Fossilization (Taphonomy)
Critical part of the process
known as taphonomy, or the study of remains of organisms after they die (the
word was derived from the Greek taphos,
meaning grave-burial, and nomos,
meaning law). The concept was introduced to paleontology in 1940 by the
prominent Russian paleontologist and science fiction author Ivan Efremov (also
spelled Yefremov) to describe the study of the transition of organic remains,
including entire intact bodies, dissembled parts or products, from the
biosphere to the lithosphere, meaning the eventual conversion of living organisms
to fossil assemblages.
The motivation behind the
study of taphonomy is to better understand biases present in the fossil record.
Although fossils may seem to some to be ubiquitous in sedimentary rocks, they
are actually somewhat rare. Paleontologists cannot draw the most accurate
conclusions about the lives and ecology of the fossilized organisms without
identifying and understanding the processes involved in their fossilization.
Therefore, taphonomy includes the organism’s life-history, type-location of
death, decomposition, post-mortem transport (if any), burial, compaction, and
other chemical, biologic, or physical activities that affect the remains of an
organism. Recognition of taphonomic processes that have taken place leads to a
more complete understanding of paleoecology and even the life-history of a
once-living organism.
Since most of those who
study geoscience know that the world is very old and that many billions of
animals have lived and died between the time when plants and animals first
appeared, why is the environment not simply awash in fossils? Here’s the answer
in a nutshell: because for organisms death is guaranteed while preservation is
not.
To understand that answer we
have to focus on the underlying problem: how does anything become fossilized?
And that’s where taphonomy comes in, by elucidating critical elements in the
taphonomic process: life—death—preservation—survival—discovery. The
fossilization process starts with life (as many young student observers of
tenured university faculty and administrators approaching retirement will
readily attest) because the way an organism lives provides a bias with respect
to its potential for preservation.
Organisms living in or in
close proximity to lacustrine or tidal marsh environments have much greater
chances of having their remains rapidly covered with sediments and fossilized
than do condors or mountain goats living in a desert far from most sources of
rapid burial and sedimentation. As an example of a life having high
fossilization potential, imagine a shore bird falling dead in a near-shore
environment where its feathers are weighed down with clay-rich water and that
night and the next many nights it is covered with an inch or more of sand, silt
or mud until it is buried under several yards of moist, heavy materials.
Moreover, an animal’s position in the food web also affects its fossilization
potential since organisms at the bottom of the web are far more numerous than
the top predators.
Real World Examples of animals whose lives and deaths and preservation contributed to very
high fossilization potentials are those so remarkably preserved in the Burgess
Shale and the recently discovered nearly complete fossil skeleton of an ancient
aquatic bird in the Gansu area of China , Gansus
Preservation encompasses
ways an organism’s remains survive after death and its transition into a
fossil. Rapid burial is the best method to ensure transition into the
preservation stage and is a common process at the Earth’s surface that occurs
on a regular basis (again, the Burgess Shale is a wonderful example). Floods,
mass wasting episodes, storms, and volcanic eruptions can deposit sediments
over periods of time ranging from minutes to days. A well-known historical
example is the sudden volcanic ash fall that buried Pompeii
and Herculaneum
Fun Stuff:
Actual examples of what might humorously be called “fossil” graffiti preserved
on the walls of the two Roman cities include:
“Atimetus
got me pregnant.”
“I
fucked a lot of girls here.”
“Phileros
is a eunuch.”
“Once
you are dead, you are nothing.”
“May
I always and everywhere be as potent with women as I was here.”
Back to more serious stuff.
Not all types of landscapes are preserved equally in the geological record,
leading to a heavy bias towards shallow, near-shore marine and lacustrine
environments. In addition, not all periods of geological history are preserved
equally, witness the paucity of fossils from periods earlier than the Cambrian.
Recent Earth history is far better preserved if only because not as much time
has elapsed during which erosion or destruction via heat and pressure can
occur. Burial also protects the dead organism from being consumed by higher or
lower food chain organisms and from mechanical (abrasion and break-up) and
chemical (decay and disintegration) processes. Although in the vast majority of
cases decay is an inevitable part of death, organisms with hard parts (shell,
bone, claws, teeth) have a much higher preservation potential than organisms
that are entirely soft-bodied or that possess fragile hard parts.
Preservation comes in
various types. An unaltered state is
the rarest form of preservation in the far greater majority of the geologic
record in which fossils are found but is more frequent in recently formed
sedimentary environments. Types of unaltered preservation where even the soft
body parts are preserved include: mummification, encasement in tar or other
hydrocarbons (as can be seen at the Rancho La Brea Pits in Los Angeles),
encasement in amber (such as frogs, insects, or leaves), encasement in
sediment, and freezing, as in the case of an 18,000-year-old frozen Siberian
woolly mammoth that was on display at the 2005 World Exposition in Aichi,
Japan. However, most commonly only the hard skeletal materials are preserved
after decomposition of soft body parts.
Preservation by means of molds and casts entails creation of a
type of replica of the organism’s hard parts. In general, a mold is an impression
of a bone or shell in lithified sediment, forming a mirror image of the
original part. Molds can be internal, an impression of the inside surface of
skeletal hard parts, or external, an impression of the outside surface. A cast
is formed when a mold is filled with fine sediment and is therefore a true
replica not a mirror image of the original hard part.
A common form of
preservation involves dissolution and
replacement or recrystallization (alteration)
of original hard materials by chemical or physical means. Replacement,
sometimes on a molecule by molecule basis, most often occurs when various
minerals, typically contained in percolating groundwater, fill in voids after
dissolution of original skeletal material. Common secondary replacement minerals
include silica (SiO2) and pyrite (FeS2). The process of
the physical re-arrangement of crystalline structure of skeletal material is
known as recrystallization, which is a common phenomenon in shells that were
originally aragonite or calcite (both forms of calcium carbonate—CaCO3).
Carbonization or the formation of carbon films is another type of fossilization and
is typical for such organisms as plants and insects. After an organism (most
typically a plant) dies, internal volatile organic compounds disperse and leave
behind residues that form a coal-like black carbon film that preserves the
outline and sometimes considerable organic details. Those thin films of carbon
are often found on planes of sandstone or shale.
Permineralization or petrifaction is a type of
preservation in which the soft tissue of the organism has decomposed and the
remaining hard parts are flooded with groundwater saturated with secondary
minerals, especially calcium carbonate (calcite) or silicate that precipitates
out and fills the gaps/pores/interstitial spaces of the dead organism but does
not replace existing material. Minute details are often well preserved, even
down to the cellular level. Cementation occurs with time and pressure as
secondary minerals lithify to form a rock (fossil) in the place of the original
bone, teeth, shell, or wood, preserving an amazing amount of detail. Real World Example: A great and very
popular illustration of this form of fossilization is petrified wood, as can be
seen in the Petrified Forest National Park in northern Arizona Vietnam
Author’s Note: The celebrated mystery author Patricia Cornwell brought worldwide
attention to human taphonomy in her best-selling 1994 novel, The Body Farm, in which she discussed
the very real Anthropological Research Facility at the University of Tennessee—Knoxville, which is engaged in an effort to apply principles of physical
anthropology to the study of human decomposition, or what is called forensic
taphonomy. Check out the book, it contains a
fictional but well-written account of the work of William Bass, founder of the
decay research facility.
[1] Hai-lu
You, et al. 2006. Nearly Modern Amphibious Bird from the Early Cretaceous of
Northwestern China, Science 312(5780): 1640-1643.
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