Neoproterozoic

Proterozoic eon
The Proterozoic eon comprises the Paleo-, Meso-, and Neo- proterozoic eras, and is preceded by the Archaean eon.
2500 - 542 millions of years ago

The Neoproterozoic is the terminal era of the Proterozoic eon (or informally, the Precambrian) extending from about 1,000 million years ago (mya) at the end of the Mesoproterozoic to 542 mya at the beginning of the Cambrian of the Paleozoic era. It is one of three major divisions (eras) of the Proterozoic in the geologic time scale. It covers one of the more 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 later parts of the era, the Ediacaran period, the earliest evidence of multi-celled life is found, including the earliest animals.

Overview

The Proterozoic is the geologic eon extending from about 2500 million years ago (mya), at the end of the Archean eon, to about 542 mya, at the start of the current Phanerozoic eon. The Proterozoic eon is the most recent division of the Precambrian supereon, which spans the roughly 4 billion years of Earth history that preceded the current Phanerozoic eon, whose first period, the Cambrian era, is marked by the appearance of abundant macroscopic hard-shelled fossils.

The Proterozoic consists of three geologic eras, from oldest to youngest:

• Paleoproterozoic
• Mesoproterozoic
• Neoproterozoic

The Neoproterozic is the unit of geologic time from 1,000 to 542 +/- 0.3 million years ago. . It is further subdivided into the Tonian, Cryogenian, and Ediacaran Periods. The most severe glaciation known in the geologic record occurred during the Cryogenian, when ice sheets reached the equator and formed a possible "Snowball Earth"; and the earliest fossils of multicellular life are found in the Ediacaran, including the earliest animals.

Proterozoic eon (2500 - 542 million years ago)
Paleoproterozoic era				Mesoproterozoic era			Neoproterozoic era
Siderian	Rhyacian	Orosirian	Statherian	Calymmian	Ectasian	Stenian	Tonian	Cryogenian	Ediacaran
----------X--------------------------Three probable snowball earth episodes.------------------------------------X—X----

There were also abundant soft-bodied, multicellular organisms during the late Proterozoic, but being soft-bodied, fossilization was rare. Another process occurring primarily during the Proterozoic was a transition to an oxygenated atmosphere. The Proterozoic thus provided the foundation for the explosion of life forms during the Cambrian explosion, when most phyla of animals appeared in the fossil record.

Geology

At the onset of the Neoproterozoic the supercontinent Rodinia, which had assembled during the late Mesoproterozoic, straddled the equator. During the Tonian, rifting commenced which broke Rodinia into a number of individual land masses. Possibly as a consequence of the low-latitude position of most continents, several large-scale glacial events occurred during the Era including the Sturtian and Marinoan glaciations. These glaciations are believed to have been so severe that there were ice sheets at the equator—a state known as the "Snowball Earth".

Paleobiology

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 20th 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 Ediacaran 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 25 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 were most commonly known as Vendian biota until the formal naming of the Period, and are currently known as Ediacaran biota. 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).

In addition to Ediacaran biota, later two other types of biota were discovered in China (the so-called Doushantuo formation and Hainan formation).

Terminal period

The nomenclature for the terminal period of the Neoproterozoic has been unstable. Russian geologists referred to the last period of the Neoproterozoic as the Vendian, and 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 630 +5/-30 to 542 +/- 0.3 million years ago. The Ediacaran boundaries are the only Precambrian boundaries defined by biologic Global Boundary Stratotype Section and Points, rather than the absolute Global Standard Stratigraphic Ages.

References

ISBN links support NWE through referral fees

• Gradstein, F. M. and Ogg, J. G.; Smith, A. G. (eds.) (2005). A Geologic Time Scale 2004. Cambridge University Press. ISBN 0-521-78673-8. 
• Knoll, A. H. and Walter, M.; Narbonne, G.; Christie-Blick, N. (2006). The Ediacaran Period: a new addition to the geologic time scale. Lethaia 39: 13–30. PDF here.

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