Precambrian Life I: Microfossils
Introduction
The Precambrian microfossil fossil record is largely a record of the cyanobacteria. These prokaryotic organisms were and are responsible for the construction of stromatolites. The mucilage-rich sheathes that enclose the cells of the cyanobacteria not only provide the material “glue” for trapping and binding sediment, but also, under certain conditions, preserve quite well in the fossiliferous cherts of the Precambrian.
Paleontologists rely heavily on the concept of uniformitarianism in their study of microfossils -- we look at living modern analogues of the organisms we hope to characterize in the fossil record. This methodology has its value, but does not profive a clear mechanism for interpreting fossils that are seriously different from modern forms. Also, today ioogists are working more with molecular characters to describe and categorize microbial groups. However, with fossil microbes, we are forced to stick with morphology, since most of the characteristic biochemistry associated with the once-living cells is gone.
Lab Assignment
Please review the images of modern cyanobacteria and look through the links for the Archaean, The Gunflint Type Biotas and the Proterozoic Cherty Biotas. For each of the these three sets of images, draw a sketch and write a descriptive paragraph for one species. Recall the previous labs where we described different fossil characteristics. Include all the components of morphology that you can muster, for some of these it won't be much. Include in your description such parameters as size and shape. For specimens that come from entire populations, try to include some information about variation as well. Your sketches and descriptions can be turned in to class or to your TA throughout the next week.
The Cyanobacteria Today
Today, the cyanobacteria can classified into five groups, based on morphology:
Chroococcales: Coccoid cell shape (either spherical or ellipsoidal), cells occur either solitary, in pairs or as colonies of unicells.
Oscillatoriales: single filamentous forms consisting of a trichome of cells surrounded by a sheath
Pleurocapsales: coccoidal cells that form a thallus (clusters of cells) via 3-dimensional divisions.
Stigonematales: filamentous forms with true branching.
Nostocales: coccoidal cells forming filaments often with specialized cells called, heterocysts and akinetes. Heterocysts are thought to be associated with Nitrogen fixation, a property of many Nostocalean cyanobacteria that provides Nitrogen in a form that can be assimilated by other organisms.
Archæan Microfossil record
First of all, ALL microfossil occurrences from the Archæan are to some extent controversial or debatable. That said, it is worthwhile to have a look at the claims on the earliest microfossils. This is the closest we get to a tangible record of the earliest forms of life in Earth.
The Gunflint Type Biotas
Proterozoic Cherty Biotas
Update, 13 November 2001.