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South China and elsewhere, together with numerous
works on molecular dating, provide an
opportunity to draw a more precise timescale
for the early metazoan evolution. This study
presents a synthesis of available molecular
and fossil dating results, placed in the palaeoenvironmental
background, which leads to
the recognition of the following time events
for early metazoan cladogenesis: (1) Animal
multicellularity occurred most probably during
early to middle Neoproterozoic times (between
1000 and 660 Ma) during the geological
period of gradual increase in oxygen content;
(2) Eumetazoans and sponges (or sponge-like
organisms) parted from each other during the
Neoproterozoic glaciation interval (ca. 750-
580 Ma), but both experienced adaptive radiation
after the glaciations as evidenced by the
Doushantuo Biota (ca. 635-551 Ma); (3) A
great metazoan radiation event (MRE) occurred
during the late Ediacaran and earliest
Cambrian periods (ca. 550-530 Ma), interpreted
here as a metazoan radiation at higher
taxonomic levels, as compared with the early
Cambrian radiation representing one at lower
taxonomic levels (lower than subphylum or
class). In order to better constrain the phylochronology
of early metazoans, we present
evidence for the divergences of arthropod
subgroups on the basis of molecular dating
results and the fossil record. It is shown that
the arthropod stem group diverged from its
sister taxon at between 700 and 565 Ma and
major arthropod subgroups originated coevally
with MRE. The MRE is probably correlatable
with the 13C negative anomaly in the
late Ediacaran Period.
Although the phylochronology of early
Metazoa is gradually emerging, a great deal of
research still will be needed in the following
directions in order to reduce uncertainties in,
and discrepancies among, dating results: (1)
Evaluate different molecular dating techniques
so as to establish a proper set of standard procedures
for dating early metazoan events; (2)
Set up a standard set of fossil calibration
points suitable for dating early metazoan divergences;
(3) Use both molecular and fossil
constraints as far as possible; (4) Place the
time sequence in the geological and palaeoecological
background so that one can
visualize a logical evolutionary scenario. We
suggest that the fossil record plus the geological
background should serve to construct a
general timeframe for evolution, while molecular
dating will supplement and probably
increase the precision of the chronology.
古 生 态 学
2008010049
委 内 瑞 拉 西 部 上 阿 尔 拜 阶 - 下 马 斯 特 里 赫 特
阶 地 层 中 记 录 的 海 洋 及 气 候 变 化 的 地 球 化
学 特 征 = Geochemical characterization of
oceanographic and climatic changes recorded
in upper Albian to lower Maastrichtian strata,
western Venezuela. ( 英 文 ). Erlich R N;
Palmer-Koleman S E; Lorente M A. Cretaceous
Research, 1999, 20(5): 547-581
Organic and inorganic geochemical data
(Rock-Eval, TOC, biomarker GC/MS, and
mineral, major and trace element analyses)
were used to characterize the palaeoceanographic
and palaeoclimatic conditions that
influenced the deposition of upper Albian to
Maastrichtian rocks of western Venezuela.
These data show that the late Albian to early
Santonian was characterized by the accumulation
and preservation of hydrogen-rich marine
algal and foraminiferal organic matter, and
was modulated, in part, by siliciclastic dilution
(via eolian and fluvial/turbidite processes).
The contemporaneous development of palaeobathymetric
barriers surrounding the Maracaibo
and Barinas/Apure Basins resulted in
stagnation and poor circulation, and along
with high evaporation rates, produced salinity
stratification and entrapment of anoxic bottom
waters. Bottom water oxygen levels increased
from the late Santonian through the end of the
Cretaceous as seasonal upwelling intensified,
and stratified and entrapped anoxic bottom
waters underwent frequent overturn and ventilation.
These major changes in depositional
patterns can now be identified and linked to
regional or global oceanographic and climatic
events. The depositional events that best demonstrate
this link include: (1) late Albian to
early Cenomanian drowning of the Maraca
Formation carbonate platform (linked to the
global mid-Cretaceous platform drowning
event); (2) Cenomanian-Turonian drowning of
the Guayacan Member (Capacho Formation)
carbonate platform (southern Maracaibo Basin)
and deposition of organic carbon-rich sediments
(linked to the global Cenomanian-
Turonian oceanic anoxic event); (3) depositions
of Tres Esquinas Member (La Luna
Formation) phosphates and glauconites in the
Santonian to Campanian (linked to a regional
increase in fluvially derived sediments); and
(4) onset of delta progradation from Colombia
into western Venezuela during the Campanian
through Maastrichtian (linked to regional tectonic
activity and cooling global climate).
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