WHAT CHARACTERISTICS OF MICROFOSSILS MAKE THEM GOOD
TOOLS FOR TIME CORRELATION?
1.Short geologic range.
2.Widespread distribution. Many microscopic organisms are part of the floating plankton
in the oceans. These species are readily distributed over large areas by ocean
currents.
3.Facies independence (not restricted to one particular rock type; present in many rock
types). For example, plankton falls into any sediment that is forming on the sea floor.
4.Distinctive and easily recognized form.
5.Preservable, fossilizable hard parts.
6.Abundance. The fossils should be abundant enough to be collected in sufficient
quantity for study. Because they are so tiny, hundreds or thousands of microfossils
may be present in small sediment samples.
The Symposium on "Biology and Paleobiology of Ostracoda" was organized by Dr. Frederick
M. Swain at
the University of Delaware in Newark in 1972.
The proceedings were published in 1975 as "Biology and Paleobiology of Ostracoda,"
BULLETINS OF
AMERICAN PALEONTOLOGY, vol. 65, no. 282, 687 pp. (Frederick M. Swain, ed.).
For information about the current availability of this volume, contact the publisher,
Paleontological Research
Institution in Ithaca, New York. The current address of Dr. Frederick M. Swain is: Department
of Geology
& Geophysics, 108 Pillsbury Hall, 310 Pillsbury Drive S.E., University of Minnesota,
Minneapolis,
Minnesota 55455. E-mail: swain002@maroon.tc.umn.edu
Conodont Morphology
The individual elements are composed from a variety of small cones, bars, and blades or
platforms some of which bear small teeth-like structures called denticles. Generally,
conodonts can be grouped into three main morphotypes that are listed below (see also
accompanying figure) and can be found on the slide (§12.13).
Coniform elements: consist of a single cone or cusp which has a small basal cavity.
(§12.15)
Ramiform elements: consist of bars with a central cusp and denticles extending on
either side of cusp. Usually, a basal cavity can be recognized. (§12.16)
Pectiniform elements: includes diverse forms bearing a platform, and numerous
denticles, one of which may be an anterior cusp. Usually, a basal cavity can be
recognized. (§12.17 and §12.18)
HYOLITHIDS
It is uncertain as to what phylum the hyoliths belong. Until recently they have been
classified as mollusks or worms, some prefer toconsider them as a separate phylum, and
your text chose to ignore the group altogether. Hyoliths have a bilaterally symmetrical
exoskeleton composed of a tapering conical (or pyramid) conch which is closed at one end
(posterior) and has an open end (anterior) which may be closed by an operculum. In cross
section, the cone of hyoliths may be either triangular or semicircular. The skeleton of
hyoliths is composed of calcium carbonate. One group of hyoliths has two anterior bar-like
protrusions called helens. Several examples are provided (§12.21).
TENTACULITIDS
Tentaculitids are a group of small animals which lived in a conical shaped exoskeleton
composed of calcium carbonate. Like the hyoliths, they have been variously classified as
mollusks or worms; unfortunately little is known about this extinct group which may belong
to its own phylum. The exoskeleton of the tentaculitids may be relatively smooth as in the
genus Styliolina (§12.22) or may have regularly spaced ribs or ridges as in the genus
Tentaculites (§12.23-§12.25)
The fossil record of foraminifera extends back at least to Early
Cambrian times (c. 550 Ma). Peaks of foraminiferal diversity
and abundance occurred during the latest Paleozoic (c.
320-250 Ma), the Late Cretaceous (c. 80-65 Ma) and the
Cenozoic (65 Ma to present day).
HOW ARE FORAMINIFERA USED?
Petroleum exploration. The petroleum exploration industry
has for a long time been a major stimulus for foraminiferal
research. During the 1930's foraminifera became the first
microfossil group to be used extensively for age assessment
of strata encountered during drilling, and still today are the
major "workhorse" microfossil for subsurface exploration. This
is because foraminiferal tests survive inside the
millimetre-sized rock chips produced by rotary drilling,
whereas larger fossils, such as molluscs, are obliterated.
Age and correlation of sedimentary rocks. Like other fossil
groups, foraminiferal species evolve over time, and stages in
their evolution provide a basis for subdividing the rock column.
Once the ranges of various species in the rocks have been
established, the age of an unknown bed can be determined
from the assemblage of foraminifers present. First and last
occurrences of species are especially important "bioevents",
marking unique moments in geological time. Where these can
be identified and their absolute age determined, perhaps by
magnetostratigraphy, these bioevents can become proxies for
absolute age determinations.
Because planktic foraminifera are able to spread quickly
throughout large areas of the Earth's oceans, their bioevents
are especially valuable for age determinations and for
correlation over intercontinental distances. Benthic
foraminiferal distribution is strongly controlled by local
conditions, but many species nonetheless can be used for
local or even regional correlations.
Foraminiferal bioevents and assemblages provide the primary
criteria for recognising New Zealand's Cenozoic Series and
Stages.
Ancient environments. Many foraminiferal species live within
a limited range of environmental conditions, and (unless
transported from another environment) occur only where
these conditions are found. Major factors influencing
foraminiferal distribution include temperature, water depth,
type and abundance of food, salinity, and oxygen availability.
Thus, it is possible to recognise, for example, assemblages
indicating deep or shallow water, nearshore or offshore or
oceanic, brackish or normal marine, and cold or intermediate
or warm-water environments.
Reconstruction of the distribution of ancient environments,
and especially of changes in water depth, provides important
information for studies of sedimentary basin evolution used in
scientific research and petroleum exploration.
Evolutionary studies. Foraminifera are ideal subjects for
testing various aspects of evolutionary theory, because large
populations of individuals, whose characteristics can be
measured and treated statistically, can be obtained from
closely spaced rock samples at carefully selected localities to
provide an evolutionary time series. It is then possible to show
how distribution of a particular characteristic changes over
time within successive populations.
Stable isotope studies. Foraminiferal tests often provide the
calcium carbonate used for oxygen and carbon stable isotope
analyses. The isotope analyses frequently will use a particular
planktic or benthic species, and a foraminiferal specialist will
be required to select sufficient specimens to provide the few
tens of milligrams of material required.