Stromatolite

From Palaeos

Stromatolites are the by-products of the most ancient forms of life on Earth. They are built up by the accumulation of layers of single-celled blue-green algae (Cyanobacteria), which form a circular, layered structure which is formed by the sticking of sedimentary particles to the sticky outer surfaces of the algae.

Stromatolites may measure up to a metre across, and most have been found in sedimentary rocks formed in shallow tidal and subtidal environments from the Precambrian times upto the present.

The earliest stromatolites fossils are known from Zimbabwe and are at least 3,000 million years old (Mesoarchean era). Somewhat younger specimens have bene found in Montana, South Africa Australia, and northern Norway. Without doubt these organsisms were common worldwide.

Although they reached their peak during the late Proterozoic era, they still continued on into the Phanerozoic, despite predation (grazing) by the then evolved animal life. Living Stromatolites are still to be found in Shark Bay, West Australia, where the extremely high salt content protects them from grazing my marine invertebrates.

There were also specialised forms of Blue-Green algae that evolved during the Paleozoic. The genus Girvanella was a subtidal "calcimicrobialite" (microbial encruster) known from Cambrian of California, from back reef and reefal settings in Labrador, East Canada, and the Lower Permian of the Southern Alps.

Credits: MAK991201 (Kheper), copied to Palaeos org MAK060928