Life on Earth: A Fossil History
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Geologic Time Scale:
The geologic time scale is a device developed to assist scientists in their
study of the earth’s history. It allows the immense story of the earth to be divide
into more manageable lengths. Once scientists understand the general
sequences of the geologic time scale, it is much easier to refer to a fossil by the
period in which it existed rather than by the actual number of years before
present time it is easier to say, “This is a Devonian fossil,” than to say, “This fossil
is that of an organism that existed from 395 million years ago to 345 million years
ago.” Our understanding of dates can change as scientists make new discoveries.
The type of dominant life forms, or major events during each time span, is
used to divide the segments of earth history. For example, the Mesozoic era was
a time when dinosaurs were the dominant form of life on land. The end of the
Mesozoic was marked by a huge extinction that wiped the dinosaurs and other
organisms from the face of the earth forever, preparing the way for new types of
organisms. The bottom of this model represents the pre-Cambrian, which is
subdivided into the Hadean, Archean, and Proterozoic Eons.
The first section of the earth’s history, from the formation of the earth to
570 million years ago, is often called the Hadean. The Pre-Cambrian is actually a
rather slang term and represents the Hadean, The Archean, and the Proterozoic
eras. An accurate scale of the Pre-Cambrian would require this timeline to be
about four meters long.
***Note*** Geologists/Paleontologists have added a new period to their
official calendar of Earth’s history - the first significant change in over 120 years.
The brand new Edicarian Period covers some 50 million years of ancient
time on our planet from 600 million years ago to about 545 million years ago. It
officially becomes part of the Neoproterozoic, when multi-celled forms started to
take hold of the earth.
The Ediacaran begins at the end of the last ice age of the Snowball Earth, or
the Cryogenian Period, a term given to a series of glaciation that covered most of
our planet around 600 million years ago.
One theory proposes that these climate shocks somehow triggered the
evolution of complex, multicelled life, and this set the stage for the explosion of
more recognizable life-forms that occurred in the subsequent Cambrian Period.
The Earth’s history since the Pre-Cambrian is called the Phanerozoic Eon
and has been divided into the Paleozoic, Mesozoic, and Cenozoic Eras. Paleozoic
means ancient or early life. Mesozoic means middle or intermediate life. Cenozoic
means recent life. The boundary of each Era has a capital “E” representing a
major extinction event that changed in the rock strata (layers). This is on the chart
you will work with tomorrow.
Each of the eras has been further subdivided into periods. The periods have
been named after the location or events in the area where the rocks were first
studied. Cambrian rocks were first scientifically described in Wales. The roman
name for Wales is Cambria. The Jurassic period was named after the Jura
Mountains, between France and Switzerland. The Cretaceous period was named
after the Latin word for chalk (creta) because the huge chalk beds of England are
of this time period.
Ages of Fossils:
There are two main ways to determine the age of fossils. Relative dating is
used to place different organisms in the proper sequence of when they lived. The
principles of superposition and cross-cutting relationships, among others, are
used to determine the relative ages of fossils. Superposition states that, in an
undisturbed sequence of rocks, the rocks on the bottom have been there longest
and are, therefore, the oldest. Younger rock layers are then sequentially
deposited on top of the older ones. Cross-cutting relationships help us
understand the order of origins of rocks when other geologic events (igneous
intrusions and faults) cut across pre-existing rock layers (strata). An intrusive
igneous rock or fault therefore must be younger than the rock layers across or
through which they cut.
The other form of dating is called absolute dating. It must commonly refer
to dating by analysis of radioactive elements, tree rings, sediment layers in the
lake bottoms, or other types of evidence that occur on a cyclical basis. Absolute
ages are given in units of time, usually years.
Fossilization:
The fossil record is sparse from the beginning of life on earth to about 570 million
years ago. Scientists hypothesize that this is because early organisms from this
time span did not have body parts that easily fossilize. With rare exceptions, only
the hard parts of an organism - like the shell or skeletons - are preserved. It is
also possible the population of organisms then were considerably smaller than
those later in the earth’s history.
About 570 million years ago, many organisms evolved skeletons, shells, and
chitinous exoskeletons. Made of harder material, these remnants were more
readily fossilized. Therefore, paleontologists and other fossil hunters have been
much more successful at finding these organisms because of the rigger incidence
of fossilization of living things with hard body parts.
Although more organisms were fossilized during the Cambrian, it would be
an error to assume that the fossils represent a complete record of all life in
existence then. Fossilization is a process that relies on a series of favorable
circumstances. When organisms die, they are usually eaten or decomposed.
Even the hard body parts that have increased the likelihood of fossilization are
usually broken down. Organisms are most likely to become fossils when the
condition for decomposition are poorest. Chances of preservation increase when
decomposition is delayed and then sediments bury the organism’s remains. Sea
or lake bottoms are often a good location for fossilization because these
environments meet most of the necessary fossilization conditions. Deceased land
organisms not only have a slight chance of being buried by sediments, but they
also face exposure to weather and scavengers. These factors contribute to the
fact that few land organisms ever fossilize.
Even when a fossil is formed, more favorable circumstances have to happen
for the fossil to survive through time. Natural processes of rock formation can
destroy fossils through heat and pressure. Fossils are often very susceptible to
weathering and erosion. Once fossils are exposed to the forces of the weather,
they usually are quickly damaged or destroyed. If an organism happens to
become fossilized and then is not destroyed by geologic processes, it may still not
be found. The layers of rock that contain the fossil need to be deformed and
exposed by weathering and erosion before a paleontologist can find them. Most
fossils have not been and may never be found.