Fossils: Window to the past
What is a fossil?
Fossils are not just a bunch of bones that
make up an old dinosaur model. Fossils are
the remains of an ancient organism or the traces of activity of such an
organism. There are two types of fossils: body fossils and trace
fossils. Body fossils include preserved remains of an organism (i.e. freezing,
drying, petrification, permineralization, bacteria and algea). Trace fossils are the indirect signs of life that give evidence of the
organism's presence (i.e. footprints, burrows, trails & other evidence
of life processes).
Life began in the sea. The earliest evidence of life on earth is of
marine animals, during the Precambrian era.
But there is only sparse evidence
of life before the Cambrian era. The oldest known Precambrian rocks, found
in Africa and Australia, are believed to be more than three billion years
old. The fossils found among them are of the oldest known organisms on earth. Usually,
fossils found in these old rocks are microfossils, such
as elongated bacteria, Eobacterium and other water environment fossils.
Bacteria represents the first stage of recognizable organized life and scientists
have found well defined remains of algae and bacteria from
nearly two billion years ago.
How old is this fossil?
The most common fossils are found in sedimentary rock. Sedimentary layers act as evidence
of the changing climate or movement of the continents during the passage
of time. The Law of Superposition or Steno's law
states that in a pile
of undisturbed sedimentary rock, the oldest bed will lie at the bottom
and the youngest on top. Layers of strata in different locations may have
the same composition but carry fossils of a different time period, therefore
a technique of zoning or an index fossil is used. Index fossils are
specific animals or plants that had a broad geographical distribution but
existed for relatively short periods of time. These fossils allow geologists
to establish a parallel between layers of sediment. Some excellent guide fossils are ammonites;
each species lived for relatively short periods of time but had
such a broad geographical distribution. They can be found today in
stratigraphic rock layers often separated by great distances. The appearance
of the same ammonite in different layers in different localities gives
evidence that those layers were deposited at the same time. Each time period
is marked by an abundant radiation of many new life forms or the mass extinction
of past life forms.
Types of fossils
Fossils undergo a variety of different fossilization processes, depending
on the characteristics of the particular organism. There are various levels
of fossil preservation, each containing its own clues pertaining to the
organism. Fossilization at the cellular level varies in all organic compounds
since not all cellular types are equally resistant to decay and decomposition.
The same hold at the tissue level, where some tissue types are more susceptible
to fossilization. The other two kinds include the organ level and organism
level which provide information in the field of morphology and biology
of the ancient organism. Although there are
an endless number of categories, we will focus on the broader mode of classification.
- Permineralization: is the occurrence of decay
of organic substances and filling of mineral material into every cavity
of the organism, still retaining most information about the fossil.
- Compressions: the two-dimensional compression
which retains organic matter of the organism.
- Impressions: the two-dimensional imprint most
commonly found in silt or clay, without organic material present.
- Casts & Molds: caused by deposits of sediment
in cavities of organism, resulting in a three dimensional model.
- Compactions: preservation of organic material
with slight volume reduction.
- Molecular fossils: deals with chemical data,
preserving organic material, but providing no information concerning the
structure of the organism.
- Freezing: ideal fossils that are rare, everything
up to internal organs are preserved in cold storage.
- Amber: biological specimen that is encased
in the hardened resin of a tree, in which the entire body may be preserved.
- Drying & Desiccation: fossils that have
been thoroughly dried.
- Wax & Asphalt: almost as good as freezing,
but with the usage of natural paraffin.
- Coprolites & Gastroliths: these categories
deal with the indigestable remnants of meals.
- Trace fossils: typically formed when an organism
moves over the surface of soft sediment and leaves an impression of its
How are fossils found?
There are certain techniques that paleontologists might exercise to
find fossils, but it mostly has to do with chance and luck.
However, all paleontologists need a place to start. Paleontologists must have
an extensive knowledge of the stromatolites, the different
eras, and which environment was most suitable for certain organisms. With this
information, a collector can eliminate certain localities in his or her search.
For example, if a collector
was interested in finding fossils of animals of ancient rocky shores, he or she
would eliminate formations and beds in which remains are likely to be rare
and poor. Paleontologists can also follow leads that other paleontologists
or collectors have left behind in published reports. These clues help narrow the focus of the search.
For example, marsh plants are most abundant in shales and sandstones
between beds of coal, and coals are found in limy shales and massive limestones,
many of which are the remains of ancient reefs. Using these techniques,
the process of elimination, and perhaps some luck, paleontologists have retrieved
many different fossils and animal remains from a variety of geological areas.
Conditions that lead to fossilization
There are many conditions that contribute to the formation of fossils.
The most common include the possession of hard parts,
such as a skeleton or shell, and a rapid burial after death. Besides being tough
and hard, the organism must come to rest in a place where it can be
buried before it decays or disintegrates. If the organism
is not buried deeply and quickly, aerobic bacteria
will reduce it to rubble.
Water, given enough time, can also dissolve it. For this reason, fossils
of some organisms are more rare than others.
The skeletons that contain a high percentage of mineral matter are most
readily preserved. The soft tissue that is not intimately
connected with skeletal parts is least likely to be preserved. Other conditions
that lead to fossilization include resting an environment that was biologically inert,
areas that are receiving a large, steady supply of sediment (deltas
of major rivers), and parts of the earth below sea level compared to those
above the sea level. The ideal place to become a fossil is at the bottom
of a quiet sea or lake where the prospective fossil is safe from damage
and can be covered rapidly with sediment. Clay offers excellent conditions where
the sediment protects the tissues and helps to exclude
predators and solvent water.
What do fossils tell us?
Answers to this question can be broad and depend upon particular fossil locations.
Nevertheless, there are certain general ideas that can be drawn
from different fossils accordingly. Different fossils, depending upon how
they were preserved, tell us different things. For example, fossils that
are preserved in amber give us an extraordinary amount of information
about the anatomy of that organism; since the organisms that are preserved
in amber, mostly insects, are usually preserved intact without any
disintegration of organs, muscles, and coloring. Even bones may tell a great deal
about the soft anatomy. For instance, the
area where the muscle attaches to the bone leaves marks that indicate size,
shape, and functions of these varied organs. Also, the cavities and the
the channels in skulls give us an idea of their intelligence, behavior,
and their principle features. Certain parts of certain fossils can also
tell us about growth, injury, disease, form, function, activities,
and instincts. Fossils also record the successive evolutionary diversification
of living things, the successive colonization of habitats, and the development
of increasingly complex organic communities. Fossils can tell a great
deal about their surroundings and the conditions under which they lived.
Finally, study of fossils contributes greatly to the study of evolution. They
are the only direct record of what has occurred in sequences of
reproducing populations and in the course of the time on an evolutionary