Neoproterozoic-Cambrian Microbialite Record
Russell Shapiro (Department of Geological and Environmental Sciences, California State University at Chico, Chico, California, USA)
Paleontological Society short course: "Neoproterozoic-Cambrian Biological Revolutions", Denver, Colorado, USA
6 November 2004
Burne & Moore (1987) give a definition of microbialite - a good general term for Shapiro.
The Bahamas and Australia have living microbialites. Subrecent microbialites occur in Nevada.
Formation of microbialites - there are 4 possible ways to make a microbialite:
1) trapping & binding of grains
2) in-situ precipitation
3) repetitive ďabiogenicĒ cementation on decaying organic matter
4) microbial stabilization of sediments
Microbialites are considered trace fossils. Their form is the result of the interplay of several factors (physical, chemical, environmental, sediments, cement, biota).
There are 3 main microbialite types at the handlens scale:
1) stromatolites - have laminations (Example: Cryptozoon)
2) thrombolites - have clots (leopard rock); have no onion-like structure in plan view/bedding plane view
3) dendrolites - have branching clusters
Oncoids - >2 mm concentrically laminated (but discontinuous) grains with assumed biologic origin; aka Osagia and Girvanella, which are the actually the names of the microbes in some oncoids.
During the Mesoproterozoic and Neoproterozoic, can see an increase in stromatolite morphologic/form diversity, probably related to extensive granitization and the formation of widespread continental shelves.
Thereís a stromatolite crash at the Precambrian-Cambrian transition, but they make a return blip in the Late Cambrian and Ordovician.
Key events in the Neoproterozoic-Cambrian:
1) appearance & rapid increase in thrombolite abundance
2) appearance & diversification of calcimicrobes
3) appearance of dendrolites
4) proliferation of all types of microbialites
5) demise of all of these at the end of the Early Ordovician
Thrombolites - the oldest are 1.9 by, and there are many in the Neoproterozoic of the Northwest Territories, Namibia, Nevada. Northwest Territories & Namibian examples are in complexly zoned reefs.
At the end of the Early Cambrian, thrombolite diversity wanes as their abundance increases through the remainder of the Cambrian and Early Ordovician. Thrombolites are only locally important in the Early Ordovician, coinciding with an increase in invertebrate taxa.
Calcimicrobes - there are differing views on how to deal with calcimicrobes. Some say they should be treated as real genera. Some say the different forms are the result of diagenesis. However, they should be named so we can build up data, in order to see patterns. Includes Renalcis and Epiphyton and Girvanella (the classic little spaghetti tubes).
Dendrolites - get conflicting information regarding their lifestyles - see 2 patterns: 1) branches going upward; or 2) branches going downward or in random directions (as in crypts). How does this affect our perceptions of whether they were photosynthetic? Dendrolites grew up or down or in all different directions. For example, can see this in archaeocyathan reefs. Dendrolites also grew with anthaspidellid sponges.
Get a microbialite proliferation in the Neoproterozoic-Cambrian. They dominated because there werenít any metazoan reefs then. Microbialites are common if you hike in the Cambro-Ordovician.
Reasons for microbialite abundance in the Cambro-Ordovician:
1) post-extinction lag hypothesis - after the archaeocyathan reefs go extinct, thereís a lag.
2) reduced bulldozers hypothesis - nothing is eating the microbes.
Problems with these ideas: there are Cambrian corallomorphs and there are Neoproterozoic skeletal reefs and there are certainly lots of grazers in the Cambrian.
1) Mg/Ca seawater chemistry hypothesis
2) nutrient deficiency hypothesis
3) warm water hypothesis
Do these factors hold back the invertebrate reefs? Or do they encourage the microbialite reefs?
Letís think of this another way - the metazoan reefs didnít dominate because the microbialite reefs were dominant. Microbialites were better. They werenít abundant because everything else was gone, necessarily.
We still donít understand the differences between stromatolites, thrombolites, and dendrolites.
They can be qualitatively recognized in the field, but why are they different?
Some of the differences are diagenetic, but not always. Some forms are atop other forms.
Still need work on using microbialites to recognize facies.
Recommendations for educators: microbialites are trace fossils - discuss their complexities. Emphasize the procaryote world! Microbialites are a natural introduction to biogeology, astrobiology, origin of life.
The diversity of stromatolite forms crashes near the Neoproterozoic-Cambrian transition.
Early Cambrian archaeocyathan reefs - the microbialites are there, but theyíre in the background. The Middle Cambrian and Late Cambrian have lots of microbialites - the Upper Cambrian especially, including kilometers of microbialite deposits. Volumetrically, even in the Early Cambrian, microbialites dominate over the archaeocyathans.