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Invertebrates-Other Invertebrates

Algae and Bacteria

Halimeda, a green alga



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]

fossilized algal mats??

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.
Kingdom Monera, Phylum Cyanophyta

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.

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.

Yes, in theory. But see the note at the bottom of the page.
dehydrating cyanobacterial mats
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.
mud cracks after a spillway flood in Brownwood, TX

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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