Extinction at the Precambrian-Cambrian boundary

The very beginning of the Cambrian is associated in every geologist’s mind with the explosive appearance and diversification of animals with hard parts.  Why this dramatic introduction to the modern biological world occurred is one of the great questions in evolution.  Some connection with the effects of “Snowball Earth” events in the late Neoproterozoic was thrown into doubt by evidence that it had little effect on micro-organisms (see Microbes showed no sign of change following a “Snowball Earth” in May 2003 EPN).  Exactly at the boundary there is a marked fall in the abundance of carbon-13, and this negative d13C excursion is so widespread that it is the best indicator of the position of the Precambrian-Cambrian boundary in stratigraphic sequences of roughly this age.  One of the places that it occurs is in Oman, reported previously in EPN (A possible fuse for the Cambrian Explosion, January 2003).  The paper describing the evidence from Oman that the carbon-isotope excursion relates to a mass extinction is now out (Amthor, J.E. and 6 others 2003.  Extinction of Cloudinia and Namacalathus at the Precambrian-Cambrian boundary in Oman.  Geology, v. 31, p. 431-434)  The disappearance of the distinctive eukaryote fossils coincides exactly with the carbon anomaly.  Luckily, so too does a volcanic ash horizon from which zircons provide a very precise U-Pb age of 542±0.3 Ma.  This matches less precise dates for the anomaly from Siberia and Namibia, and seems likely to become accepted as the definitive age for the start of the Phanerozoic.

“Snowball Earth” and evolutionary diversification: Australians speak out

By comparison with the vast amounts of Australian diamictites that span a range of Neoproterozoic ages, the sites elsewhere, from which evidence in support of the “Snowball Earth” hypothesis and possible effects on evolution have been drawn, are puny.  Besides that, the Late Precambrian of Australia has the best record of biological change, including the type locality for the Ediacaran fauna that presaged the Cambrian Explosion.  Although somewhat less hasty than the flurry of papers on the “Snowball” hypothesis, since 1998, the appearance of published data from the “Red Continent” is sure to push the debate decisively one way or another.  Palaeontologists from the Geological Survey of Western Australia, Macquarie University and Mineral Resources Tasmania have just unveiled details of acritarchs from late-Neoproterozoic sediments that overlie the Marinoan (~600 Ma) glaciogenic rocks in South Australia (Grey, K. et al. 2003.  Neoproterozoic biotic diversification: Snowball Earth or aftermath of the Acraman impact?  Geology, v. 31, p. 459-462).  Acritarchs are spore-like fossils, that probably represent encysting algae.  Their rapid diversification makes them useful biostratigraphic indicators from the Late Precambrian to the present.  Grey et al. Found that the same assemblage of acritarchs occur before the Marinoan glaciogenic strata and after the succeeding “cap” carbonate.  They are part of a group that can be traced back to the Mesoproterozoic  However, higher in the sequence that they examined there is a distinctive layer of debris that contains evidence of impact-induced shock.  This can be correlated with little doubt to the 90 km Acraman structure in South Australia, which formed at 580 Ma with an energy likely to have had a major influence on life.  Sure enough, in the strata above this ejecta layer a completely new type of acritarch group appears and diversifies rapidly, while the pre-impact groups simply disappear.  Clearly, the Acraman impact is implicated in this sudden biological change; an extinction followed by rapid diversification.  Acritarchs are thought to represent the phytoplanktonic base of the Neoproterozoic food chain.  Immediately above the strata in which the post-impact acritarchs diversified lie sandstones that contain the famous Ediacara fauna of the first large, soft bodied animals.  The Marinoan “Snowball” event seems disconnected from this evolutionary leap.

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