in the heavy isotope of carbon and, as it went intocirculation in marine water, caused the precipitous drop or boundary excursion in the d13C value.. It therefore seems reasonable to int
Trang 1in the heavy isotope of carbon and, as it went into
circulation in marine water, caused the precipitous
drop or boundary excursion in the d13C value
It therefore seems reasonable to interpret the
Ven-dian carbon isotope curve as a record of gradually
increasing biotic productivity, with sequestering of
much of the organic matter within and below the
seafloor microbial mats, followed by a relatively
sudden release of part of this organic matter
de-posited at the end of the Vendian as a result of
increased burrowing intensity Other factors, such
as destabilization of gas hydrates in seafloor
sedi-ments, may also have been involved in these isotopic
excursions
A number of other phenomena can perhaps be
traced to what has been called the Cambrian
Sub-strate Revolution The Vendian and Cambrian both
saw an increase in the proportion of calcified
fila-mentous microbes (such as Girvanella), which were
perhaps less palatable to mat grazers than
filament-ous cyanobacteria and unprotected algae With all the
new sediment and organic matter in suspension, filter
feeding probably became more possible throughout
the water column, leading to the evolution of the first
tiered filter feeders in the Cambrian The only
Ven-dian organisms that were likely to engage in
suspen-sion feeding were the cloudinids, which lived close to
the sediment–water interface
Finally, assuming that we are interpreting the
secu-lar carbon isotopic curve correctly, it is entirely
pos-sible that oxygen levels increased in the Vendian due
to the sequestration of organic matter Whether or not
increasing oxygen levels influenced metazoan
evolu-tion is not known, although it seems fair to say that
early burrowing animals would not have required
high levels of oxygen The earliest animal habitat
appears to have been the submicrobial mat
environ-ment, where oxygen levels would probably have
been rather low considering the relative abundance
beneath the mats of hydrogen sulphide and other
reduced compounds
Glossary
abiogenically A term applied to rocks formed
by processes not directly influenced by living
organisms
anactualistic processes Processes that occurred at
one time in the Earth’s past, but which are no
longer operational today
cloudinid A late Vendian calcareous shelly fossil
con-sisting of closely nested, thin-walled tubes or cones
Thought to represent one of the earliest examples
of a shelly animal fossil Includes the genera
Cloudina and Sinotubulites
coelomic spaces The compartments that house the rigid, fluid-filled body cavity present in many animals The coelom serves as a hydrostatic skeleton
conulariid Any member of an enigmatic group of Vendian/Cambrian to Triassic shelly organisms They formed conical, often pyramidal, tapering cones with transverse ribbing, composed of cal-cium phosphate
Ediacaran Any member of a group of marine, mega-scopic fossils with a metacellular growth pattern Found primarily in strata deposited before the Cambrian period Assigned to extinct Kingdom Vendobionta
frondose forms Ediacarans with a leaf, palm, or frond body form
Garden of Ediacara A palaeoecological theory that holds that the marine ecosystems of the Vendian were largely free of megascopic predators and thus allowed organisms such as Ediacarans to survive unmolested using photosymbiotic, chemoauto-trophic, and osmotrophic life styles
holobiont A single integrated organism, as opposed
to a colonial organism
hydrostatic skeleton A fluid-filled internal organ or support structure within an animal’s body that can
be kept rigid or made limp by control of internal water pressure
metacell A single or isolated modular unit of a metacellular organism; usually consists of a single enlarged cell
metacellularity Term applied to organisms that are either multicellular (such as animals and plants) or consist of clusters or metacells (such as Ediacarans and certain types of aquatic algae)
Mirovia The Precambrian superocean that sur-rounded Rodinia
molecular clock Any gene or gene sequence used by biologists in an attempt to determine the evolution-ary time of divergence from a common ancestor between two or more groups of organisms belonging to different species
osmotrophy A feeding strategy utilizing osmosis or direct absorption of nutrients
peristaltic burrowing A burrowing strategy in meta-zoans that consists of rhythmic muscular contrac-tions along the length of the body
Rodinia A supercontinent consisting of all or nearly all of the continents Consolidated one billion years ago (in an event referred to in North America as the Grenville Orogeny), this supercontinent broke up into smaller continents by the process of plate tec-tonics and continental drift before the Cambrian Sinian The Precambrian geological period immedi-ately preceding the Vendian period
380 PRECAMBRIAN/Vendian and Ediacaran