End-Ediacaran extinction

From Wikipedia, the free encyclopedia
Jump to: navigation, search

Evidence suggesting that a mass extinction occurred at the end of the Ediacaran period, 542 million years ago, 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[edit]

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.[1] Because both these groups disappear abruptly at the end of the Ediacaran period, 542 ± 0.3 million years ago, their disappearance cannot simply represent the closure of a preservational window,[2] as had previously been suspected.[1]

Post-Ediacaran organisms[edit]

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 are more consistent with a radiation of animals to fill in vacant niches, left empty as an extinction cleared out the pre-existing fauna.[3]

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.[4] 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 (or at least of biological origin, perhaps a microbial colony),[5] just not one that is related to the Ediacara biota.[6]

Survivors[edit]

Some organisms clearly survived the extinction since life on Earth has continued. However, very few organisms are known from both sides of the Ediacaran-Cambrian boundary. One such organism is the agglutinated foramanifera Platysolenites.[7] Other apparent survivors, such as Thaumaptilon, have been reinterpreted as unrelated to the Ediacaran biota.

Geochemical evidence[edit]

A negative δ13C excursion—a geochemical signal often associated with mass extinctions—is observed at the end of the Ediacaran period.[8]

Sedimentary evidence[edit]

This period is reflected in the geological record by an increase in black shale deposition,[9] representing global anoxia.[10] This may be related to global changes in oceanic circulation.[3][9]

References[edit]

  1. ^ a b Amthor, Joachim E.; Grotzinger, John P.; Schröder, Stefan; Bowring, Samuel A.; Ramezani, Jahandar; Martin, Mark W.; Matter, Albert (2003). "Extinction of Cloudina and Namacalathus at the Precambrian-Cambrian Boundary in Oman". Geology 31 (5): 431–434. Bibcode:2003Geo....31..431A. doi:10.1130/0091-7613(2003)031<0431:EOCANA>2.0.CO;2. ISSN 0091-7613. 
  2. ^ Marshall, Charles R. (2006). "Explaining The Cambrian "Explosion" Of Animals". Annual Review of Earth and Planetary Sciences 34: 355. Bibcode:2006AREPS..34..355M. doi:10.1146/annurev.earth.33.031504.103001. 
  3. ^ a b Wille, M; Nägler, T.F.; Lehmann, B; Schröder, S; Kramers, J.D; Nägler; Lehmann; Schröder; Kramers (June 2008). "Hydrogen sulphide release to surface waters at the Precambrian/Cambrian boundary". Nature 453 (7196): 767–9. Bibcode:2008Natur.453..767W. doi:10.1038/nature07072. PMID 18509331. 
  4. ^ Antcliffe, Jonathan B.; Brasier, Martin D. (2007). "Charnia and sea pens are poles apart". Journal of the Geological Society 164 (1): 49–51. doi:10.1144/0016-76492006-080. 
  5. ^ See Ediacaria
  6. ^ MacGabhann, B. A.; Murray, J.; Nicholas, C. (2007), "Ediacaria booleyi: weeded from the Garden of Ediacara?", in Vickers-Rich, Patricia; Komarower, Patricia, The Rise and Fall of the Ediacaran Biota, Special publications 286, London: Geological Society, pp. 277–295, doi:10.1144/SP286.20, ISBN 9781862392335, OCLC 156823511 191881597 
  7. ^ Kontorovich, A; Varlamov, A; Grazhdankin, D; Karlova, G; Klets, A; Kontorovich, V; Saraev, S; Terleev, A; Belyaev, S et al. (2008). "A section of Vendian in the east of West Siberian Plate (based on data from the Borehole Vostok 3)". Russian Geology and Geophysics 49 (12): 932. Bibcode:2008RuGG...49..932K. doi:10.1016/j.rgg.2008.06.012. 
  8. ^ Zhu; Babcock, L; Peng, S (2006). "Advances in Cambrian stratigraphy and paleontology: Integrating correlation techniques, paleobiology, taphonomy and paleoenvironmental reconstruction". Palaeoworld 15 (3–4): 217. doi:10.1016/j.palwor.2006.10.016. 
  9. ^ a b Schroder, S.; Grotzinger, J.P. (2007). "Evidence for anoxia at the Ediacaran-Cambrian boundary: the record of redox-sensitive trace elements and rare earth elements in Oman". Journal of the Geological Society 164 (1): 175–187. doi:10.1144/0016-76492005-022. 
  10. ^ Fike, D.A.; Grotzinger, J.P.; Pratt, L.M.; Summons, R.E.; Grotzinger; Pratt; Summons (2006). "Multi-Stage Ediacaran Ocean Oxidation and Its Impact on Evolutionary Radiation". Geochimica et Cosmochimica Acta 70 (18S): 173–173. Bibcode:2006GeCAS..70Q.173F. doi:10.1016/j.gca.2006.06.347. Retrieved 2008-06-30.