intro to fossils

1. Fossilization
BY SALAHUDIN KHURSHEED

2. Fossils
The term “fossil” is used for any trace of past life. Fossils
are not only the actual remains of organisms, such as teeth,
bones, shell, and leaves (body fossils), but also the results of
their activity, such as burrows and foot prints (trace fossils),
and organic compounds they produce by biochemical
processes (chemical fossils). Occasionally, inorganically
produced structures may be confused with traces of life,
such as dendrites. These are called pseudofossils. The
definitions below explain the types of fossils found in the
context of fossilization processes. You will find there is some
overlaps in the terminology commonly used in paleontology
and geology.

3. Body Fossils
The processes of fossilization are complex with many stages from
burial to discovery as a fossil. Organisms with hard parts such as a
mineralized shell, like a trilobite or ammonite, are much more likely to
become fossilized than animals with only soft parts such as a jellyfish
or worms. Body fossils of plants and animals almost always consist
only of the skeletonized or toughened parts because soft tissues are
destroyed by decay or by scavengers. Even hard parts can be
destroyed by natural processes such as wave action or can be eaten or
destroyed by other organisms like fungi and algae. Many species of
plant and animal fossils are known only from their fragments.

4. The remains of an organism that survive natural biological and physical
processes must then become quickly buried by sediments. The probability
for an organism to become fossilized increases if it already lives in the
sediment , and those on the sea floor are more readily fossilized than those
floating or swimming above it. Catastrophic burial with a rapid influx of
sediment is necessary to preserve delicate complete animals such as
crinoids or starfish. This explains why most crinoids, for example, are found
only as stem pieces. Since crinoids were not usually buried quickly, their
hard stem parts are far more frequently found as fossils. Observations of
rare living crinoids have shown that they will rapidly disarticulate within a
few days of death. Rapid burial, in contrast, prevents this disintegration, and
thus explains a few localities where beds of delicate crinoids, starfish and
brittle stars are preserved in their entirety. Thus many factors affect
of chances for fossilization.

5. The common processes occurring after burial include chemical alteration or
replacement and compaction. Most marine invertebrates have calcareous
skeletons containing calcium carbonate (CaCo3) that occurs in one on two
crystal forms, calcite or aragonite. Aragonite is comparatively unstable and
will convert to calcite or dissolve over time. As a result, aragonite becomes
progressively rarer in older rocks. If the calcite or aragonite is dissolved away
the result is a fossil being preserved as a mold or cast. In contrast, the
original calcite or aragonite might be replaced with other minerals such as
silica or pyrite or a similar iron-containing mineral called hematite. Calcium
phosphate is another important, but less common, skeletal material occurring
in some arthropods, inarticulate brachiopods and conodonts. Apatite, a
calcium phosphate mineral, is also found in bones and teeth of vertebrates.
These are the most common replacement minerals other than calcite

6. Soft Tissue Fossilization
It is possible to infer a certain amount about the missing soft parts of fossils
by comparing them to living relatives. Information can be so deduced from
traces such as muscle scars left on a skeleton for example. The preservation
of soft parts is rare but scattered examples are found throughout the world
at various localities. Examples of soft part fossil preservation include the
frozen Siberian mammoths and ground sloth fur and feces. Preservation in
this case is dependent on local climatic conditions and such fossils are
unlikely to survive any significant amount of geological time since climate
changes. Older soft bodied preservation due to protection from decay and
scavenging under anaerobic conditions (without oxygen) especially at low
temperatures rarely occurs. Decay is slowed allowing more time for soft parts
to be buried and preserved. Despite the rarity, there are hundreds of fossil
sites worldwide where soft tissue parts are preserved. Such a fossil site is
known as a Lagerstätte. Examples include the early Cambrian Burgess
Shale of Canada and Maotianshan Shales of China, the Carboniferous Mazon
Creek Formation, and the Eocene Messel Pit of Germany.

7. Simple Burial
Limy shells and plant remains often lie in the ground without much change.
Cones, stems, stumps, and fern roots in peat bogs have been known to exist
up to 40 million years with little change, except for some discoloration and
slight decay. The remarkable preservation in these peat bogs is due to the
high concentration of tannic acid. Mollusk shells, sand dollars and sea urchins
with ages ranging from a few thousand years to 75 million years have been
known to survive with little change, except the loss of color. Occasionally,
ammonite fossils show the original iridescence present when they were alive.
The clam fossil of the Spanish Point Formation of California is a good example
of shells that have undergone little change.

8. .
Imprints
Imprints are simply the external
molds of very thin organisms,
such as leaves and trilobites.
They are often found in rocks
such as sandstone, shale and
volcanic ash. Trilobites of the
Marjum Formation in Utah are
often found as impressions

9. Trace Fossils or Ichnofossils
Trace fossils, also called ichnofossils are structures preserved in sedimentary rocks that
record biological activity. Though trace fossils are often less interesting to view, they are very
important because they represent both the anatomy of the maker in some way as well as its
behavior. Sedimentary structures made by empty shells rolling along the sea floor are thus
not trace fossils because they do not represent the anatomy of their maker. Trace fossils
include footprints, tracks and trail marks, burrows, borings, feeding marks, and coprolites
(fossilized droppings). The conditions under which animal remains are found differ from those
favoring the survival of trace fossils they produce. The two are rarely found together. It is
often difficult to determine what animals made a trace fossil with confidence. Traces made by
wildly different animals can be very similar in appearance. Therefore trace fossils are
classified according to the activity producing them rather than the animal that made them:
resting, crawling, feeding, dwelling, etc. The majority of these trace fossils were made by
infaunal (living in sediment) animals, especially deposit feeders like worms. Worm trails in
Cambrian sediments are common. Bird tracks at some locations in the Green River Shales of
Wyoming and Utah are also common.

10. Trails, Tracks and Burrows
Tracks, trails and burrows are a particular form of trace fossil. These
traces range from the worm trails to dinosaur tracks and even the
footprints of Stone Age people. The tracks of worms, amphibians,
reptiles and birds are common at some localities. A great variety of
invertebrate’s tracks have been found. Trilobite and even insects tracks
are found commonly at some localities. Burrows of worms, snails and
crabs are known as well as their petrified remains. Worm trails are
often found in the Cambrian Wheeler Shale of Utah. Bird tracks are
common in the Green River Formation of Utah in some locations

11. Freezing
Freezing is a type of preservation in which an animal falls
into a crevasse or pit and remains frozen. Such ideal
remains are rare and almost always never very old. Animals
have been restricted to ice age rhinoceros and hairy
mammoth. These remains have preserved bone, skin,
muscle, hair and even internal organs.

12. Drying or Dessication
Remains of animals that have been found thoroughly dried
include camel, ground sloth and even marsupial wolf. These
remains were found in caves in arid and semi-arid areas of the
Southwestern United States, South America, New Zealand and
Australia. The dried dung of cave dwelling giant ground sloths
have also been found in caves.

13. Petrification
Petrification is a geology term denoting the processes by which
organic material is converted into stone or a similar substance.
It is approximately synonymous with fossilization. Petrified
wood is the most well known result of this process. Petrification
takes place in two related
ways, replacement and permineralization, described below.

14. Replacement
Replacement takes place when water dissolves the original hard parts and
replaces them with mineral matter. This chemical action may take place
slowly, reproducing the microscopic structures of the original organism. Bone,
shells and wood are often well preserved in this manner. The most common
replacement minerals are calcite, silica, pyrite and hematite. The snails of the
Green River Formation in Wyoming are often replaced by silica, a variation of
quartz. The ammonites and goniatites of Europe and North Africa are
commonly replaced by hematite, which is an iron mineral similar to, but more
stable than pyrite. When the original hard parts are replaced quickly they
often loose all trace of their original structure, leaving the original shape, but
no detail. Agatized woods are often preserved in this manner, (agate is a
form of quartz)

15. Perimineralization
Permineralization takes place when ground water carrying dissolved minerals
infiltrates the microscopic pores and cavities in bone, wood or shell. The
minerals being deposited produce stony fossils that still contain a good deal
of their original solid material. Bones, teeth and many marine organisms are
preserved in this way. The fossil wood from the Petrified Forest of Arizona are
a famous example of this type of preservation. The fossil teeth and bones of
the Oligocene badlands of South Dakota and Nebraska are also common
example of this type of fossilization, as well as the extensive deposits of
Jurassic dinosaur bones in Utah and Colorado

16. Molds and Casts
An organism will lie in sediment until the surrounding sediment becomes firm. Later the
organism dissolves away. If there is no infilling of the cavity with mineral, sand or clay this is
called a natural mold. The outside of the mold, which would have been the outer surface of
the animal, is referred to as an external mold. This often has the fine detail of the surface of
the original organism. The inside surface of the mold is referred to as the internal mold,
(sometimes miscalled casts). The internal cast forms when sand or clay fills such things as
empty shells of snails and clams, which are common. If the cavity is filled with grains of sand
or clay, duplicating the original inner surface of the organism, this is referred to as a cast.
The Procheloniceras ammonite fossils of the coastal Sahara Desert in Morocco are a fine
examples of external and internal molds; however, since the ammonites shell is gone, local
artisans often fake these ammonites by carving them out of rock. The inside molds of
turritella snails are a common example an internal mold. Many ammonites are found with the
animals original shell dissolved away, leaving only the internal mold (also see body
fossils above)