Taphonomy: Death Is A Sure Bet, Fossilization Is A Long Shot

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.    


REFERENCES

Briggs, Derek E. G. and Christopher Nedin.  Jan97.  The taphonomy and affinities 
of the problematic fossil Myoscoloex from the lower Cambrian.  Journal of 
Paleontology. V.71 (1) p22.

Plotnick, Roy E.  Nov93.  Taphonomy: Perfecting the fossil record.  Geotimes.  
V.38 (11) p.14.

Prothero, Donald R.  1998.  Bringing Fossils to Life, An Introduction to 
Paleobiology.  WCB/McGraw-Hill.  U.S.A. p.4-11. 

Savrda, Charles E. and David J. Bottjer.  1989.  Trace fossil model for 
reconstructing oxygenation histories of ancient marine bottom waters: 
application to upper cretaceous Niobrara formation, Colorado.  Palaeogeography, 
Palaeoclimatology, Palaeoecology.  74 p.49 - 74.

Trueman, C.N. and M.J. Benton.  Mar97.  A geochemical method to trace the 
taphonomic  history of reworked bones in sedimentary settings.  Geology.  V.25 
(3) p. 263.