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 “abiogenic” 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.
There’s
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 weren’t 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,
there’s 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.
Other ideas:
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?
Let’s
think of this another way - the metazoan reefs didn’t dominate because
the microbialite reefs were dominant. Microbialites were
better. They weren’t abundant because everything else was gone,
necessarily.
We
still don’t 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 they’re 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.