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S. Aaron Spriggs
Colorado State University
Fort Collins, CO, 80523
email: saarons@lamar.colostate.edu
ABSTRACT
Studying the process of fossilization, or taphonomy, can involve several
varying directions. After an organism has died (regardless weather
it was flora, fauna or miscellaneous others), a rare event may occur
leading to the possibility of
fossilization. A brief discussion of the multitude of biased events
which limit the possibilities of fossilization will then be followed by
the basic types of fossils that may be formed. Carbonization is one
form of fossilization and is typical for such organisms as plants and
insects. These fossils are a coal black film formed when the
volatile organic compounds disperse from the decomposing organism and end
up leaving a thin residue of carbon. Permineralization is a second
type of fossil formed. The soft tissue of the organism decay away
and the remaining hard parts are flooded with ground water.
Dissolved with in the water is calcium carbonate (calcite) or
silicate. Which ever mineral is present precipitates out and fills
the pores of the long gone
organism. Cementation occurs and a "rock" is left in the
place of the wood or bone or what-have-you with an amazing amount of
detail preserved as well. Dissolution and replacement is a third
type of fossilization and can be a step wise progression from
permineralization. In some cases, when the ground water flows into
the space previously occupied by the soft tissues of the organism, the
original material may dissolve away, leaving a void in the surrounding
sediments. This space, which is in the shape of the organism like a
jell-0 mold, quickly becomes filled with minerals and an internal mold or
"stone cast" is formed. Replacement can occur if it is a per
mineral fossil which is dissolved and replaced by a secondary type of
mineral. Finally,
recrystallization can be the fourth type of fossil. Shells are often
recrystallized because of the relatively unstable minerals that they
comprise of to begin with. While each type of fossilization is
different and thus differing degrees of detail remain to be scrutinized by
the paleontologist, all fossils are subject to the capricious whims of the
environment before, during, and after fossilization has occurred.
This paper is a syntheses of these forces as well as the chemistry
involved in forming fossils.
INTRODUCTION
A writer's worth or talent is judged by the test of time. If a novel
continues to be read, even after major social change (or maybe because of
it), then it is said the author was a talented writer. Some writers
are so talented that stories and myths spring up around them about how
they wrote, not just what they wrote. For instance, Jack Kerouac, it
has been said, wrote On the Road in seven days upon tele-type paper that
he scrolled into his type writer so that he was not slowed down by having
to change paper. This is actually partially true (he was rewriting
from the original manuscript that had taken him years to live and write).
Organisms, like writing styles, are widely varied, but they all end as a
Greek tragedy. Every body dies. Taphonomy is the
"written" record of life preserved in rocks and like writing
styles, preservation methods vary widely, and that is the actual point of
this review article. However, after an organism dies, be it moss,
jellyfish, or mayfly, it must now beat the odds to be immortalized as a
fossil to join the ranks of the few to represent historic life.
Fossilization, unlike death, is not a guarantee. The depositional
environment must fall between narrow parameters to give the organism even
slim odds for fossilization. If the specimen does beat the odds and
is fossilized, there is no assurance that it will remain that way.
Other minerals, diagenetic forces, or simple erosion may alter the
specimen to the point that it is worthless to be included in the fossil
record ( or worse yet, misleading!). Each of these topics as well as
the fossil formation types will be discussed further.
DISCUSSION
FOSSILIZATION
When it comes to fossilization and gambling, unaltered remains is the jack
pot.
These fossils are freeze-dried mammoths upon the Siberian tundra, Ice Age
wooly
rhinos pickled in Polish oil seeps, or amber inclusions (such as leaves,
insects, or frogs). These are probably the least likely fossil to
find but can usually be the most informative for biohistoric
content. Unlike most mineralized fossils, unaltered remains include
the soft tissues of the organism and many still have hair, skin color, and
its last meal intact. Unaltered remains are a royal flush when it
comes to fossils. A second type of fossil is carbon films or
carbonization. These are often thin films of carbon which are upon
planes of sandstone or shale. After death, volatile organic
compounds disperse and leave a residue behind. This residue is a
coal-like carbon film which preserves the outline and sometimes some
detail of the organism (Prothero 1998). Plant material is the most
common tissue preserved in this manor.
Another fossil type is permineralization. Less detail is preserved
(or less content in the writing) compared to unaltered remains, but as for
altered tissue samples, it still contains the most detail for
paleontologists. After death and burial, the remaining hard parts
(such as bones, teeth, and shells because soft tissues do not stay around
for very long) are flooded with ground water containing dissolved calcium
carbonate or silica. These dissolved minerals precipitate out of
solution and fill the pores of the bones, wood, etcetera. This new
material is filling in the gaps, not replacing existing material and this
is a fine point to be observed when discussing recrystalization
next. With time and pressure, these minerals solidify to form the
fossil. Minute details
are often well preserved in this manor, even down to the cellular
level!
Petrified wood is a common example of this form of fossil.
Recrystalization may not be the "strait flush" that
permineralization is, but a
"full house" is not to shabby. This fourth type of
fossilization is a step wise
procedure, meaning that permineralization must have occurred first.
The first
bath of minerals can be relatively unstable and may undergo
recrystalization
processes to more stable mineral forms. If this occurs, details are
lost but
the overall shape of the organism remains.
Finally, the fifth fossil type is dissolution and replacement. Going
back to the original tissue, baried and being bathed in water seeping to
it through the sediments, this tissue may well dissolve all
together. This may sound like the fossil is now a lost cause, but
such is not the case. The space or void left behind can act like a
mold to form an internal mold or steinkern ("stone cast").
This internal shape is often formed from sediments seeping in. A
variation upon this theme is going from a permineralized fossil that is
then dissolved away and the void is then filled with a second, differing
mineral substance (Prothero 1998). This "shell game" of
bait and switch is beast dealt with a firm knowledge of geology and the
hierarchal nature of minerals.
I would be negligent if, at this opportunity, I did not at least mention
trace fossils. Trace fossils are foot prints, tail prints, leaf
impressions, clam burrows.....Evidence left behind by the organism is
question rather than a fossil of the organism itself (Savrda and Bottjer
1989). Trace fossils are formed from the solidification of
sedimentary depositions under oxygen-deficient, or anoxic,
conditions. These layers can later be separated to disclose some
behaviors of the deceased. A rare thing among silent stones.
TAPHONOMIC BIAS
Taphonomy may be the study of when "the remains of the organism make
a passage
from the biosphere to the lithosphere" (Plotnick 1993), but that is
not limited
to the study of chemistry and geology. Depositional environments
(the environmental conditions existing at the time the future fossil is
placed among the accumulating sediments) plays a key role in it as
well. Potential number of fossils is greatest when total number of
living organisms is counted, but as these numbers of individuals pass
through each successive "filter" towards becoming a fossil,
their numbers are reduced (thus the term filter). The paleoecology
is the first filter encountered when individuals pass from the "life
assemblage" to the "death assemblage" (Prothero
1998). This first
screening process, or bias, involves location of the organism. Is it
terrestrial or aquatic (and marine or fresh water?)? Terrestrial
organisms are less likely to be fossilized. Drier conditions tend
not to transport the minerals needed for fossilization. Both
environments have decay, scavengers, and erosion that can destroy
remaining hard-parts that would other wise be candidates for
fossilization. Trees are targeted by bacteria, fungi, molds, ants,
and termites. Bones may be broken up by scavengers and/or predators
for bone marrow and other soft tissues that are edible. Besides the
biological agents which affect a corps, there are mechanical agents as
well. Wind, rain, and dust are a few examples that exist in a
terrestrial habitat, while waves,
currents, and storms (and of course water itself) exist within an aquatic
habitat. Mix in chert (small pebbles and other abrasive material)
and most thin shelled and soft tissued items do not last long (Prothero
1998 and Trueman and Benton 1997). Rapid burial is the key in either
habitat for fossilization.
Large items verses small items is another bias which exists. When it
comes to
discovering a fossil or for the tissue to withstand the environment to be
fossilized in the first place, large items tend to last compared to that
of small items. Which has been found in larger abundance, any of the
three inner ear bones or jaw bones? Another bias is sheer numbers
verses rare organisms. Diatoms are global and easily found (but not with
the naked eye!) While Archeopteryx are comparatively rare (even though
this is counter to the size bias just mentioned) due to the sheer
population size differences (as well as the previously mentioned
terrestrial vs. aqueous bias).
Another big bias against fossils being discovered is their location.
The best place for fossils to form is in the ocean in areas like
continental shelves (for rapid burials to take place with mud slides and
the like). The problem is, most paleontologists are poking around in
dry, warm environments and while these places may at one time have been
sea front property, many locations that were aqueous are still
aqueous. While actually finding the fossil may be the final filter
biasing the fossil records, it is not the last bias to be discussed.
DIAGENESIS
Geological forces that may destroy the fossil or prevent their
preservation in the first place. Volcanism and metamorphism are two
prime examples of diagenesis (Prothero 1998) as are tectonic plate
movements which may result in subduction zones (where one plate's edge is
sliding beneath that of the other plate playing a very slow game of limbo)
and as the rock formations cycle beneath and thus deeper into the Earth's
mantle, they heat up, becoming plastic in nature. This new found
mobility of the rocks destroys fossils, erasing the biological history
book, page by page, thus the age of the fossil is yet another bias!
Adding up all of these biases could easily result in an inaccurate account
of the past, and often has. With the latest technology, and a
blending of geologic, biologic, biochemical, and geochemical studies
(Briggs and Nedin 1997 and Plotnick 1993), the hurdles of taphonomy can be
over come. Roy
Plotnick reprinted a diagram from "Taphonomy's Contributions to
Paleobiology"
(by A. K. Herensmeyer and S. M. Kidwell) in his taphonomy paper that well
illustrates the passage from the biosphere to the lithosphere for any
given individual hopeful-fossil.
These biases not only subtract information from the fossil record, but
they can add to it as well, creating false images. If, for example,
there are two types of clam of equal population size in a given habitat
and one of them is thinner shelled and of a slightly different chemical
composition than the other and thus more prone to being dissolved after
death, it's eventual reduction in the fossil record ( the ratio between
the two clams are no longer 1:1) may make the other species of clam to
appear to have been more numerous (relatively speaking) than it really
was.
CONCLUDING REMARKS
While the chemistry of fossilization is not as interactive or flashy as
some
biochemistry found in nature is, the ultimate result of an historic record
of biology is vastly intriguing. Along with the knowledge of
organisms of the past can also include the types of environments that
existed at the time, given the types of fossils which form or reform, as
the case may be. The term "prehistoric" refers to a time
prior to written history and it is a term used all to often when
discussing the fauna of any specified era. I say that with the
fossil record as a written text of life, set in stone, the term
prehistoric is an antiquated word in an evolutionary point of view and it
should be reserved for human sociological references only.
While bias do exist within the fossil record, and the record is not yet
complete
(the optimist that I am), they are not automatically to be considered a
negative
aspect of this litho-record of life. If read properly, a bias may be
more informative rather than less and since more species have died off
than are extant presently, there is much to read. Do not judge this
book by its cover; and since there is no book review to proclaim
"powerful stuff" or other such parroted phrases, taking the time
to read it may be a gamble, but think of the odds you have already
overcome just by holding the fossil in your hands.
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