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TAPHONOMY
& PRESERVATION
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FORMS
OF PRESERVATION
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UNALTERED
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This form of preservation is rare
in most of the geologic column, but becomes more frequent in
younger sedimentary rocks. Types of unaltered preservation
where even the soft body parts are preserved include: (i)
mummification, (ii) encasement in tar, (iii) encasement in
amber, (iv) encasement in sediment, and (v) freezing. More
frequently, however, only the hard skeletal material is
preserved after removal of soft body parts.
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Examples of unaltered preservation
include the skeleton of a horseshoe crab,
whose shell is composed of interlocking plates and jointed
appendages which quickly disarticulate after death; cockle
bivalved molluscs,
whose outer-most shell layer has been removed by abrasion,
yet the original shell material of the inner layers remains;
an ammonoid
from the Cretaceous period in
which you should note the pearly luster which is original
aragonite shell material; and an insect encased in amber
.
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MOLDS
& CASTS
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This general class of preservation
entails making "replicas" of the skeletal hard parts of
organisms. In general, a mold is an impression in the
sediment of a skeleton or shell. Once encased in lithified
sediment, the dissolution of skeletal material leaves behind
the impression or mold of original skeletal form. Thus, a
mold is a "mirror image" of the original skeleton. An
internal mold (sometimes called a steinkern) is the
impression of the inside surface of skeletal hard parts. An
external mold is the impression of the outside
surface of skeleton or bone. An example of both types of
molds can be seen in this image of a trilobite
.
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A cast is formed by the
filling-in of a mold. It is thus a true replica (not a
"mirror image") of the original skeleton or shell. By this
definition, the cast one gets for a broken limb is not
really a cast at all but an external mold.
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A graphical representation of the
formation of casts and molds is provided in
Figure 2
below.
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Figure 2 -
Different
diagenetic processes leading to different preservational
styles in skeletal materials.
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* Note that
molds are produced directly as imprints of the shell and
casts are produced from molds.
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Modified from McRoberts (1998)
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REPLACEMENT
& RECRYSTALLIZATION
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This common form of preservation
involves chemical and/or physical alteration or replacement
of original skeletal material. To properly identify
replacement and recrystallization, one must know what the
original constituents of the organism's skeleton were. These
are provided in Figure 1.3. Replacement occurs often
by the filling in (by various minerals) of the void space
after dissolution of original skeletal material. Sometimes,
the replacement occurs on a molecule by molecule basis.
Common replacement minerals that you should be able to
recognize include Silica (SiO2) as shown in the coral,
and Pyrite (FeS2) shown in the ammonoid.
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Recrystallization involves
the physical re-arrangement of crystalline structure of
skeletal material. This is a common phenomenon in shells
which were originally aragonite and/or calcite (both forms
of calcium carbonate- CaCO3). Examples, both of which are
now calcite, include a gastropod which was originally
aragonite and a brachiopod which was originally calcite.
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CARBONIZATION
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As organic remains decompose in
sediments, volatile constituents such as oxygen, hydrogen,
and nitrogen are slowly lost to the surrounding sediments
frequently leaving behind a carbon film. This process is
carbonization (or sometimes called distillation), and occurs
most frequently in oxygen deficient, organic-rich
environments such as basinal black shales, and coal swamps.
The carbon films often show exquisite details of plants and
soft-body parts of animals not readily preserved, and can
often be recognized by a dark gray or black film with a
metallic sheen such as these fern-like fossil plants.
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PERMINERALIZATION
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Permineralization involves the
filling-in of pore and/or void spaces in shell or bone by
secondary mineral matter in solution. With
permineralization, the tiny pore spaces in the fossil are
filled and the original skeletal material is still retained.
However, it is often common for other types of preservation
(e.g. replacement) to occur during and/or after
permineralization. Because of its porous nature, bone and
wood
is especially prone to permineralization.
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