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:::Index>Evolutionary Course of Life>Knowledge Acquisition> Evidence of Evolution> How the
Fossil is Formed
How the Fossil is Formed
The stratum and the fossil included in the stratum are the important source of
information to study the history of earth. Where to look for the fossils? Most fossils
exist in rock stratum, and the rock stratum belongs to the sedimentary rock, which is
laminated layer by layer. The word of English fossil is originated from Latin Fossus,
which means the excavation.
Most of rock on the earth is the sedimentary rock, which is piled by the
sediments. These sediments are the eroded or weathered grit particles produced by the
water or wind for a long time. They are piled in the concave place gradually. The
particulate matter is pressed and glued to form the sedimentary rock at the bottom. If
there is biological remains settled with the particles, the fossils might be found in the
rock stratum in the future. The stratum of sedimentary rock should be nearly
horizontal, but it might be lifted upwards due to the change of earth crust change
(called the geological incident). The fossils will expose after erosion. It is not
surprising to find the fossil of ocean invertebrate in the mountain area of Himalayas.
It is not all ancient biological remains can form fossils. The formation process of
fossil is quite difficult, because most remains are either eaten by other living beings,
or decomposed by the microorganism. Normally, the condition for the formation of
fossil is the biological remains are covered by the sediment at once, or frozen, dried,
or left in the oxygen deficient environment.
Fig1：Cast fossil
According to different formation process of fossils, the fossils can be divided
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into the several categories. The first one is formed by the interchange process. The
biological remains are left in the stratum which is rich of mineral substance in
groundwater. The water penetrates into the remains, and the mineral substance
deposits in the cell tissue to form the fossil, such as the silicon wood fossil and most
skeleton fossils. The second one is called the mould and cast fossils. The erosion of
biological remains in the stratum forms the cavity with biological form, which is
called the mould fossil. If the cavity is filled with other mineral substances, this kind
of fossil is called the cast fossil. The third on is called the resin fossil or amber. Many
plants will secrete the resin. Some small-scale invertebrate such as insect, scorpion or
bacterium etc. will be enclosed by this sticky material to form the sample, and the
DNA of organism will be still involved.
Reviewed by：Zhang, Yong-Ta、Liu, Hsin professor
All rights reserved by National Taiwan Science Education Center