End-Ediacaran extinction


End-Ediacaran extinction

Evidence suggesting that a mass extinction occurred at the end of the Ediacaran period, Ma|542, includes:
*A mass extinction of acritarchs
*The sudden disappearance of the Ediacara biota and calcifying organisms;
*The time gap before Cambrian organisms "replaced" them.

Pre-Ediacaran organisms

During the Ediacaran period, two main groups of organisms are found in the fossil record: the "Ediacara biota" of soft-bodied organisms, preserved by microbial mats; and calcifying organisms such as "Cloudina" and "Namacalathus", which had a carbonate skeleton.cite journal
title = Extinction of Cloudina and Namacalathus at the Precambrian-Cambrian Boundary in Oman
year = 2003
journal = Geology
volume = 31
pages = 431–434
doi = 10.1130/0091-7613(2003)031<0431:EOCANA>2.0.CO;2
author = Amthor, Joachim E.
] Because "both "these groups disappear abruptly at the end of the Ediacaran period, Ma|542.0|error=0.3, their disappearance cannot simply represent the closure of a preservational window,cite journal
year = 2006
doi = 10.1146/annurev.earth.33.031504.103001
title = Explaining The Cambrian "Explosion" Of Animals
author = Marshall, Charles R.
journal = Annual Review of Earth and Planetary Sciences
volume = 34
pages = 355
] as had previously been suspected.

Post-Ediacaran organisms

The fossil record of the earliest Cambrian, just after the Ediacaran period, shows a sudden increase in burrowing activity and diversity. However, the Cambrian explosion of animals that gave rise to body fossils did not happen instantaneously. This implies that the "explosion" did not represent animals "replacing" the incumbent organisms, and pushing them gradually to extinction; rather, the data is more consistent with a radiation of animals to fill in vacant niches, left empty as an extinction cleared out the pre-existing fauna.cite journal
doi = 10.1038/nature07072
year = 2008
month = Jun
author = Wille, M; Nägler, Tf; Lehmann, B; Schröder, S; Kramers, Jd
title = Hydrogen sulphide release to surface waters at the Precambrian/Cambrian boundary
volume = 453
issue = 7196
pages = 767–9
issn = 0028-0836
pmid = 18509331
journal = Nature
]

The concept that all Ediacarans became extinct at the start of the Cambrian is dealt a death knell if any post-Ediacaran survivors are found. Organisms from the lower Cambrian, such as "Thaumaptilon", were once thought to be Ediacarans, but this hypothesis no longer has many adherents.cite journal
author = Antcliffe, Jonathan B.; Brasier, Martin D.
year = 2007
title = Charnia and sea pens are poles apart
journal = Journal of the Geological Society
volume = 164
issue = 1
pages = 49–51
doi = 10.1144/0016-76492006-080
url = http://jgs.geoscienceworld.org/cgi/content/abstract/164/1/49
] One possible Ediacaran survivor whose status is still open to scrutiny is "Ediacaria booleyi", a purported holdfast structure known from the "upper" Cambrian. If this does turn out to be a true Ediacaran, the biota cannot have disappeared completely. Disbelievers have claimed that the fossils don't actually have a biological origin, which doesn't seem to be the case - evidence is mounting to suggest that it "is" an organism, just not one that is related to the Ediacara biota.the Rise and Fall of the Ediacaran Biota
MacGabhann, B. A.; Murray, J.; Nicholas, C.
"Ediacaria booleyi": weeded from the Garden of Ediacara?
277
295
20
]

Geochemical evidence

A negative d13c|link excursion - a geochemical signal often associated with mass extinctions - is observed at the end of the Ediacaran period.cite journal | author = Zhu | year = 2006 | doi = 10.1016/j.palwor.2006.10.016 | title = Advances in Cambrian stratigraphy and paleontology: Integrating correlation techniques, paleobiology, taphonomy and paleoenvironmental reconstruction | journal = Palaeoworld | volume = 15 | pages = 217 ]

edimentary evidence

This period is reflected in the geological record by an increase in black shale deposition,cite journal
title = Evidence for anoxia at the Ediacaran-Cambrian boundary: the record of redox-sensitive trace elements and rare earth elements in Oman
year = 2007
journal = Journal of the Geological Society
volume = 164
issue = 1
pages = 175–187
doi = 10.1144/0016-76492005-022
author = Schroder, S.; Grotzinger, J.P.
] representing global anoxia.cite journal
url = http://linkinghub.elsevier.com/retrieve/pii/S0016703706006569
doi = 10.1016/j.gca.2006.06.347
author = Fike, D.A.; Grotzinger, J.P.; Pratt, L.M.; Summons, R.E.
journal = Geochimica Et Cosmochimica Acta
volume = 70
issue = 18S
pages = 173–173
year = 2006
title = Multi-Stage Ediacaran Ocean Oxidation and Its Impact on Evolutionary Radiation
accessdate = 2008-06-30
] This may be related to global changes in oceanic circulation.

References


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