The Early Cambrian Experiment in Reef-Building by
Metazoans
Melissa Hicks & Stephen Rowland
(Department of Geoscience, University of Nevada, Las Vegas, Nevada, USA)
Paleontological Society short course:
"Neoproterozoic-Cambrian Biological Revolutions", Denver, Colorado,
USA
6 November 2004
Who
are the Neoproterozoic players?
Grotzinger,
Walter & Knoll (2000) described Namacalathus.
Wood
et al. (2002) described Namapoikia (was large).
Who
are the Early Cambrian players?
1)
archaeocyathans
(and
see, also
see) (2 single occurrences in the post-Early Cambrian - one in the Middle
Cambrian, and one in the Late Cambrian).
2)
radiocyaths (similar to receptaculitids; may be an alga, rather than
having sponge affinities).
3)
corals/corallimorphs (occur within archaeocyathan reefs; most are late
Atdabanian to Botomian; make up a low % of reefs - make up 5% of Esmeralda
County reefs).
4)
calcimicrobes
(and see)
There
are 2 varieties of archaeocyathans - regulars and irregulars.
Regular
archaeocyathans have thin borders with inner and outer walls.
Irregular
archaeocyathans have much less symmetry, similar to modern sphinctozoan
sponges; have chambers; more common in reef environments.
There
are 180 genera of archaeocyathans at their maximum diversity.
Archaeocyathans had an 11 m.y. duration of reef building activity (~same
duration as the Paleocene).
Archaeocyathans
are traditionally classified as incertae sedis, but they are now
considered a class of Phylum Porifera. Archaeocyathans had aspiculate,
originally high-Mg calcite skeletons. If they’re aspiculate, they can’t
be sponges, right? But, 10-15 years ago, a living aspiculate sponge was
found, similar to archaeocyathans. So, this objection isn’t valid
anymore. Archaeocyathans can be sponges.
Archaeocyathans
were put in Kingdom Archaeata for a while (also known as Kingdom
Inferibionta).
The
uncertain status of archaeocyathans is no longer. The problem is now
resolved.
Scuba
allowed observation of a modern aspiculate archaeocyathan-like sponge with
chambers, the demosponge Vaceletia crypta (known to be a demosponge from
embryological studies).
It
is possible to have a sponge without spicules. Vaceletia is
not a descendent of archaeocyathans, however.
Archaeocyathans
are a class-level clade of sponges.
Archaeocyathan reefs -
occur from the Tommotian to Toyonian. Reefs greatly vary in size
from 1 meter high & wide to 100 meters in width and 50 meters in
height. Archaeocyathan reefs are loaf-shaped to lens-shaped, and range
from patch reefs to compound reefs. The reefs are a consortium of
archaeocyathans and calcimicrobes.
Tommotian reefs -
520-521 m.y.; generally small patch reefs (~2 meters wide); examples are in the
Anabar-Sinyaya Basin of Siberia; reefs include archaeocyathans and Renalcis.
They formed rigid, cavernous frameworks. They were the 1st
true metazoan reefs. They were low diversity, but contained all guilds
(bafflers, dwellers, constructors). Tommotian reefs did not contain
“simpler” forms.
Atdabanian-Botomian reefs
- 511-520 m.y.; known in Morocco, Sardinia, Yukon, Nevada, northern Mexico,
China, Australia. Apex reefs - 180 genera of archaeocyathans (high
diversity of reefal & peri-reefal organisms); occurred in low to high
energy environments; occurred in shallow to deep shelf.
Dolomite-filled
primary cavities have trilobites, echinoderm plates, calcimicrobes,
archaeocyathans.
Sinsk
(mid-Botomian) and Toyonian regression extinctions hit archaeocyathans.
The
Sinsk regression has not yet been seen in Laurentia. The Sinsk event is
recorded in non-bioturbated black shale.
Toyonian reefs -
510-511 m.y.; see a global decline in reef building; Siberian reefs are
basically gone; a few scattered reefs around the world; 35 genera of
archaeocyathans; 1 or 2 genera of archaeocyathans per reef, commonly; reefs
still can be fairly large & complex; high diversity of reef dwellers -
hyoliths, Salterella, ostracods, trilobites, corals, brachiopods,
chancelloriids, echinoderms; reefs got hit hard by the Toyonian regression (=
Hawke Bay event) - a global regression is seen in Morocco, Laurentia, Baltica,
South China, Iran; sessile organisms were hit (such as archaeocyathans) &
vagrant organisms were hit (such as trilobites).
Why
didn’t reefs recover?
There
is a 30 to 40 m.y. hiatus in metazoan reef building after this. See lots
of microbialites
instead.
See
the book Phanerozoic Reef Trends.
Hypotheses:
1)
post-extinction lag - 40 m.y. is longer than it should take to recover,
however.
2)
photosymbiosis recovery - archaeocyathans probably did have photosymbionts
(there’s no evidence, though).
3)
reduced grazing - cyanobacteria + microbialites went crazy; the fossil record
doesn’t support this hypothesis.
4)
nutrient deficiency - there was a lack of much exposed terrestrial rocks on the
continents (Example: high transgression on Laurentia),
5)
high level of CO2atm. - doubling CO2 gives corals trouble
(it changes the pH of the water); the Berner (1997) curve shows CO2atm.
was 20 times higher than now.
6)
global warming
7)
Mg/Ca seawater chemistry - seafloor spreading resulted in increased Ca and
decreased Mg levels; therefore, calcite organisms do well and aragonite or
high-Mg calcite organisms (such as archaeocyathans) have trouble; this is a
time of changeover from aragonite seas to calcite seas.
Audience comment:
there are problems with some of these hypotheses - reefs rebound when
temperatures and sea level are still high; so, causative factors have to
be more complex.
