Neoproterozoic Era

This was the last era of the Proterozoic, and lasted from about 1.0 billion to 542 million years ago. This era and its periods are all dated chronometrically rather than stratigraphically, except for the very last part (the Ediacaran), which has been defined bio-stratigraphically. Stratigraphic labels depend upon the establishment of Global Boundary Stratotype Sections and Points (GSSP's), which are globally prevalent markers in the strata that distinguish one period from another. The stratigraphy of the Precambrian is not well established, but it is a work in progress and new developments continue to be made. A list of currently accepted GSSP's for different ages of the Earth's history can be found here.

The last period of the Neoproterozoic (the Ediacaran) has been accepted by International Committee on Stratigraphy, and was published in Science (volume 305, pages 621-22, 2004). The base was defined as the base of the Marinoan cap carbonate (Nuccaleena Formation) in the Enorama Creek Section of the central Flinders Ranges, Adelaide Rift Complex, South Australia. The principal observed correlation events are (1) the rapid decay of Marinoan ice sheets and onset of distinct cap carbonates throughout the world, and (2) the beginning of a distinctive pattern of secular changes in carbon isotopes. Other locations around the world include Marinoan-type cap carbonates above the Nantou Formation in China, the Blaini Formation in the lesser Himalayas (India), the Smalfjord Formation in North Norway, the Icebrook Formation in Canada, the Ghaub Formation in Namibia. The Yudoma Group in Siberia and the Vendian succession in Ukraine, both begin with a transgression within the Ediacaran Period. More details can be found in Knoll, et al., 2004a and Knoll, et al., 2004b.

The second period of the Neoproterozoic, the Cryogenian, is currently defined chronometrically, but will soon be defined stratigraphically as well. GSSP replacement of the chronometric system is expected in 2009.

The Neoproterozoic is marked by several developments in the Earth's climate, geology, and biota. Some of the highlights include:

The Neoproterozoic era is divided into three periods: the Tonian, the Cryogenian, and the Ediacaran, which are described in more detail below.

Acritarch Vandalosphaeridium walcottii from the late Tonian, found in the Grand Canyon. From the Precambrian Paleobiology Group, UCLA.

Tonian

This was the first period of the Neoproterozoic, dating roughly from the end of the Grenville orogeny about 1.0 billion years ago, to 850 million years ago. This period has been chronometrically defined, with moderately positive 13C values, while 87Sr/86Sr values remain below 0.7065 with minimal variation. The name is derived from the Greek word meaning "stretch" possibly from the rifting and stretching of continental plates, which led to the breakup of Rodinia. The stretching created a thinning of the crust in the stretched regions, leading to the formation of inland seas. There is a lot of evidence of drowned cratons covered with stromatolites.

An interesting feature of the Tonian is the first radiation of the Acritarchs (meaning "of uncertain origin"). Acritarchs are small organic microfossils. They are found predominantly in sedimentary rock of marine origin, and the term applies to how they are extracted from the sediments. The rock is pulverized and then dissolved in hydrofluoric acid. Acritarchs do not dissolve (showing that they have non-carbonaceous and non-siliceous skeletons) and are extracted. They are clearly organic-walled unmineralized structures, and are the remains of several different organisms, usually in their cyst stages.

Acritarchs first appear in the geological record about 1.4 billion years ago, in the beginning of the Ectasian. They include the remains of a wide range of quite different kinds of organisms - ranging from the egg cases of small metazoans to resting cysts of many different kinds of chlorophyta (green algae). It is likely that some acritarch species represent the resting stages (cysts) of algae that were ancestral to the dinoflagellates. The nature of the organisms associated with older acritarchs is generally not clear, though many are probably related to unicellular marine algae. In theory, when the biological source (taxon) of an acritarch does become known, that particular microfossil is removed from the acritarchs and classified with its proper group.

The Tonian produced a great increase in the diversity of Acritarchs, which was sustained through the Neoproterozoic, other than a small decrease during the Cryogenian glaciations.

Rodinia was the dominant super continent during the Tonian. It was centered somewhat south of the equator, and surrounded by the vast Mirovian ocean. There were many inland seas, and more appeared during the Tonian. Stromatolites were found both in the inland seas (freshwater) and along the outer coasts (marine environment).

Cryogenian

This was the second period of the Neoproterozoic, and lasted from about 850 million to 635 million years ago. The period is chronometrically dated, though a stratigraphy based definition is expected in 2009. The name "Cryogenian" derives from the Greek word for "cold". This period includes at least 2 major glaciations: the Sturtian and Marinoan. It seems possible that these glaciations covered the entire Earth, including the equator, and led to the "snowball Earth" scenario.

Rodinia during the Cryogenian, with landmasses clustered near the south pole. Red asterisks mark the site of glacial deposits from this period. Reprinted by permission from Macmillan Publishers Ltd: Nature: Hyde, Crowley, Baum and Peltier, copyright 2000.

The Cryogenian is also sometimes referred to as the Varangian, based on the Varanger glaciation, which occurred roughly from 800 million to 630 million years, which is roughly the same period as the Cryogenian. However, further refinement of the dates now separates this from the Sturtian glaciation, and the Marinoan glaciation. Despite the ICS approved term "Cryogenian", the term Varangian is still in common use to denote the same period of the Neoproterozoic.

Evidence for extensive glaciation during the Neoproterozoic has been accumulating since the 1960's, but the theory of snowball Earth gained much momentum following the publication of Hoffman's paper on the subject (Hoffman PF and Schrag DP, Snowball Earth, Scientific American, January 2000, pages 68-75). Three glaciations are thought to have produced a snowball Earth during the Neoproterozoic, starting with the Sturtian (800 million years ago), the Varanger (600 million years ago) and the Marinoan (580 million years ago). Evidence for the first of these, the Sturtian remains the strongest.

The idea of snowball Earth comes from the theory (substantiated by computer models) that when glaciation is intense enough for the glaciers to reach about 30° North and South latitudes, the Earth's albedo rises sufficiently to reflect a large part of the incoming sunlight back into space. This causes a series of cascading events which lead to further glaciation, with glaciers eventually covering the entire Earth, right down to the equator. The Earth in fact, looks like a snowball from space.

It has been speculated that the presence of the super continent Rodinia in the southern oceans disrupted the flow of ocean currents, that normally transfer heat between the equator and the poles. The figure above shows a possible configuration of Rodinia in the middle of the Cryogenian (when Rodinia had started to break apart, around 750 million years ago) used in computer simulations. Note the position of Australia near the equator. The red asterisks indicate glacial deposits from this period, and the presence of such deposits in Australia has been offered as evidence that glaciation extended to the equator.

The snowball Earth scenario does not require glaciation of the continents -- only the oceans. The state of the continents in a snowball Earth is a matter of debate. In temperate and polar regions, the glaciers likely extended over the land as well, to depths of several thousands of feet. However, the ice cover on the oceans prevented water from evaporating, and therefore the climate must have been very dry. Lack of precipitation likely caused at least parts of continents to be bare rock, as ice was sublimated or eroded or flowed into the sea, and was not replaced due to the lack of precipitation.

Snowball Earth produced a compartmentalization of many of the Earth's geographical zones. The water cycle was severely hampered, as explained above, by the ice cover on the oceans which prevented sea water from evaporating and coming down as rain or snow. Volcanism underneath the sea changed the ocean chemistry, since the gases could not easily escape into the atmosphere. The dry climate inhibited the normal flow of sediments into the sea, carried by rainwater. The carbon cycle was affected as well. Rain normally washes carbon dioxide out of the atmosphere into the ground, where carbonic acid reacts with rocks to form carbonates/bicarbonates, which are washed into the sea and eventually deposited on the sea floor. These processes must have been affected during snowball Earth. Carbon excursions during the Neoproterozoic indicate that unusual things were happening to the carbon cycle at the time, but their interpretation has not been settled.

The commonly proposed scenario for the end of snowball Earth is through the accumulation of carbon dioxide. Volcanism produces carbon dioxide, which accumulates until it reaches a point where it triggers warming through its greenhouse effect. The ice sheets are melted rapidly and temperatures rise, perhaps reaching as high at 50 °C temporarily, before the carbon dioxide is removed from the atmosphere. There is strong evidence of such extreme rises in atmospheric carbon dioxide, in the form of cap carbonates.

Rodinia started to break up in the middle of the Cryogenian, around 750 million years ago. Rift zones opened up between Laurentia (which occupied the central position in Rodinia) and continents to both the north and south. In the north lay Ur, which was separated from Laurentia as Laurentia started to drift eastwards. This opened up an ocean along the west coast of Laurentia, which was later to become the Panthallasic Ocean (the proto-Pacific, which lies on the western side of North America today). Rifts also appeared along the southern edge of Laurentia, separating it from Baltica and Atlantica (Amazonia and West Africa), which continued to drift southwards. These rifts opened up the ancient Iapetus Ocean between Laurentia and Atlantica.

Life in the Cryogenian seemed to proceed as usual. One would expect that massive glaciations and a snowball Earth would impact life, and there are some signs that perhaps it did. A number of reports claim a dip in both the diversity and abundance of life during the Cryogenian, but these are contradicted by other reports which show no significant change. This goes against expectations, and is hard to reconcile with the theory of a snowball Earth.

Several groups may have evolved or undergone significant developments during the Cryogenian, including red and blue algae, dinoflagellates, ciliates, and testate amoebae. The amoebae are specially important, because they are the first fossil evidence of eukaryotic heterotrophs, the stem group of fungi and animals.

Ediacaran

This was the last period of the Neoproterozoic era, lasting from about 653 million years ago until 542 million years ago. Although "Ediacaran" is the name officially recognized by the International Committee on Stratigraphy, the older name Vendian also has widespread use. Its status as the last period of the Neoproterozoic was established in March 2004 by the International Union of Geological Sciences. Today, the term "Vendian" is sometimes used to describe the later part of the Ediacaran, from about 565 to 542 million years ago.

The Ediacaran is defined stratigraphically. The lower limit is determined geo-stratigraphically, with the base of the cap carbonates from the Marinoan glaciation, at 653 million years ago at the Nuccaleena Formation. The upper limit is determined bio-stratigraphically. Conventionally, "precambrian" time was separated from the Cambrian by the appearance of trilobites. However, the current accepted boundary for the end of the Ediacaran is the first appearance of Trichophycus pedum, an organism that left burrowing fossils.

The early part of the Ediacaran consisted of a number of glaciations, commonly called the Varanger-Marinoan ice ages (605 - 585 million years ago). These may or may not have been snowball Earth events; the evidence is inconclusive. The second part of the Ediacaran, from about 585 million years ago to 542 million years ago is characterized by a warm, humid climate, even up to very high latitudes, specially in the southern hemisphere. Towards the very end of the Ediacaran and beginning of the Cambrian, there was a cooling, resulting in glaciation at high latitudes.

Rodinia had started to break up 750 million years ago. Temporarily the continents drifted together to form Pannotia, a hypothetical super continent that existed briefly between the Pan-African Orogeny (600 million years ago) to about 540 million years ago.

The Ediacaran is known for the first large scale radiation of multicellular life. Among the earlier fossils are the Aspidella disks. These are relatively simple disc like impressions left on the rock from soft bodied organisms. The earliest date from about 610 million years ago, found in the Twitya Formation of the MacKenzie Mountains in northwestern Canada. They have been interpreted as being the basal attachments (foot pedals) of sessile organisms. They are often considered "cnidarian grade", which is a comparison to modern cnidarians. More cnidaria include very simple organisms like corals, sea anemones, hydras, jellyfish, etc. They are the simplest metazoa - they do not even have organs, just a body cavity or stomach, with a mouth, usually surrounded by tentacles. Aspidella is notable for being the only cnidarian-grade metazoans found below the Varangian glacial deposits, meaning they are older.

Artist's Impression of life in the Ediacaran. From a diorama at the National Museum of Natural History, courtesy of the Smithsonian.

Later snapshots of Ediacaran life include:

Drook Formation

In southeast Newfoundland, 575 million years old: roughly dating from around 595-575 million years old, they include frond-like organisms including Charnia masoni, and a newer species Charnia wardi, which grew up to 2 meters in length. These were marine organisms attached to the floor, with frond like appendages.

Doushantuo Formation

In south central China, 570 million years old: includes algae, cnidarians and bilaterians - the last two known mostly from fossil embryos. These are about 570 million years old, though some reports say they may be as much as 590 million years old. However, they are definitely post-Varangian.

Mistaken Point

In east Newfoundland, 565 million years old: this is a large collection of spectacularly preserved fossils which have been well-dated to 565 ±3 million ago. They contain some cosmopolitan taxa, such as Aspidella and Charnia, but most of the organisms seem to be endemic to this area, or shared only with the Charnwood Forest locality in central England.

Zimnie Gory

White Sea coast of Russia, and the Ediacara Hills in the Flinders Ranges, south Australia, 555 million years old: these two sites have some of the most abundant and diverse collections of Ediacaran fauna. Approximately 60% of all the well-described Ediacaran taxa are found at these two sites. They include cubozoans (box jellies) such as Kimberella quadrata, as well as a wide range of other body fossils.

Nama Group

Namibia, 545 million years old: In addition to typical Ediacaran taxa, such as the cosmopolitan Pteridinium, the shelly fossil Cloudina first appears slightly below the earliest Ediacaran fossils, extends throughout the Ediacaran range, and into the Cambrian.  Moreover, a second, unnamed, shelly taxon ("goblet-shaped shelly fossils") coexists with Cloudina from at least 545 Ma through into the Cambrian.

To quote from a passage from Benchley and Harper's Paleoecology (pages 121-123):

"The fauna is entirely soft-bodied and was probably adapted to relatively low oxygen conditions in a variety of usually nearshore marine environments. ..... Reproduction may have been by spores or gametes, and growth was achieved by both isometric and allometric modes. The skin or integument had to be flexible, although it could crease and fracture. Moreover the skin must have acted as an interface for diffusion processes, whilst providing a water-tight seal to the animal. ..... There is little doubt that the Ediacara biotas dominated the latest Precambrian marine ecosystem, occupying a range of ecological niches and pursuing varied life strategies probably within the photic zone. It is also possible that these flattened animals hosted photosymbiotic algae, maintaining an autotrophic existence in the tranquil 'Garden of Ediacara' (McMenamin, 1986). The ecosystem, however, was dominated by medusoid pelagic animals and attached, sessile benthos; infaunal animals were sparse; food chains were probably short and the trophic structure was apparently dominated by suspension- and deposit-feeders."

The relationship between Ediacaran metazoa and modern taxa is uncertain. Most of them were probably taxa that did not survive into the Cambrian. However, other organisms did survive. For example, sponges are today considered to be the most primitive of metazoans. They are found in the fossil record in the Doushantuo phosphates, dating from about 570 million years ago. The earliest described species is Paleophragmodictya reticulata, from the Ediacara Hills, about 555 million years old. Some of the burrowing fossils were probably made by primitive worms. The worms also survived the end of the Ediacaran, and took part in the Cambrian explosion of life.

Most Ediacaran biota, however, did not survive into the Cambrian. Some people have interpreted this as an extinction event, in which Ediacaran fauna became extinct and were completely replaced by Cambrian fauna. Very few of the taxa survived. We do not currently know how abrupt this extinction event was. Stromatolites are also one of the casualties of the Ediacaran. They had survived for billions of years, but at the end of the Ediacaran they had practically been wiped from the fossil record. One evidence for this extinction event is the presence of a carbon excursion in the geological record, which can be somewhat precisely dated to about 542 +/- 0.3 million years ago. This date is often used to mark the beginning of the Cambrian, the first period of the phanerozoic eon.

Much more information about Ediacaran fauna can be found at the Palaeos site here.

 

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