The first macroscopic life forms were the enigmatic bag-like and
quilted fossils in sedimentary rocks dating back to 635 Ma in Australia,
eastern Canada and NW Europe. They are grouped as the
Ediacaran
Fauna named after the Ediacara Hills in South Australia where they are
most common and diverse. Generally they are not body fossils but
impressions of soft-bodied organisms, often in sandstones rather than
muds. Some are believed to be animals that absorbed nutrients through
their skin, whereas others are subjects of speculation. One thing seems
clear; these first metazoans arose because of some kind of trigger
provided by the
global glacial conditions
that preceded their appearance. It has always been assumed that,
whatever they were, Ediacaran organisms lived on the sea floor, probably
in shallow water.
New sedimentological evidence found in the type
locality by
Gregory Retallack of the University of Oregon seems set to
force a complete
rethink about these hugely important life forms.
So momentous are his conclusions that they form the subject of a Nature editorial in the 13 December 2012 issue.
Retallack, a specialist on ancient soils of the Precambrian, examined
reddish facies of the Ediacara Member of the Rawnsley Quartzite of
South Australia, whose previous interpretation have a somewhat odd
background. Originally regarded as non-marine, before their fossils were
discovered, when traces of jellyfish-like organisms turned up this view
was reversed to marine, the red coloration being ascribed to deep
Cretaceous weathering. A range of features, such as clasts of red facies
in grey Ediacaran rocks, the presence of feldspar in the red facies –
unlikely to have survived deep weathering – bedding surfaces with
textures very like those formed by subaerial biofilms, and desiccation
cracks, suggest to Retallack that the red facies represents
palaeosols
in the sedimentary sequence. Moreover, some features indicate a land
surface prone to freezing from time to time. The key observation is that
this facies contains Ediacaran trace fossils representing many of the
forms previously regarded as marine animals of some kind, including
Spriggina,
Dickinsonia and
Charnia on which most palaeontologists would bet good money that they were animals, albeit enigmatic ones.
Dickinsonia (credit:Wikipedia)
If Retallack’s sedimentological observations are confirmed then
organisms found in the palaeosols cannot have been animals but perhaps
akin to lichens or colonial microbes, and survived freezing conditions.
As they occur in other facies more likely to be subaqueous, then they
were ‘at home’ in a variety of ecosystems.
As the Nature
editorial reminds us, from the near-certainty that early macroscopic
life was marine there is a chance that views will have to revert to a
terrestrial emergence first suggested in the 1950s by Jane Grey.
Uncomfortable times lie ahead for the palaeontological world.
This article is from 'earth -pages - Research News from the Earth Sciences'.
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