Life In The Precambrian

The earliest known fossils are found in 3.5 billion-year-old cherts of the Warrawoona Group at North Pole, Australia. The fossils consist of stromatolites that formed reeflike mounds made of small, single-celled organisms called cyanobacteria, which are still common today. Stromatolites are known from many other Archean locations, and they are essentially the only macro-scale fossil found in Archean rocks. There are a variety of microscopic organisms known, but most of these fall into varieties of single-celled organisms such as cyanobacteria.

The first truly diverse fauna is found in the 1.81.6 billion-year-old Gunflint Formation on the north shore of Lake Superior. The Gunflint Formation contains layers of stromatolites, but between the stromatolite layers many new species of cyanobacteria have been found. The Gunflint contains the first possible eukaryotic cells, containing a nucleus, as opposed to all earlier cells, which were prokaryotic and contained no organized nucleus. This represents a very significant evolutionary advance.

The late Precambrian had a more diverse fauna and saw the first definitely eukaryotic cells, found in the 1.3 billion-year-old Beck Springs Dolomite of Death Valley, California. In this period stromatolites became very abundant, and some grew to enormous sizes, some tens of feet (several m) across. Trace fossils also first appeared in the late Precambrian in the form of worm burrows from the late Precambrian rocks at the base of the Grand Canyon Series.

Rocks of the Ediacarian (also known as the Vendian, Sinian, and Eocambrian) range between 700 million years and 543 million years old at the base of the Cambrian and the Paleozoic. This period is named after fossil-rich beds in the Ediacarian Hills of Australia, containing a wide variety of fossils of jellyfish (Coelenterate medusae), sea fans (octocorals), worms, and other species without skeletons. All of the Ediacarian fauna became extinct at the end of the period, and none shows any relationship to any younger fauna. There is a 200-million-year break in the fossil record between Precambrian stromatolites and the diverse plant and animal fossils in the Edia-caran Hills, and in this short gap of time for which no record is known, life had to change dramatically from the single-celled organisms that had inhabited the Earth for the past 3 billion years to complex life-forms found in these strata. To accomplish this the eukaryotes had to clump together in colonies; become consumers of food rather than manufacturers; develop specialized cells for reproduction, locomotion, and other functions; and develop a body sack with tissues and organs.

Continue reading here: Late Precambrian Paleogeography And Tectonics

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