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 1. Home→
 2. Digital Encyclopedia of Ancient Life→
 3. Nature of the Fossil Record→
 4. 2. The process of fossilization


2. THE PROCESS OF FOSSILIZATION

CHAPTER CONTENTS:

Nature of the fossil record
– 1. Body fossils and trace fossils 
– 2. The process of fossilization ←
– 3. Types of fossil preservation
– 4. Completeness of the fossil record

Two fundamental natural factors govern the process of fossilization:

 1. The environment where an organism died.
 2. The materials that made up the organism's body when it was alive.

FOSSILIZATION AND THE ENVIRONMENT

The rapid burial of remains beneath a blanket of sediment is critical to the
process of fossilization because burial separates the remains from the
biological and physical processes that would otherwise destroy them.

Paleontologists use the term taphonomy (from the Greek taphos—meaning burial or
grave—and nomos—meaning rules or law) to describe all the events that happen to
the remains of an organism between the time that it dies and the time that it is
buried. Many events can impact remains between these two events. For example,
the remains may be scavenged, broken, or transported long distances before being
buried.

Geologists characterize habitats where burial occurs as depositional
environments. Depositional environments are almost always basins, or depressions
on the Earth’s surface. Basins can range from very small (like a pothole in a
road) to medium sized (like a lake) to global scales (like the Pacific Ocean
Basin). Because of gravity, water flows downhill until it reaches a basin that
it cannot move past; think of a river flowing downhill from a mountain that
fills a lake below. Fast moving water may have enough energy to transport large
amounts of sediment, including grains of mud, silt, sand, and pebbles. When the
fast moving water collides with the slow moving water in a basin, the available
energy decreases dramatically and the sediment can no longer be carried by the
water and it is knocked out of suspension. This causes the sediment to be
deposited in layers, with younger layers of sediment covering the older layers
below (this is the Principle of Superposition).

The remains of organisms are typically only fossilized in depositional
environments where sedimentation—and therefore burial—is frequent. Examples of
common depositional environments are lakes, river deltas, and ocean basins.
Organisms that live in these types of environments—or are transported to these
types of environments soon after death—are much more likely to be preserved as
fossils than organisms that live elsewhere. In general, organisms that live in
or near depositional environments have much better fossil records than organisms
that live far from such habitats. For example, consider the fossil-rich layers
of marine shells shown in the photograph below.

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An exposure of the Purisima Formation (Neogene Period) at Capitola Beach, near
Santa Cruz, California. The handle of the hammer rests upon a particularly rich
fossil layer (see below). Photograph by Jonathan R. Hendricks.  This work is
licensed under a Creative Commons Attribution-ShareAlike 4.0 International
License.

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Left: Closer view of the image above. Right: Examples of the kinds of fossilized
bivalves found in the dense shell layer. Photograph by Jonathan R. Hendricks. 
This work is licensed under a Creative Commons Attribution-ShareAlike 4.0
International License.

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When a paleontologist or geologist views fossil deposits like the one above,
many questions come to mind:

 * Why are most of the shells restricted to individual, densely packed layers?
 * Are the shells in life position, or were they transported to this location
   prior to burial?
 * Did shells found next to each other live at the same time?

Answering these questions depends upon interpreting the fossils in their context
in the field and recognizing the roles that taphonomy plays in generating the
fossil record that we observe in nature. In the case of this example, many of
the shells were likely swept up from the seafloor by large waves during a rare,
massive storm event and then dropped into this single, densely packed layer.
This interpretation is supported by other lines of evidence. First, the fossil
assemblage includes exclusively remains of marine organisms. Second, the shells
are all mixed together and do not appear to be in life position (we know this by
comparing their shell orientations in the rock with the resting positions of
their live relatives). Finally, we do not have enough information available to
say for sure whether or not two shells found next to each other were
contemporaneous, though it may be unlikely: the robust shells of some mollusks
may spend many years in habitats--long after their owners died--before they are
finally buried. This final process is referred to by paleontologists as "time
averaging." Because many fossil deposits contain multiple generations of
organisms, it is useful to view such assemblages--borrowing some terminology
from photography--as time exposures rather than snapshots.

Because mollusks like the ones shown above often live their lives in
depositional environments favorable to burial, they have an excellent fossil
record (this is also, of course, favored by their mineralized shells; see
below). On the other hand, the fossil record of dinosaurs is--by any
measure--comparatively poor.

Even though dinosaurs often had robust, mineralized bones, they frequently did
not live in habitats suitable for rapid burial. Indeed, many dinosaur species
are only known from incomplete skeletons, sometimes only consisting of a few
discovered bones. An excellent example of this is provided by the recent
discovery in Argentina of massive bones that belonged to a dinosaur recently
described as Patagotitan mayorum. This species of Titanosaur was one of the
largest dinosaurs that ever lived and details about this “megadino” and its
discovery are outlined on the website of the Museo Paleontológico Egidio
Feruglio (MEF) in Trelew, Argentina.

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The approximate size of Patagotitan mayorum compared to two humans. Image by
Steveoc 86 - Own work, CC BY-SA 4.0,
https://commons.wikimedia.org/w/index.php?curid=61638555.

--------------------------------------------------------------------------------

The skeletons of seven different Titanosaurs have been found at the dig site,
but all are incomplete. What happened to the rest of the bones?

Taphonomy provides the answer. While the official scientific report for this
discovery has not yet been published, it seems that—perhaps due to a combination
of physical and biological factors—only a subset of the bones reached a suitable
depositional environment. Burying large bones—including in this case single leg
bones that are longer than a full-grown man—requires a lot of sediment and such
extreme burial events are very rare in most habitats. Imagine that you had the
job of burying a dinosaur that was more than 130 feet long – it would take a lot
of sand (and a lot of hard work)! On the other hand, burying a small shell in
the sand would take almost no work at all.

--------------------------------------------------------------------------------

The massive femur of Patagotitan mayorum (compare with inset photograph of two
individuals collecting this bone). Image by Gastón Cuello - Own work, CC BY-SA
4.0, https://commons.wikimedia.org/w/index.php?curid=41267726.

FOSSILIZATION AND THE BODIES OF ORGANISMS

The bodies of most (but not all) organisms are made from a variety of different
types of tissues. These range from soft tissues like those that make up skin and
internal organs to hardened materials like bones and shells that provide an
animal with either internal (skeleton) or external (shell) support. Hardened
tissues—often referred to informally as “hard parts”—are frequently mineralized,
meaning that they are composed of a particular kind of mineral. For example,
many snails and clams make their shells out of the minerals calcite and
aragonite. Likewise, the bones in your body are mostly comprised of the mineral
calcium phosphate. Hard parts are not always mineralized, however. Consider
wood, for example, which is not mineralized when a tree is alive, but may later
become petrified (turned to stone) during the fossilization process.

The fossil halls of natural history museums are primarily made up of displays of
bones and shells, reflecting the fact that the fossil record is comprised mostly
of the mineralized hard parts of organisms. The minerals that make up bones and
shells and other hard parts are usually highly resistant to biological decay and
physical weathering: they break down at a much slower rate than soft tissues,
which often provide food and nutrients to predators, scavengers, and
decomposers. Because they are already made of geologically resistant minerals,
hard parts have a much higher likelihood of making it to the burial stage of the
fossilization process. Once buried, these mineralized structures have a very
good chance of becoming fossilized, at least relative to soft parts. Because of
this, the fossil record is biased in favor of documenting organisms with hard
parts.

--------------------------------------------------------------------------------

This specimen--from the Eocene Green River Formation--shows the fossilized
skeleton of a larger fish (right) that apparently died while eating a smaller
fish (left). Despite the exquisite preservation of fine skeletal features, no
soft parts are preserved. Specimen on exhibit at Fossil Butte National Monument,
Wyoming. Photograph by Jonathan R. Hendricks.  This work is licensed under a
Creative Commons Attribution-ShareAlike 4.0 International License.

--------------------------------------------------------------------------------

The majority of species alive today have no hard parts, however, and this was
almost certainly the case in the past as well. For example, insects are the most
biodiverse group of animals alive today (i.e., they are the group with the most
species), but since they do not have hard parts, their fossil record is poor
relative to groups like snails and clams that have shells.

--------------------------------------------------------------------------------

The bodies of insects like this butterfly--along with those of many other kinds
of animals--lack mineralized hard parts, resulting in a comparatively poor
fossil record. Photograph by Jonathan R. Hendricks.  This work is licensed under
a Creative Commons Attribution-ShareAlike 4.0 International License.

--------------------------------------------------------------------------------

This is not to say, however, that soft parts are never preserved as fossils. In
rare circumstances, soft parts are preserved and fossil collecting localities
with this type of preservation (termed lagerstätten, a German word meaning
“storage place”) are of great interest to paleontologists. This is because they
provide important windows on ancient biodiversity and the body forms of ancient
species (beyond the typically preserved hard parts). The most famous lagerstätte
is the Burgess Shale of British Columbia, Canada, which shines a light on
weird-looking species from the beginning of animal life; an excellent overview
of this deposit was developed by the Royal Ontario Museum. Similarly aged
fossils from Utah may be viewed on the Utah’s Cambrian Life webpage. A listing
of important lagerstätten from other regions and time intervals can be found on
Wikipedia.

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A specimen of the soft-bodied arthropod Emeraldella from the Cambrian Marjum
Formation of Utah (KUMIP 204791). The red spots along the middle of the specimen
are organs. Image from the Utah's Cambrian Life Webpage.

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Why are soft parts sometimes preserved in such lagerstätten? The answer is
difficult because the circumstances and styles of preservation of soft parts
vary from place to place, but a key factor seems to be very rapid burial in
sediments with very little oxygen present; such environments are referred to as
anoxic (meaning “without oxygen”). The low amount of oxygen is important because
it slows the decomposition of soft parts by bacteria, favoring the preservation
of the soft parts as thin carbon films or allowing the soft parts to be replaced
with geologically stable minerals like pyrite.

Next Section: Types of fossil preservation

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Previous Section: Body fossils and trace fossils


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The Digital Atlas of Ancient Life project is managed by the Paleontological
Research Institution, Ithaca, New York.

Development of this project was supported by the National Science Foundation.
Any opinions, findings, and conclusions or recommendations expressed in this
material are those of the author(s) and do not necessarily reflect the views of
the National Science Foundation.



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