One of the most severeglaciation events known in the geologic record occurred during the Cryogenian period of the Neoproterozoic, when globalice sheets may have reached theequator and created a "Snowball Earth" lasting about 100 million years. The earliest fossils ofcomplex life are found in the Tonian period in the form ofOtavia, a primitivesponge, and the earliest fossil evidence ofmetazoanradiation are found in the Ediacaran period, which included the namesakedEdiacaran biota as well as the oldest definitivecnidarians andbilaterians in the fossil record.
At the onset of the Neoproterozoic the supercontinentRodinia, 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 Neoproterozoic Era including theSturtian andMarinoan glaciations of the Cryogenian Period.
These glaciations are believed to have been so severe that there were ice sheets at the equator—a state known as the "Snowball Earth".
In the regional timescale of Russia, the Tonian and Cryogenian correspond to the LateRiphean; the Ediacaran corresponds to the Early to middle Vendian.[5] Russian geologists divide the Neoproterozoic ofSiberia into theMayanian (from 1000 to 850 Ma) followed by theBaikalian (from 850 to 650 Ma).[6]
Russian timescale for Proterozoic. Neoproterozoic is equivalent to the time span from Late Riphean to Late Vendian.
The idea of the Neoproterozoic Era was introduced in the 1960s. Nineteenth-century paleontologists set the start ofmulticellular life at the first appearance of hard-shelled arthropods calledtrilobites andarcheocyathid sponges at the beginning of theCambrian Period. In the early 20th century, paleontologists started finding fossils of multicellular animals that predated the Cambrian. A complex fauna was found in South WestAfrica in the 1920s but was inaccurately dated. Another fauna was found in South Australia in the 1940s, but it was not thoroughly examined until the late 1950s. Other possible early animal fossils were found in Russia, England, Canada, and elsewhere (seeEdiacaran biota). Some were determined to bepseudofossils, but others were revealed to be members of rather complex biotas that remain poorly understood. At least 25 regions worldwide have yieldedmetazoan fossils older than the classical Precambrian–Cambrian boundary (which is currently dated at538.8 million years ago).[7][3]
A few of the early animals appear possibly to be ancestors of modern animals. Most fall into ambiguous groups of frond-like organisms; discoids that might be holdfasts for stalked organisms ("medusoids"); mattress-like forms; small calcareous tubes; and armored animals of unknown provenance.
These were most commonly known asVendian biota until the formal naming of the Period, and are currently known as Ediacaran Period 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 unknown animal types.
In addition to Ediacaran biota, two other types of biota were discovered in China. TheDoushantuo Formation (of Ediacaran age) preserves fossils of microscopic marine organisms in great detail.[8][contradictory] TheHuainan biota (of late Tonian age) consists of small worm-shaped organisms.[9]
The widespread proliferation of marine algae during the Neoproterozoic caused an increased flux of algal particulate matter to benthic environments, stimulating the evolution of microbial eukaryotic predators.[11]
The nomenclature for the terminal period of the Neoproterozoic Era has been unstable. Russian and Nordic geologists referred to the last period of the Neoproterozoic as theVendian, while Chinese geologists referred to it as theSinian, and most Australians and North Americans used the name Ediacaran.
However, in 2004, the International Union of Geological Sciences ratified the Ediacaran Period to be a geological age of the Neoproterozoic, ranging from 635 to 538.8 (at the time to 542) million years ago.[12][13] The Ediacaran Period boundaries are the only Precambrian boundaries defined by biologicGlobal Boundary Stratotype Section and Points, rather than the absoluteGlobal Standard Stratigraphic Ages.
^Rino, S.; Kon, Y.; Sato, W.; Maruyama, S.; Santosh, M.; Zhao, D. (2008). "The Grenvillian and Pan-African orogens: World's largest orogenies through geologic time, and their implications on the origin of superplume".Gondwana Research.14 (1–2):51–72.Bibcode:2008GondR..14...51R.doi:10.1016/j.gr.2008.01.001.
^Muscente, A.D.; Hawkins, Andrew D.; Xiao, Shuhai (September 2015). "Fossil preservation through phosphatization and silicification in the Ediacaran Doushantuo Formation (South China): a comparative synthesis".Palaeogeography, Palaeoclimatology, Palaeoecology.434:46–62.Bibcode:2015PPP...434...46M.doi:10.1016/j.palaeo.2014.10.013.
^Malenkov, A. G. (May 2018). "13C, Ontogeny, and the Paradox of Evolution".Biophysics.63 (3):477–484.doi:10.1134/S0006350918030156.
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