Australia: The Land Where Time Began

A biography of the Australian continent 

Ediacaran Life on Land - Critique2

There were apparently major sea level changes over the last 700 My of the Precambrian during which the margins of the continents were submerged and exposed to erosion alternately over a number of cycles. According to Knauth1 over this period the soils were being eroded into the ocean (Kennedy et al., 2006) but there are expected to be some that remained as palaeosols as is observed in more recent strata, though at the time of writing none from the Late Precambrian. This lack of known palaeosols from the Ediacaran may be the result of the difficulty of recognising them when there are no root traces that are so obvious in rocks from more recent deposits. Only researchers experienced in the study of palaeosols are generally capable of recognising other, more subtle diagnostic features, and Retallack is one such specialist.

The biota of the Ediacaran has been studied by Retallack for a number of years, based on which he has proposed they were actually large lichens rather than animals (Retallack, 1994). Knauth1 suggests that if Retallack has found examples of the Ediacaran organisms that were terrestrial the evidence for his heterodox interpretation would be greatly strengthened. Such a discovery would indicate that at least some organisms had overcome the difficulties they faced moving from the ocean to land much earlier than is currently believed, or even the possibility that life actually evolved on land, only then moving to the ocean, probably accounting for the Cambrian Explosion (Knauth, 2005).

As to the strength of his arguments, it is difficult to interpret ancient environments, especially in the case of a stratigraphic layer that has no root  fossils that has been proposed to be a palaeosol. Retallack's suggestion that structures he has interpreted as geological relics of soil deformation  have been interpreted by other researchers as water-escape features (Callow et al., 2012). Knauth1 suggests that other researchers will resist Retallack's cogent arguments for the red colour of the Ediacaran Member representing weathering in the Precambrian, as they would be familiar with the red colour imparted to Australian rocks by present day weathering. Without comparisons with examples from the Ediacaran that Retallack accepts as being of marine origin it is unlikely that a lack of non-overlapping specimens actually precludes wash-ups on the shore as he suggests. Another point that is made by Retallack is that the isotope data for carbonate nodules represent subaerial exposure, though this is also compatible with rainwater recharge into subsurface aquifers, and this is known to often occur far offshore Hathaway et al., 1979). The sedimentary structures that Retallack interprets as sand that has been deposited in a terrestrial valley are difficult to distinguish from sand deposits in a marine canyon.

According to Knauth1 in sedimentology it is usual for observations to be interpretable in more than 1 way, and interpretations of these strata have historically covered a wide range of possibilities, lacustrine to lagoonal, coastal to open marine, the interpretations being changed as new evidence is discovered, and Knauth1 suggests for these reasons this publication should be considered seriously. The interpretations of Retallack are at odds with the generally accepted dogma, but it is not necessary for the 2 to be mutually exclusive. Knauth1 suggests that in the Ediacaran there is no reason to believe that organisms lived only on the land or in the sea. He suggests that at other sites Ediacaran organisms may be found in marine deposits, even if those of the Ediacaran Member are found to have been deposited in a terrestrial environment. Knauth1 suggests eliminating a possible habitat based on whether the fossils in them are animals or not is unwarranted, as uncertainty still exists over just what kind of organism they were. Retallack has provided comprehensive reasoning for all of his points, and has considered new observations and data. Knauth1 suggests that other researchers need to become experts in the characteristics of palaeosols to convincingly mount counter-arguments as he has carefully considered his case.

In the light of the carbon isotope data that, according to Knauth1, probably indicates a greening of the land surface in the Late Precambrian(Retallack, 2005) making the search for biological evolution in non-marine environments of the Precambrian an exciting new frontier. It is commonly assumed at the present that the land surface of the Ediacaran time was a biologically barren place and if it eventually found that Ediacaran organisms living in soils would be more evidence that the land surface was not as biologically barren as has been believed. Knauth1 suggests that as it will never be known for certain what the environment actually was like in the Ediacaran there is no reason for multiple hypotheses not to thrive until the evidence for a particular one becomes strongly compelling.

Sources & Further reading

  1. Xiao, Shuhai, and L. Paul Knauth. "Palaeontology: Fossils Come in to Land." Nature 493, no. 7430 (01/03/print 2013): 28-29.
Author: M. H. Monroe
Last Updated 07/02/2013

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