INTRODUCTION TO FORAMINIFERA
WHAT ARE FORAMINIFERA?
Foraminifera (forams for short) are single-celled organisms (protists) with shells or tests (a technical term for
internal shells). They are abundant as fossils for the last 540 million
years. The shells are commonly divided
into chambers that are added during growth, though the simplest forms are open
tubes or hollow spheres. Depending on the species, the shell may be made of
organic compounds, sand grains or other particles cemented together, or crystalline CaCO 3(calcite or aragonite).
Fully grown individuals range in size
from about 100 micrometers to almost 20 centimeters long. Some have a symbiotic
relationship with algae, which they "farm" inside their shells. Other species eat foods
ranging from dissolved organic molecules, bacteria, diatoms and other single-celled algae, to small animals
such as copepods. They catch their food with a network of thin pseudopodia (called reticulopodia) that extend from one or more apertures
in the shell. Benthic
(bottom-dwelling) foraminifera also use their
pseudopodia for locomotion.
WHERE DO THEY LIVE?
There are an estimated 4,000 species
living in the world's oceans today. Of these, 40 species are planktonic,
that is they float in the water. The remainder live on or in the sand, mud, rocks and plants at
the bottom of the ocean. Foraminifera are found in
all marine environments,
from the intertidal to the deepest
ocean trenches, and from the tropics to the poles, but species of foraminifera can
be very particular about the environmentin which they
live. Some are
abundant only in the deep ocean, others are found only on coral
reefs, and still other species live only
in brackish estuaries or intertidal salt marshes.
Foraminifera are
among the most abundant shelled organisms in many marine environments. A cubic centimeter of sediment may hold
hundreds of living individuals, and many more dead shells. In some environments
their shells are an important component of the sediment. For example, the pink sands of some
In regions of the deep ocean far from land the
bottom is often
made up almost entirely of the shells of planktonic
species.
WHY ARE THEY IMPORTANT?
The study of fossil foraminifera
has many applications beyond expanding our knowledge of the diversity of life. Fossil foraminifera are useful in biostratigraphy,
paleoecology, paleobiogeography,
and oil exploration.
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BIOSTRATIGRAPHY
Foraminifera provide evidence of the relative ages
of marine rocks
There are several resons
that fossil foraminifera are especially valuable for
determining the
relative ages of marine rock layers. They have been around since
the Cambrian, over 500
million years ago. They show fairly continuous evolutionary
development, so different
species are found at different times. Forams
are abundant and widespread, being found in all marine environments. Finally, they are
small and easy to collect, even from deep oil wells.
PALEOECOLOGY AND PALEOBIOGEOGRAPHY
Foraminifera provide evidence about past
environments
Because different species of foraminifera are found in different environments, paleontologists can
use the fossils to determine environments in the past. Foraminifera
have been used to map
past distributions of the tropics, locate ancient shorelines, and track global
ocean temperature changes during the ice
ages. If a sample of fossil foraminifera contains many extant species,
the present-day distribution of those species can be used to infer the environment at that site when the fossils
were alive. If samples contain all or mostly extinct species, there are still numerous
clues that can be used to infer past environments. These include species diversity, the
relative numbers of planktonic and benthic species, the ratios of different shell types, and shell chemistry.
The chemistry of the shell is useful
because it reflects the chemistry of the water in which it grew. For example, the ratio of stable
oxygen isotopes depends on the water temperature, because warmer water tends to
evaporate off more of the lighter isotopes. Measurement of stable oxygen isotopes in planktonic and benthic foram shells from hundreds of deep-sea cores worldwide have been used to map past
surface and bottom water temperatures. This data helps us understand how climate
and ocean currents have changed in the past and may change in the future.
OIL EXPLORATION
Foraminifera are used to find petroleum
Some species are geologically
short-lived and some forms are only found in specific environments. Therefore, a
paleontologist can examine the specimens in a small rock sample like those recovered during the drilling
of oil wells and determine the geologic age and
environment when the rock formed. As a result, since the 1920's the oil
industry has been an
important employer of paleontologists who specialize in these
microscopic fossils. Stratigraphic
control using foraminifera is so precise that these
fossils are even used to direct sideways drilling within an
oil-bearing horizon to increase well productivity.
BIOLOGY OF FORAMINIFERA
Very little is known about how most
species of foraminifera live. The few species that have been studied
show a wide range of behaviors, diet, and life cycles. Individuals of some species live
only a few weeks, while other species live many years. Some benthic
species burrow
actively, though slowly, through sediment at speeds up to 1cm per hour, while
others attach themselves to the surface
of rocks or marine plants. Foraminifera are abundant
enough to be an important part of the
marine food chain, and their predators include marine snails, sand dollars and small fish.
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CLASSIFICATION OF FORAMINIFERA
Traditionally, classification of foraminifera has been based primarily on characters of the shell or test.
Wall composition and structure, chamber shape and arrangement, the shape and position of any
apertures, surface ornamentation, and other morphologic features of the
shell are all used to define taxonomic
groups of foraminifera. New research is adding molecular data on
relationships among species that may greatly affect how these organisms
are classified.
Chamber arrangements commonly found in
living species are shown in figures 1-6 . The
following terms are used: Unilocular refers to
a shell made of a single chamber Uniserial refers to chambers added in a single linear
series Biserial refers to chambers added in a double linear series Triserial
refers to chambers added in a triple linear series Planispiral refers to chambers added in a coil within a
single plane like the chambered nautilus
Trochospiral refers to chambers added in a
coil that forms a spire like a snail shell Milioline refers to an arrangement where each chamber
stretches the full length of the shell and each
successive chamber is placed at an angle of up to 180 degrees from the
previous, relative to the central axis
of the shell Arborescent refers to an erect,
branching series of tubes. Terms such as planispiral-to-biserial
and biserial-to-uniserial are used when the mode of
chamber addition changes during growth.
Of the various kinds of wall
composition and microstructure found in foraminifera,
three basic
types are common among living species. Agglutinated shells may be composed of very small particles
cemented together and have a very smooth surface, or may be made of larger particles and have a rough surface. Hyaline
shells are made of interlocking microcrystals of CaCO
3, and
typically have a glassy appearance and pores that penetrate the wall. Porcelaneous shell
walls are composed of microscopic rod-shaped crystals of CaCO
3. These have a milky,
translucent to opaque look and generally lack pores beyond the initial chambers. In some porcelaneous
species, small depressions in the surface ornamentation give the appearance of pores. Another type
of wall structure, called microgranular, is made of tightly packed equidimensional rounded grains of calcite. This wall type
is found in many
Paleozoic foraminifera including the fusulinids.
Figures
1-6. The text
and these images were authored by Karen Wetmore at the UC Museum of
Paleontology,
