An outline of the pathways
affecting the preservation of once living organisms can be
found in Figure
1 below. As discussed
below, this encompasses both the processes of
biostratinomy and diagenesis.
Figure
1 -
The
field of Taphonomy as it relates to steps in transformation
from living organisms to fossils.
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Modified from McRoberts (1998)
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| Processes that occur between the
death of an organism and its subsequent burial in the
sediment are termed biostratinomy. Generally, this
includes the decomposition and scavenging of the animal's
soft parts, and at least some amount of post-mortem
transport. Such things as the amount of shell breakage and
the concentration of shells in layers often indicate the
level of water energy and post-mortem transport. For
example, the shell-hash or coquina has experienced a significant amount of shell breakage and
probably post-mortem transport suggesting deposition in high
energy environments; whereas, the articulated plant remains are intact suggesting little or no post-mortem transport and
deposition in a very low energy and oxygen-free environment.
In Table
1 below are various
taphonomic indicators and their environmental
implications. |
| The physical and/or chemical
effects after burial are called diagenesis. This is
the realm in which dissolution, replacement, or
recrystallization of original shell material occurs, as can
the formation of molds and casts. A more detailed
description of diagenesis with regards to fossil
preservation in the next section. |
| Table
1 |
|
Summary
of Taphonomic Indicators and TheirPaleoenvironmental
Implications
|
| TAPHONOMIC
FEATURE |
IMPLICATIONS
|
| Abrasion |
The wearing-down of skeletons owing
to differential movement with respect to sediments is an
indicator of environmental energy. Significant abrasion is
most commonly found on skeletal material collected from
beaches, or areas of strong currents or wave
action. |
| Articulation |
Multi-element skeletons are soon
disarticulated after death. Articulated skeletons, then,
indicate rapid burial or otherwise removing the skeleton
from the effects of energy of the original
environment. |
| Bioerosion |
Bioerosion encompasses the many
different corrosive processes by organisms. The most
pervasive causes of degradation are boring and grazing.
Bioerosion erases information from the fossil record, but it
also leaves identifiable traces made by organisms on
remaining hard skeletons or surfaces. Therefore, trace
fossils produced by bioerosion add information on the
diversity of ancient assemblages. |
| Dissolution |
Skeletal remains commonly are in
equilibrium with surrounding waters, but changes in chemical
conditions can cause skeletons to dissolve. Dissolution
represents fluctuation in temperature, pH or pCO2 in calcium
carbonate skeletons. Siliceous skeletons also can dissolve
because normal sea water is usually undersaturated with
respect to silica. |
| Rounding |
Broken edges of skeletons become
rounded owing to dissolution and/or abrasion of exposed
surfaces. Processes that control edge rounding probably
include a combination of dissolution, abrasion, and
bioerosion. Rounding gives an estimate of time since
breakage. |
| Encrustation |
The growth of hard skeleton
substrates by other organisms is a common occurrence.
Besides indicating exposure of the skeleton above the
sediment-water interface, encrustation can specify a
particular environment. Different patterns of encrustation,
as well as different biota, occur in different
environments. |
| Fragmentation |
Breakage of skeletons is usually an
indication of high energy resulting from wave action or
current energy. Fragmentation also can be caused by other
organisms through either predation or scavenging. |
| Orientation |
After death, skeletal remains are
moved by the transporting medium and oriented relative to
their hydrodynamic properties. Fossil skeletons in life
position indicate rapid burial, attachment to a firm
substrate, or death of in-place infauna. Hard parts tend to
orient long-axis parallel to unidirectional flow in
current-dominated areas and perpendicular to wave crests on
wave-dominated bottoms. |
| Size |
After death and if not rapidly
buried, a skeleton behaves as a sedimentary particle and is
moved and sorted with respect to the carrying capacity of
the flow of currents, waves, or tides. Size can, therefore,
be an effective indicator of flow capacity in a hydraulic or
wind-driven system. |
From McRoberts (1998)
A Brief Introduction to
TAPHONOMY
© Gastaldo, Savrda, & Lewis. 1996. Deciphering Earth History: A
Laboratory Manual with Internet Exercises. Contemporary
Publishing Company of Raleigh, Inc. ISBN 0-89892-139-2
Not every organism that ever lived could become part of the fossil record. If you eat an average
of three meals a day, you test and prove this hypothesis daily. A large percentage of all biological
entities end up as food for other organisms higher on the food chain. This fact alone may
prevents these organisms from being preserved. Even those organisms that avoid being eaten
have a low probability of becoming fossilized because most of them undergo decay and recycling
of their chemical components. For example, you can examine any forest-floor litter and find that
beneath the top layer of leaves, the organic matter has been degraded to an unrecognizable form
(humus -- not hummus, the garlic-laden spread served in health-food restaurants). This recycling
keeps the carbon, nitrogen, and sulfur cycles operating. In fact, many taphonomic biases impact
the odds of any organism being preserved.
The paleontological subdiscipline called Taphonomy, from the Greek taphos (death), is
concerned with the processes responsible for any organism becoming part of the fossil record and
how these processes influence information in the fossil record. Many taphonomic processes must
be considered when trying to understand fossilization. These include events that affected the
organism during life (changes in rainfall, availability of food, and behavior for maximum growth,
etc.), the transferral of that organism (or a part of that organism) from the living world
(biosphere) to the sedimentary record (lithosphere; compare the death of a herd of vertebrates
with the autumnal leaf fall from a forest), and the physical and chemical interactions that affect the
organism from the time it is buried until the time it is collected in the field.
Any organism must successfully pass through three distinct, and separate, stages in order to be
seen in a museum display. These stages span the entire time from death of the organism to
collection. Necrology is the first stage, and involves the death or loss of a part of the organism.
The vast majority of animals must die before they can become introduced to the next phase. It's
true that if a starfish is cut in half, each half will regenerate itself. The result will be two animals.
Not many animals have this capability. We suggest that you don't test this hypothesis with your
beloved pet. On the other hand, most plants do not have to die to contribute one or more of their
parts to the potential fossil record. When autumn leaves fall in temperate climates, the trees don't
die. The oldest living organism, bristlecone pines, are more than 5300 years old (as determined by
counting tree rings). Their present leaves are not the same ones that grew 5300 years ago. When
plants disperse their reproductive bodies (spores, pollen, or seeds), most do not die thereafter. Of
course there are exceptions, but these are a small percentage of all extant (living) plants.
Once an organism has died or sheds a part, all the interactions involving its transferral from the
living world to the inorganic world (including burial) constitute the second taphonomic stage.
This is the Biostratinomy stage. Besides the conspicuous fossil characteristics that you will be
able to observe during this laboratory (those external and internal features of the fossilized
remain), less-obvious details often record what happened to the organism (or part) before it
became a fossil. By studying these details paleontologists are able to understand, in a Sherlock
Holmesian way, the mode of death or disarticulation (breakup of an organism), any biological
processes that may have modified the remains before burial (such as scavenging), the response of
the part to transport (by animals, water and/or wind), and the amount of time the organism sat
around in the environment before it was finally entombed.
Ultimately the organic matter is buried. Burial plays an important role in potential preservation of
the organic matter. Very specific chemical and physical conditions must exist in the burial
environment to allow preservation in a form recognizable to us. It is here that biological (e.g.,
enzymatic and bacterial) and chemical (e.g., enzymatic and dissolution) processes must be slowed
or eliminated. Once buried, the organic material is subjected to the third taphonomic phase, or
Diagenesis. Diagenesis involves all of the processes responsible for lithification of the sediment
and chemical interactions with waters residing between clasts. The processes of fossilization
appear to be site specific with respect to depositional settings, resulting in a mosaic of
preservational traits in the terrestrial and marine realm. Few fossil assemblages are exactly
identical, especially with regard to the way in which they were formed, but general patterns do
exist. An understanding of taphonomic assemblage features within an environmental context
allows for a more accurate interpretation of the fossil record.
Most organic matter on Earth is used by some organism higher on the food chain and is,
therefore, ultimately recycled. This is the fate of almost all biomass on Earth. Most organic
matter is composed of easily degraded and digested compounds that are not likely to be preserved
even under the most favorable conditions. Those parts of an organism that are already
mineralized (such as your calcium-fortified skeleton) and, hence have made the first step in the
transition to "stone", have a higher probability of preservation than any of the soft, fleshy tissues
either around or within the skeleton. The early inhabitants of Paris, France, the bones of whom
are now stacked neatly in catacombs beneath the city streets, attest to this fact.
Although the fossil record is incomplete, it still provides a useful survey of the history of life
because of the vast amounts of time represented within the rock record. Even if the conditions for
preserving organic matter existed only once every 10,000 years in each contemporaneous
depositional environment around the globe, a lithology that was 100 meters thick (330 feet) and
encompassing 1 million years of time would contain 100 fossil assemblages. Such conditions are
not unrealistic, particularly within the ocean basins. If we then consider contemporaneous
depositional settings around the globe, the number of fossil assemblages that would be preserved
during this 1 million years of time increases dramatically. Of course, not all of these fossil sites
are or would be accessible for collection and study. Mountain-building processes associated with
plate tectonic activity (metamorphism of fossil-bearing sedimentary rock beyond recognition) and
the erosion of these folded (metasedimentary) and faulted (sedimentary) rocks depletes the
number of fossil localities available at the Earth's surface through time. The quantity of
fossiliferous rocks beneath ground level far exceeds those available at the surface to be sampled
and studied. Nevertheless, there are far more fossils than paleontologists, which will continue to
be the case far into the future. Paleontologists are not wanting in their search for the history of life
on Earth.
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Links used:
More detailed study:
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