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...more News in Brief

EoandromedaEight-armed Ediacara


A recent report in Geoscience World 1 describes the preservation of carbonaceous compressions of an eight-armed Ediacaran fossil in the Doushanto black shale of South China; and also as casts and moulds in the Rawnsley Quartzite of South Australia. Eight compressions were collected by Drs Maoyan Zhu of Nanjing Institute of Geology and Palaeontology and Dr Jim Gehling (South Australian Museum, Adelaide) and colleagues. Seven specimens were collected by them from the Flinders Ranges , Australia, leaving 31 more on two excavated and reassembled beds. This fossil has been named Eoandromeda octobrachiata. The contrasting preservational styles in two now geographically widely separated taphonomic windows (once closer to each other in Gondwana) indicate that it may have had a relatively recalcitrant taphonomic integument, which rules out comparison with giant agglutinated foraminifera such as xenophyophores. Its octahedral symmetry and dextral spiral arms suggest that it may be a diploblastic-grade animal sharing features with cnidarians and ctenophores, though its phylogenetic affinity remains uncertain .It is the first unambiguously identified Ediacaran fossil (from the period 635-541 Ma2 ) that occurs in two drastically different taphonomic windows, thus bridging the biological and taphonomic gap between the renowned Ediacara3 and Miaohe4 biotas, which together record the earliest known macroscopic and complex life. The Miaohe biota consists of carbonaceous compression fossils in the Uppermost Doushanto Formation in the Yangtse Gorges area. More than 100 species have been described from it, but more recently it has been reduced to 20 distinct taxa. Most of these fossils can be interpreted unambiguously as colonial prokaryotes or multicellular algae4.

The arms were tubular, and in close contact but not joined. The organism was soft-bodied and dome shaped and fed by absorbing nutrients from the ambient environment. Dr Zhu and his team suspect that these organisms represent now-extinct diploblastic animals, creatures that possessed only two cellular layers separated by jelly-like substance. They could be related to corals, sea-anemones or jellyfish. They display radial symmetry but lacked complex organs. They became extinct about 542Ma, leaving niches for the Cambrian explosion of complex animals, in which relatives of all known animals appeared.

Refs:
  1. Maoyan Zhu, Gehling, J G, Shuhai Xiao, Yuanlong Zhao, Droser, M L 2008. Eight-armed Ediacara fossil preserved in contrasting taphonomic windows from China and Australia. http://geolog.geoscienceworld.org/
  2. Viegas, J 2009. Eight-armed animal preceded dinosaurs. http://www.abc.net.au/science/
  3. McCall, G J H 2006.The Vendian (Ediacaran) in the geological record: enigmas in geology’s prelude to the Cambrian explosion. Earth Science Reviews 77; 1-229.
  4. Xiao, Shuhai; Yuan, Xunlai; Steiner, M; Knoll, A H 2002. Macroscopic ompressions in a terminal Proterozoic shale: a systematic reassessment of the Miaohe biota, Southern China. Journal of Paleontology, March 1 2002.


Impact structures, and the Paasselkä crater


I have been involved in supplying snippets of news to Geoscientist since 1992, and in this have been entirely concerned with other scientists’ research. However, time, I think, to talk about myself for a change. Now available online through Science Direct is my 18-page review of the subject in this title1.

It is nearly 50 years since I flew out from Perth to Derby and drove in an ancient Land Rover to map Wolfe Creek Crater near Halls Creek for nine days2. Since then I have maintained my interest in this remarkable and (then) newly discovered family of enigmatic structures. Though there can now be little doubt that they originated extraterrestrially, anomalies remain. The global distribution is anomalous: Sixty-two (!) possible such structures have been recognised in Fennoscandia3, and, even if just over half of these are confirmed by association with shock products, this is a remarkable number of the 170 or so structures now globally recognised. Various explanations have been offered for the global anomalies of distribution, but they remain inadequate.

Secondly there is the question of how these structures relate to mass extinctions. I studied the literature for and against such involvement and was surprised to find that if one accepts Keller et al.’s foraminifera-based dating of the Chicxulub structure, Yucatan, as ~300 Ka prior to the K/T boundary4 (as seems now to be confirmed), there is little evidence at all of impact cause for Mass Extinctions, which are complex and apparently relate to a variety of causes, each case being different. The research by Lloyd and others5, to which I called attention in Geoscientist 19.1 p6, must change our views about the departure from the planet of the dinosaurs? My conclusions will be no doubt argued against, but they are based on a sober review of statements of both sides of the arguments. It appears that in the excitement of the Alvarez statement and the discovery of Chicxulub, not enough weight was given to palaeolontological counterarguments. Or so it seems to me, after careful research and nearly 50 years’ involvement in this topic.

Refs:

  1. McCall,G J H 2009. Half a century of progress in research on terrestrial impact structures: a review. Earth Science Reviews (Science Direct), 18 pp.
  2. McCall, G J H 1965. Possible meteorite craters: Wolf Creek, Australia, and analogs. Annals of the New York Academy of Science 123 (Art 2); 970-998.
  3. Henkel, H, Pesonen, L J 1992. Impact craters and craterform structures in
  4. Fennoscandia. Tectonophysics 216, 31-40.
  5. Keller, G, Stinnesbeck W, Adaite, T, Holland, B, Harting, M, De Leon, C,De La Cruz J 2003. Spherule deposits in Cretaceous-Tertiary boundarysediments in Belize and Guatemala. Journal of Geology 160; 1-13.
  6. Lloyd, G T, Davis, G E, Pisani, D, Taver, J E, Ruta, M, Sakamoto, M, Hone, D W E, Jennings, R, Benton, M J 2008. Dinosaurs and the Cretaceous revolution. Proceedings of the Royal Society, B. 275; 2483-2490.

Paasselkä crater


Eleven impact structures have been recognised and confirmed in Finland1, and the deeply eroded (especially by the Pleistocene glaciation) Paasselkä structure, the third largest with a diameter of ~10km, situated in SE Finland at 62o 09’ N, 29o 25’E, was drilled by the Geological Survey of Finland in 1999, recovering a 200m drill core; though no impact melt rocks were reported and the only age date given was that of the 1.9 Ga Proterozoic target rocks. However, we now have a splendid, well illustrated description2 of melt rocks recovered from a till pit near Sikosärkät, about 1 km SE of the southeastern shore of Lake Paasselkä. These are in all probability derived from the crater. Shock features comprise shocked feldspar grains, intensely shocked and ‘toasted’ quartz, marginally molten and recrystallised clasts thought to have been diaplectic quartz, largely fresh and recrystallised feldspar glasses, decomposed biotite flakes, recrystallised fluidal silica glass (?originally lechatelierite) in partly molten sandstone clasts, all set in a glassy to cryptocrystalline melt matrix. Peak shock pressures were probably ~ 35GPa and temperatures of ~1500o C. The melt rocks might be suitable for radiometric dating of the actual impact. The geochemical character of the melt rocks is similar to that of other impact structures on the Baltic Shield.

Henkel and Pesonen3 listed 62 possible impact structures in Fennoscandia, but McCall4 remarked that only 26 appear to be confirmed by the displaying of shock effects. There will be a few more confirmed like Paasselkä since 1992. At that time, 15% of all confirmed terrestrial structures were situated in Fennoscandia, an extraordinary bias. There have been various explanations for this and other biases in global distribution of impact structures5, but like the recurrence of L chondrite shower products in the Orthoceritid Limestone of the Ordovician in Southern Sweden, indicating repeated showers falling at the same location again and again over several million years6, this bias seems to be something that is unexplained, and contrary to meteoritic theory.

Refs:

  1. Pesonen, L J, Hietala, S, Poutanen, M, Moilanen, J, Lehtinen, M, Ruotsaleinen, H E 2005 The Keurusselkä meteorite impact structure, Central Finland: geophysical data XXII Geofisikan päivät – Proceedings of the 22nd Geophysical Days, May 19 20 2005, Helsinki, Finland; 165-170.
  2. Schmieder, M, Moilanen, J, Buchner, E 2008 Impact melt rocks from the Paasselkä impact structure (SE Finland): Petrography and Geochemistry Meteoritics and Planetary Science 43(7); 1189-1200.
  3. Henkel, H, Pesonen, L J 1992 Impact craters and craterform structures in Fennoscandia Tectonophysics 216, 31-40.
  4. McCall, G J H 2006 Meteorite cratering: Hooke, Gilbert, Barringer and beyond In McCall, G J H, Bowden, A J, Howarth, R J, eds The History of Meteoritics and Key Meteorite Collections: Fireballs, Falls and Finds Geological Society of London, Special Publications 256; 443-469
  5. McCall, G J H (in the press) Half a century of research on terrestrial impact structures Earth Science Reviews.
  6. Schmitz, B 2003 Shot stars: a rain of meteorites in the Ordovician Geoscientist 13(5); 4-7 .