Stromatolites Prokaryotes Algae Cyanobacteria Fossils mm 22 x 21 x 15 gr 11 Photosynthetic benthic microorganisms Bacteria Fossilized eukaryotes Extinct Paleozoic Cambrian Collectibles Paleontology Museum.
Pleasant calcareous fossil find with concentric laminated layers of Stromatolites from the Cambrian of Morocco. The layers originating from Algal Mat and Cyanobacteria alternating with particulate sediment of carbonate mud are perfectly visible in the rock. Only a piece, as in photo.
Stromatolites (from the Greek στρώμα, strōma, carpet, blanket and λὶθος, lithos, stone) are sedimentary rock structures, with a striated appearance laminated in alternate bands, belonging to the group of bioconstructed non-particle limestones due to the presence of materials deposited by activity of benthic photosynthetic microorganisms such as Prokaryotes (for example Calcimicrobes similar to the current Cyanobacteria) and also microscopic Eukaryotic Algae.
They were among the main responsible for the
presence of oxygen in the Earth's atmosphere and were the organism that dominated the Earth for almost
three billion years.
With the
appearance of photosynthesis in these organisms, oxygen began to accumulate in the Earth's atmosphere and this process determined at least two important consequences:
- the birth of aerobic metabolism, i.e.
cellular respiration;
- the formation of the
ozone layer, which protects living things from ultraviolet radiation coming from the Sun.
Oxygen currently makes up 21% of the atmosphere and it is believed that about 2 billion years ago it reached a concentration of
1%, preceding the appearance of the first
eukaryotes and allowing them to develop an aerobic metabolism.
Stromatolites are formed by periodic entrapment in the mucilage produced by blue-green algae and/or bacteria, of very fine
particulate sediment, consisting mainly of
carbonate mud. After the first
algal mat has fixed the sediment particles on its gelatinous surface, until it is completely covered, the algal filaments develop above it and thus form another carpet. By doing so, a succession of
alternating levels of organic substance with others rich in sediment is formed. Due to diagenetic processes, the remains of microalgae are rarely preserved and
lamination is the only evidence of their activity.
The
morphology of stromatolites is very variable, from flat-parallel to wavy laminae, up to mamellonar and columnar, depending on the type of algae or bacteria that constitute them and the environmental conditions. In an environment with high water energy, algal mats can form
concentric envelopes around objects ("nuclei") subjected to continuous rolling by waves and currents, giving rise to isolated objects called
oncolites, which constitute a particular case of stromatolite.
They are fossils widely distributed throughout the
Precambrian.
The oldest stromatolites of proven microbial origin date back to
2,724 million years ago, but new discoveries show strong evidence of stromatolites from microbial activity dating back to approximately
3,450 million years ago and in 2016, stromatolites dating back to
3,700 million years ago were found in the Inua region in Greenland. Among the best known, those found in the Gunflint Iron Formation in Canada, dating back to about 2 billion years ago.
In the
Archean, stromatolites are rather rare and structurally simple; become abundant and diverse in the
Proterozoic. The stromatolites definitively
diminished about 680 million years ago (Neoproterozoic); a modest
recovery occurred only in the Cambrian-Ordovician transition (485 million years ago). This decline is attributable to the appearance of the first
grazing Metazoans and digging organisms that bioturbated the seabed, destroying the texture of the plates.
In
Italy they are quite frequent in the
Lower Cambrian formations of Sardinia and the
Triassic formations of the Alps and the Apennines, in a
carbonate platform environment; they have never been reported in rocks of Precambrian age.
Currently, stromatolites form in marginal marine environments of tidal flats, in the intertidal zone with normal or high salinity but also in subtidal environments of some areas of Australia, Florida, the Bahamas and the Persian Gulf. These structures are also widely present in lake environments, especially in temperate-tropical climates, as a product of bacterial activity.
They have an important paleoenvironmental significance and, at least in the Phanerozoic, are good indicators of shallow-water environments with high environmental stability. The current ones are morphologically different from those of the Proterozoic.
For more about Stromatolites and more generally on the Carbonate Platforms of this planet, we recommend the article
"The first forms of life: prokaryotes and stromatolites," published on our site in the space
"Naturalist Portal",
at this link.
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