Fossils found in rocks of the Ediacaran period in Australia have been previously characterized as early marine organisms. But a report suggests that these rocks are fossilized soils. So did some of these Ediacaran organisms in fact live on land, like lichens? A palaeontologist and a geologist weigh up the evidence. See Letter p.89
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J. GEHLING/SOUTH AUSTRALIAN MUSEUM
Notes
*This article and the paper under discussion1 were published online on 12 December 2012.
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Xiao, S., Knauth, L. Fossils come in to land. Nature 493, 28–29 (2013). https://doi.org/10.1038/nature11765
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DOI: https://doi.org/10.1038/nature11765
Peter Gibson
Possibly the Ediacara species were not marine but lived in shallow fresh water such as ponds or streams and would have needed a holdfast. The great advantage of shallow water is that it is strongly illuminated. These early organisms probably never existed in deep water. At depth they would have been cut off from the superficial photic zone by a thermocline which is the same as that found today in the summer in lakes and above continental shelves. Winter storms break these up and re-circulate nutrients. The deep oceans were in all probability anoxic. The iron bearing sands, brown, of the Ediacara were oxidised unlike the black sulphurous muds of the deep. The reconstruction of the evolutionary path of Ediacaran species may be of little interest. What occurred prior to this era is crucial when the balance was tipped from single celled to multicellular organisms. Afterwards there was a slow decline in the size of taxa if not the number of species and may have come with the introduction of sexual reproduction. The sacrifice is that sexual reproduction is less efficient than asexual reproduction. It could only be achieved by an increase in complexity of cell requiring the annexing of other cell to provide organelles such as mitochondria and chloroplasts. Sex may have had little to do with providing genetic variation. Genes, as such, are small in number. Sex may have been concerned with more advantageous factors for the individual rather than increased variation, which was a by-product. Species are an accumulation of individuals and as such increase species variation. The taxon, species, is a taxonomic convenience and from an evolutionary standpoint is meaningless. Each individual produced may benefit from assortment of genes – sex. Following the Precambrian explosion evolution due mutation may have been superseded by the positioning of structure though epigenetic evolution.
The evolution of Ediacaran species, or rather individuals, was possibly because of their relatively large size which enabled them to carry more gametes. This increase in size may have only been possible because of the shallow water and general increase in oxygen levels. A problem was a reduction of oxygen through increased temperature, during the day. Overcoming this may have been through physiological mechanisms. The apparent move to land may have resulted from resistance to drought. Ratallack (pp 89-92), who the two authors are commenting on, may be half right in saying that Ediacaran individuals are soil living ? they were drought resistant. They were like lichens and mosses which can withstand desiccation. In addition, draught will have killed off completion which explains the relative paucity of Ediacaran species.
Paleo Recipes
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