Geology Reference
In-Depth Information
Geo-Focus
Palynology: A Link Between Geology and Biology
Palynology is the study of organic mi-
crofossils called
palynomorphs
. These
include such familiar items as spores and
pollen (both of which cause allergies for
many people) and also such unfamiliar
organisms as acritarchs (organic-walled
microfossils of probable algal origin),
dinofl agellates (marine and freshwater
single-called phytoplankton, some spe-
cies of which in high concentrations
make shellfi sh toxic to humans) (see
Figure 22.22c), chitinozoans (vase-
shaped microfossils of unknown origin),
scolecodonts (jaws of marine annelid
worms), and microscopic colonial algae.
Fossil palynomorphs are extremely
resistant to decay and are extracted from
sedimentary rocks by dissolving the
rocks in various acids.
A specialty of palynology that attracts
many biologists and geologists is the
study of spores and pollen. By examin-
ing the fossil spores and pollen preserved
in sedimentary rocks, palynologists can
tell when plants colonized Earth's sur-
face, which in turn infl uenced weather-
ing and erosion rates, soil formation,
and changes in the composition of at-
mospheric gases. Furthermore, because
plants are not particularly common as
fossils, the study of spores and pollen
frequently reveal the time and region
for the origin and extinction of various
plant groups.
Analyses of fossil spores and pollen
are used to solve many geologic and
biologic problems. One of the more im-
portant uses of fossil spores and pollen
is determining the geologic age of sedi-
mentary rocks. Because spores and pol-
len are microscopic, resistant to decay,
deposited in both marine and terrestrial
environments, extremely abundant,
and part of the life cycle of plants
(Figure 21.33), they are very useful for
determining age. Many spore and pol-
len species have narrow time ranges that
make them excellent guide fossils.
Rocks considered lacking in fossils by
paleontologists who were looking only
for megafossils often actually contain
thousands, even millions, of fossil spores
or pollen grains that allow palynologists
to date these so-called unfossiliferous
rocks.
Fossil spores and pollen are also use-
ful in determining the environment and
climate in the past. Their presence in
sedimentary rocks helps palynologists
determine what plants and trees were
living at the time, even if the fossils of
those plants and trees are not preserved.
Plants are very sensitive to climatic
changes, and by plotting the abundance
and types of vegetation present, based on
their preserved spores and pollen, paly-
nologists can determine past climates
and changes in climates through time.
An interesting study by C. B. Foster
and S. A. Afonin published in 2005
related morphologic abnormalities in
gymnosperm pollen grains to deteriorat-
ing atmospheric conditions around the
Permian-Triassic boundary—that is, at
the time of the global Permian extinc-
tion event. One cause of morphologic
abnormalities in living gymnosperms
and angiosperms is environmental stress
on the parent plant. Plants are sensitive
indicators of environmental change,
and studies have shown that pollen wall
abnormalities are caused by atmospheric
pollution, ultraviolet (UV) radiation, or
a combination of both.
Processing of samples from nonma-
rine sedimentary rock sequences that
span the Permian-Triassic boundary
from the Junggar Basin, Xinjiang Prov-
ince, China and Nedubrovo, Russia,
yielded a diverse, abundant, and well-
preserved pollen assemblage. Examina-
tion of the assemblage revealed that
more than 3% of the pollen showed
morphologic abnormalities (
Figure 1).
Based on this fi nding, the authors con-
cluded that these abnormalities were the
result of atmospheric pollution, includ-
ing increased UV radiation, caused by
extensive volcanism. Recall from
Chapter 20 that one of the possible
causes of the Permian extinction was
increased global warming resulting from
◗
but rather at different times, a pattern known as
mosaic
evolution
. This diversifi cation and adaptive radiation took
place during the Late Silurian and Early Devonian and re-
sulted in a tremendous increase in diversity (
plants from their dependence on moist conditions and
allowed them to spread over all parts of the land.
Seedless vascular plants require moisture for success-
ful fertilization because the sperm must travel to the egg
on the surface of the gamete-bearing plant (gametophyte)
to produce a successful spore-generating plant (spo-
rophyte). Without moisture, the sperm would dry out
before reaching the egg (
Figure 21.32).
During the Devonian, the number of plant genera remained
about the same, yet the composition of the fl ora changed.
Whereas the Early Devonian landscape was dominated by
relatively small, low-growing, bog-dwelling types of plants,
the Late Devonian witnessed forests of large tree-sized
plants up to 10 m tall.
In addition to the diverse seedless vascular plant fl ora
of the Late Devonian, another signifi cant fl oral event took
place. The evolution of the seed at this time liberated land
◗
Figure 21.33a). In the seed
method of reproduction, the spores are not released to the
environment, as they are in the seedless vascular plants, but
are retained on the spore-bearing plant, where they grow
into the male and female forms of the gamete-bearing
generation.
◗
Search WWH ::
Custom Search