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Algae and bacteria are important members of the biological community, however, they are often very small and/or composed of few preservable components. They are underrepresented in the fossil record.
This green alga, Halimeda sp., precipitates calcium carbonate internally. When the
plant dies the grains of calcium carbonate are left behind as part
of the sand. White beaches of Caribbean islands are composed of a mixture of shell and coral fragments along with these algal remnants.
[On the other hand, beach sand on some Hawaiian islands is can be black because it comes from erosion of lava that has flowed into the sea and cooled] |
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Taxon: 'algae'?
Specimen number: UT 47859
Age: Cretaceous
Horizon: Glen Rose Limestone
Location: Blanco County, Texas |
The wavy structure of this
slab may represent the fossilization of bacterial mats similar
to the cyanobacterial mats shown below. |
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This cyanobacterial mat came
from the salt marshes near Port Aransas, on the Gulf coast.
It is still hydrated and bears a characteristic wavy surface,
similar to what is seen on the slab shown above. |
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The mat is thin, and will peel
away quite easily from the mud below. Within the
layers of these particular modern mats one finds thousands of
tiny diatoms (single-celled algae).
Could fossil mats from the
Cretaceous include diatoms? Place the
cursor over the hand to find the answer. |
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When the mats dehydrate, they
look similar in shape to mud cracks that you might see after a
flood. |
| This garfish, stranded after
a flood at Lake Brownwood spillway, suffers from the same
dehydration as the mud around it. If they were to be
covered by a fine grained, gently deposited layer of mud, that
would exclude air and prevent further decay, both the mud
cracks and the fish could be preserved as fossils. |
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| Note: Diatoms
have been recorded from as early as the Jurassic
period, so they could theoretically be found in Cretaceous bacterial
mats. However, we are not certain whether benthic living diatoms existed at that time. The problem of finding
fossilized diatoms is made more difficult by the fact that the
diatom's frustule
('skeleton') is made of an unstable form of silica (opal),
which may not be preserved. |
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