Window to the past
Fossils pages in this web site introduce the common readers to types
of fossils, conditions leading to fossilization, and the information contained
in fossils. Last updated 8th Dec '96.Your feed
back is welcome.
What is a fossil ?
There is a misconception that fossils are just a bunch of bones that
make up an old dinosaur model, but there are many other aspects to be considered
when defining a fossil. The most general definition of fossils refers to
the remains of an ancient organism or the traces of activity of such an
organism. There are two types of fossils- the body fossils and the trace
fossils. Body fossils include preserved remains of an organism (i.e. freezing,
drying, petrification, permineralization, bacteria and algea). Whereas
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).
Ages covered by fossils
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, and the fossils among them the oldest known organism on earth. The
fossils found in rocks dating so far back are usually microfossils, such
as elongated bacteria, Eobacterium and other water environment fossils.
There have also been well defined remains of algae and bacteria found from
nearly two billion years ago. Bacteria represents the first stage of recognizable
organized life. The most common fossils are found in sedimentary rock.
Sedimentation is the process of the accumulation of particles originating
from the break up of pre-existing 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. The 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 using the presence of similar index
fossils. Some excellent guide fossils are ammonites, whose evolution was
such that each species lived for relatively short periods of time but had
such a broad geographical distribution that 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. These levels are preserved by different processes
that will be explored individually by this web site. 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 & Dessication: 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 fossils are found ?
There are certain techniques that paleontologists might exercise to
find fossils, but mostly finding a fossil has to do with chance and luck.
However, all paleontologists need a place to start. No person would ever
go on any kind of a hunt without having at least some clues about the general
locations of the object that they are looking for. For paleontologists,
this is where an extensive knowledge of the stromatolites, the different
eras, the knowledge of which era(s) certain organisms dominated the earth,
and which environment was most suitable for certain organisms. With these
informations, the collector can eliminate certain localities depending
on what kind of fossils they are searching for. For example, if a collector
was interested in finding fossils of animals of ancient rocky shores, he/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. Soon every collector
realizes that fossils of certain types are found in particular kinds of
rocks. 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,
process of elimination, and perhaps some luck, paleontologists have retrieved
all sorts of fossils and animal remains from all sorts of geological areas.
Conditions that lead to fossilization
There are many conditions that contribute to the formation of fossils.
However, the most common conditions include the possession of hard parts,
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 stands a good
chance of being buried before it decays or disintegrates. If the organism
is not buried deeply and quickly, aerobic bacteria will reduce it to rubble;
or water given enough time, will dissolve it. For this reason, fossils
of some kinds of organisms are rarer than others. As for the skeletons,
the skeletons that contain a high percentage of mineral matter are most
readily preserved, and in contrast the soft tissue that is not intimately
connected with skeletal parts is least likely to be preserved. Other conditions
that lead to fossils include an environment that was biologically inert,
areas that are receiving a large, steady supply of sediment, such as 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 where it is covered rapidly with sediment. For this purpose, clay is
an excellent option. The sediment protects the tissues and helps to exclude
predators and solvent water.
What do fossils tell us?
The realm of which this question can be answered is very broad. The
answers depend upon the fossils found in particular places, and upon the
questioner. Nevertheless, there are certain general ideas that can be drawn
from different fossils accordingly. Different fossils depending upon how
they were preserved tells us different things. For example, fossils that
are preserved in amber can tell us an extraordinary amount of information
about the anatomy of that organism; since the organisms that are preserved
in amber, mostly insects, are preserved as a whole usually without any
disintegration of organs, muscles, and its coloring. Even bones devoid
of flesh may tell a great deal about the soft anatomy. For instance, the
area where the muscle attaches to the bone leaves marks that indicate sizes,
shapes, 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 their 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 also tell a great
deal about their surroundings and the conditions under which they lived.
Finally, fossils also contribute greatly to the study of evolution. They
are the only direct record of what has in fact occurred in sequences of
reproducing populations and in the course of the time on an evolutionary
Amber || Casts & Molds
|| Compactions || Compressions
|| Coprolites & Gastroliths
Drying & Dessication || Freezing
|| Impressions || Molecular
Fossils || Permineralization
Reference || Trace
Fossils || Wax & Asphalt