals of unknown provenance. These are generally called Ediacaran or Vendian animals. Most were soft bodied. The relationships, if any, to modern forms are obscure. Some paleontologists relate many or most of these forms to modern animals. Others acknowledge a few possible or even likely relationships but feel that most of the Ediacaran forms are representatives of (an) unknown animal type(s).

Geologically, the Neoproterozoic is thought to comprise a time of complex continental motion as a supercontinent called Rodinia broke up into perhaps as many as eight pieces. Several massive worldwide glaciations occurred during the Era including the Sturtian and Marinoan glaciations, the most severe the Earth has ever known. These are believed to have been so severe as to bring icecaps to the equator, leading to a state known as the "Snowball Earth".

The nomenclature for the Neoproterozoic has in the past been unstable. Russian geologists referred to the final period of the Neoproterozoic as the Vendian, while the Chinese called it the Sinian, and most Australians and North Americans used the name Ediacaran. However, in 2004, the International Union of Geological Sciences ratified the Ediacaran age to be a geological age of the Neoproterozoic, ranging from 600 to 544 million years before present.