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New in Brief - April 2009

Trace fosssils from the Stirling Range Formation (Myxomitoides stirlingensis) (from Rasmussen, B. ref.1 cited here).Very flat, Australia

In Western Australia, it is so flat that a 50m eminence is often named a “mountain” on the map! So surprisingly, north of Albany on the south coast, a line of spines rises to the sky - the Stirling Range. Fieldwork there is horrendous, as the vegetation is as spiky as the topography. During my decade at the University of Western Australia, the age of the range’s sedimentary rocks - mudrocks, siltstones and sandstones - was unknown, nor were any fossils or traces.

Since then, questionable body fossils and some more convincing trace fossils have revealed themselves to hardy field workers. There are simple disks and millimetre-sized trails on several of the peaks1. In my review of the Neoproterozoic Ediacara fauna and trace occurrences throughout the world2, I included the Stirling Range, but noted that despite similarities of the disks to Ediacara fossils, the age of the rocks is probably Mesoproterozoic. It is now known that these rocks were deformed and metamorphosed weakly more than 1.2 billion years ago, but the rocks are 2.0-1.8 billion years old, or Palaeoproterozoic1,3.

Recently, a unicellular eukaryote organism, named Gromia, has been discovered off the Bahamas. This, by rolling across the sediment surface, generates trails with a prominent elevated central groove and ridges, 50 cm long. At the time in the Palaeoproterozoic when the Stirling Range trails were made, both unicellular and multicellular eukaryotes are known to have existed, and such organisms may have made the Stirling Range trails. They may also have been unicellular like Gromia1.

The importance of these new discoveries is that they show that modern unicellular eukaryote organisms are capable of movement and of producing centimetre-scale trails, and that similar organisms may have been roaming the sea floor in the late Palaeoproterozoic times. Unicellular organisms may or may not have produced the Stirling Range traces, but those represent the oldest evidence for movement known among eukaryote animals, be they unicellular or multicellular1. Either way, these are most important discoveries. The Stirling Range is such difficult terrain to ‘toothcomb’, that there are likely to be remarkable further discoveries there.

  1. Rasmussen, B 2009 The Stirling Range trace fossils: an update West Australian Geologist 475; 8-9
  2. 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
  3. Rasmussen, B , Bengtson, S , Fletcher, I R , McNaughton, M 2002 Discoidal impressions and trace-like fossils more than 1200 million years old Science 296; 1112-1115


     

Paul HoffmanSnowball’s chances


A brief review of the latest state of the ongoing Snowball Earth controversy1, unfortunately without references, would seem to leave it in a state of limbo. The controversy was described by myself in a review of the Ediacara occurrences world wide2. Philip Allen, with Ruben Rieu and others3 showed that sedimentary rocks in Oman, dated as Cryogenian (Neoproterozoic, but older than the Ediacaran), with ripple marks that could only have been made by surface waves, seem to indicate that there were areas of open water in the period when the whole Earth was supposed to be glaciated, according to the hypothesis. The rocks in Oman show alternating glacial and non-glacial sediments, glaciation apparently advancing and retreating. The water cycle apparently did not shut down. Philip Allen considers that this evidence sounds the death knell for the frozen snowball hypothesis1. He accepts that there are other examples in China, Australia, Canada and California1. There is also the report of giant cross-bedded sandstones within the Port Askaig Formation, Islay, a Dalradian glacial formation which has extension in Donegal in the Fanad ‘Conglomerate’, and has been connected to the hypothetical Snowball Earth4, 2.

Paul Hoffmnan, one of the architects of the Snowball Earth hypothesis, is reported as replying that this is not fatal to the hypothesis1. Philip Allen admits that a debate-ending falsifying test is elusive. At present the Snowball Earth is only firmly placed prior to the Ediacaran and a range of 710-640 Ma is all that can be stated1. These rocks are very old and hard to date. This wide range could well cover the separate Marinoan and Sturtian glaciations that have been recognised, rightly or wrongly in the southern hemisphere (the Marinoan is generally accepted as equivalent to the Varangian or Laplandian of the Northern Hemisphere, being at the base of the Ediacaran,and in the Cryogenian)2.

Whereas, the evidence adduced by Allen severely weakens the case for the frozen snowball, and I personally favour his case, the need for accurate dating of the Neoproterozoic glaciations is as acute as the need for a method of age dating samples from other planets, such as Mars, without bringing the sample back to Earth (a probable impossibility). Possibly more accurate dating of Neoproterozoic rocks can be obtained by comparisons of isotopic variations (C,O) with those of rocks of known date?
  1. Morton, M C 2009 Sedimentary rocks reignite heated debate over the Snowball Earth Earth February 2009; 22-23
  2. 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
  3. Rieu, R , Allen, P , Plötze,M , Pettke, T 2007 Climatic cycles during a Neoproterozoic ‘snowball’ glacial epoch Geology 35; 299-302
  4. Arnaud, E , Eyles, C H 2001 Giant cross-bedded sandstones in the Neoproterozoic Port Askaig Formation, Scotland: palaeogeographic implications Abstracts with Programs – Geological Society of America 33; 75

     

High speed photometer HSP data binned in 200ms intervals. The narrow background trace is the signal fro the star and the broad downward ‘spike’ represents the occultation with Enceladus’s plume.Enceladus water-spout


Encaladus, a satellite of Saturn, (radius ~251 km) is one of two known active satellites in the solar system, the other being Jupiter’s extremely eruptive Io. Enceladus is known to project a plume of vapour, which has been related to fissures on the satellite’s surface (named Alexandria, Baghdad, Cairo and Damascus, for obscure reasons!).

Research experiments can require enormously expensive equipment, but clever scientists can use nature herself as the equipment. The experiment on Enceladus recently described by Hansen et al.1 exemplifies this. They used the Utltraviolet Imaging Spectrograph (UVIS) aboard the Cassini spacecraft to observe the plume during its occultation of the star γ Orionis. The gas jets on Encaladus, coinciding with previously reported dust jets, are the source of the large general water vapour flux from the satellite. The figure (above) shows the high speed photometer (HSP) data binned to 200m intervals, the un-occulted signal from the star being contrasted with the signal during occultation.

Without delving too far into technical details, the spectrographic evidence reveals the dominance of water in the plume: the source of the plume is believed to be liquid water, with gas accelerated to supersonic velocity in nozzle-like channels. There is believed to be a subsurface water reservoir in Enceladus, like Jupiter’s satellite Europa, which is also believed to harbour liquid water. The plumes are tidally activated.
  1. Hansen, C J , Esposito, L W , Stewart, A I F , Meinke, B , Wallis, B , Colwell, J E , Hendrix, A R , Larsen, K , Pryor, W, Tan, F 2008 Water vapour jets inside the plume of gas leaving Enceladus Nature 456; 477-479

Broodysaurus in Bayreuth. Photo - Ted NieldBroodysaurus


Dinosaurs are full of surprises. Recent research by Varrichio et al. reported in Science1 covers the bone history and egg clutch volume of brooding adult troodontid and oviraptid adults. Troodon, Oviraptor and Atipati scale most closely with bird paternal care. This care system evidently emerged before the emergence of birds and represents the birds' ancestral care system. Avian males participate in the care of eggs in over 90% of extant bird species, compared with only 5% in the case of mammalian males.
  1. Varricchio, D J , Moore, J R , Erickson, G M , Norell, M A , Jackson, F D, Borkowski, J J 2008 Avian paternal care had dinosaur origin Science 322; 1826-1828