(unknown genus and species, possibly Girvanella sp.) Geological
Time: Holocene to Pleistocene
Size (25.4
mm = 1 inch): Stromatolite is 90 mm by 185 mm by 20 mm thick (maximum)
Fossil Site:
Marion Bay, York Peninsula, South Australia
Code: AS13005
Price: Sold
Description:
Stromatolites are remnants of the most ancient of colonial organisms.
Stromatolites are fossils that are the result of the work of
simple blue-green “algae” or Cyanophytes, which lived
in chains or mats covered in a jellylike substance. By taking
in carbon dioxide as a food source, the precipitate limy deposits
on the jelly that builds up in layers. Thus these organisms build
up stony supports for their colonies. These mound like structures
can be anywhere from several centimeters to several meters in
height. The production of oxygen is thought to have led to the “rusting
of the seas” which brought about deposition of extensive
iron deposits such as the Mesabi Range. Hematite is an iron mineral,
and is one of the components of this banded structure (dark bands).
Stromatolites have persisted to the modern day in such places
as Shark Bay, Australia where they continue their billions of
years old lifestyle. The oval to circular structure in these
images is an entire stromatolite colony that dates from the Pleistocene
to the Holocene, a time when stromatolites were no longer prevalent
and abundant. By the Cambrian, photosynthetic bacteria responsible
for the biogenic formation of stromatolite structures no longer
had the earth to themselves. The oxygenated atmosphere had become
toxic to some bacteria, and they had to compete with other organisms,
some of which would have been predaceous to this most ancient
of life forms. No wonder that this stromatolite shows the Girvanella-like
shape that was a group of small independent colonies, never reaching
the large domes of the Proterozoic. The location is now a park
with no access to collectors, making this example from an old
collection impossible to replace.
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