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clearly we have a problem with the terms fossil and fossilization.
probably, these have resulted from the fact that we use this extremely
general term to categorize the many products from some 20-odd different
geological processes that can affect animals, plants, and their traces.
those processes include:
- weathering, including cracking, drying (dehydration), rehydration,
frost, heating, exfoliation, what for rocks is called `spheroidal
- chemical weathering, hydrolysis, dehydrolysis, acid dissolution, aqueous
dissolution, precipitation, etc.
- organic chemical weathering, including hydrolysis, dehydrolysis, organic
degradation, formation of new organic compounds, leaching, etc.
- cast formation (which includes many processes noted above)
- mould formation (ditto)
- remineralization, including bacterially mediated precipitation of new
minerals, simple precipitation in porosity elements, dissolution &
reprecipitation on a molecular level, etc.
- ingestion by other organisms
- biological weathering (including acid dissolution by roots, insect boring,
fungal boring, algal boring, sponge boring, bacterial boring, (i
know bo-o-o-ring), ingestion by insect larvae, mechanical degradation
such as by echinoderm spines, etc.)
- crushing due to trampling
- gnawing (i.e. bones by dogs, wolves, etc.)
- dispersal of the various parts of the animal by mechanical or biological
- pyritization (again a combination of many processes noted above)
- encasement in amber, calcite, aragonite, dolomite, various salts
(of which amber and sometimes calcite are the only ones that
really have any hope of long term perservation)
- impregnation by silica, calcite, ferromanganese minerals
- impregnation by clays (or in situ formation of clays in porosity elements
I suspect that I have forgotten several here, but you get the idea.
The point is that the basket term "fossil" and "fossilization"
have been used to cover all these processes.
Some occur very rapidly, producing very noticeable changes in the fossil
within a few days to weeks. Some ensure preservation, such as amber
encasement,but only require hours to days to complete. Should we not consider
insect encased in amber that is only 5 ka to be not a fossil, but consider
a Tr coral with its original aragonite to be a fossil, PURELY ON THE BASIS
OF THEIR AGES? Clearly, this is not good science. In fossilization,
age confers no special characters. Most fossils that are going to be
preserved have been preferentially affected within their first few
days to months in the geological environment. Taphonomic studies have
clearly demonstrated that bones that are not buried within 3 months
to 5 years in 99% of environments will not be preserved for perpetuity.
Similarly, carbonate shells not buried or inundated in near-neutral waters
within 3-10 years become dust in the wind. Plants we all know must
be perserved within days to weeks (except pollen which is designed to
be impervious to almost every known agent of destruction).
Tracks, feeding traces, burrows, etc., all must be covered by
more sediment in appropriate manners within days to perhaps as long as
a few years in some very quiet environments.
Ergo, anything potentially able to become a fossil (sensu quasi stricto),
namely surviving for more than 10 ka remineralized or not, has actually
had the necessary geochemical or sedimentological conditions occur
LONG before that, usually within 1 year. Hence, the insistence on
10 ka, is meaningless, as would an insistence on 1 Ma, or 1 Ga, as
a 'prerequisite age'.
Maybe it is time to abandon the term fossil and use more rigorous
definitions for such geological phenomena that more precisely describe
their potential for preservation for long times, or that more carefully
detail their experienced geochemical and sedimentological histories leading
to/resulting in what we now loosely call 'fossilization'.
Bonnie A.B. Blackwell, firstname.lastname@example.org
Dept of Geology, off: (718) 997-3332
Queens College, City University of New York, fax: (718) 997-3299
Dept of Earth \& Environmental Sciences, messages: (718) 997-3300
The Graduate Center, CUNY,
Flushing, NY, 11367-1597, USA