FOSSILS AND FOSSILIZATION
FOSSIL:
remains or traces of an ancient organism (e.g. bones, teeth, shells, casts, tracks). The
likelihood of fossilization is a frequency of only .001 (1 in every 1,000 organisms leave
behind any fossil evidence).
 among the reasons for this low probability are the many changes that occur
over time, after an animal or a human dies: weathering, scavengers moving or
gnawing on bones, trampling by hoofed animals, movement along fault lines,
movement along streams or rivers, etc.
 all of these events scatter, crush and decompose bones, decreasing the
likelihood of fossilization.
FOSSILIZATION:
the process in which minerals from the surrounding sediment replace the organic
materials (calcium, collagen and other proteins) in the bone – “bone turns to stone”.
FACTORS THAT CONTRIBUTE TO FOSSILIZATION:
 mineral content: teeth, bones and shells are the most common fossils (soft
tissues, which are low in minerals, are very rarely preserved in the fossil record).
 low humidity: dry/arid or very cold climates are ideal for fossil preservation
(high humidity, like that found in the tropics, causes decomposition to occur
quickly, and soil is often very acidic)
 how quickly corpse is buried: the probability of fossilization increases greatly
when the body settles in stagnant water, like that of a lake (less bacterial decay,
low acidic levels, lack of currents, etc. minimizes movement of bones)
WHAT CAN FOSSILS TELL US? A FEW EXAMPLES:
 fossilized plant parts: (e.g. fossilized pollen granules or seeds, casts of plant
parts, etc.) may indicate the type of habitat or climate (e.g. the modern
relatives of many of the plant fossils found at a site in Messel, Germany grow
only in tropical regions today, indicating that Messel must have been tropical
rain forest habitat some 50 million years ago)
 fossilized teeth: shape and size indicates the type of diet the animal had; may
help to reconstruct the environment in which it lived (e.g. large, flat molars
indicate an herbivorous diet and may indicate the types of plants present in the
habitat); in mammals, the eruption patterns of deciduous (baby) teeth and
permanent (adult) teeth indicate the animal’s approximate age, etc.
 fossilized bone: muscle attachments can be seen on the surface of bone (may
indicate possible shape, size and function of the animal’s muscles); unusual
features may link ancient animal to modern animals (e.g. the “bulb” in the ear
region of the skull of Pakicetus linked it to today’s whales); relative size of eye
sockets, nasal cavities, etc. can indicate which senses were most important to
the animal when it was alive.
PROBLEMS WITH INTERPRETING INFO. FROM THE FOSSIL RECORD:
 sampling error: the fossil record is not complete; more fossils might be found
in one area versus another, meaning that some organisms are better
represented than others; because of differential preservation, the frequency of
fossils doesn’t necessarily reflect the size of living populations (e.g. birds are not
well represented in the fossil record, most likely due to fragile bones, not
because there were less birds in the past; extinct whales and dinosaurs are very
well represented in the fossil record due to their large, dense bones).
 species must sometimes be defined on the basis of one specimen: since
probabilities of fossilization are so low, numerous specimens of one species are
not always recovered; sometimes one must represent a whole species
 variation within one species may be misinterpreted: a group of fossils
may be interpreted as representing one species/population by one scientist, yet
another scientist may interpret slight morphological differences among the
specimens as representing two or more species. Who is right?
DATING METHODS:
There are various dating methods, most of which involve measuring the rate of decay
in a given element. Certain elements are known to be unstable, or radioactive. This
means they will decay, or deteriorate, at a steady rate into another type of element.
Paleontologists and paleoanthropologists normally send the specimen to a dating lab,
where chemists test the item.
 Potassium-argon dating: used most often to date hominids; can provide a
reliable date going back several billions of years.
- based on the radioactive decay of potassium 40 into a stable argon gas (a
type of gas that accumulates within certain minerals).
- as time goes on, the amount of potassium 40 decreases while the amount of
argon gas increases.
- Argon increases at a steady rate; scientists test for the amount of argon
present, and work backwards to come up with a date (i.e. applying
mathematical formulas)
- the drawback is this method cannot be applied directly to bones, or other
organic materials, only volcanic ash (i.e. when a bone is found in a habitat
where there were once active volcanoes ); any bones found in the same layer
as the volcanic ash, are as old as the ash
 Radiocarbon dating: carbon is found in all organic materials (e.g. bone, teeth,
shells, charcoal, seeds, etc.)
- carbon 14 decays at a steady rate into nitrogen 14
- the drawback of this method is that it can only provide a reliable date to
approximately 50,000 years ago (i.e. this method cannot be applied to most
hominid fossils)
 Other methods exist, that measure the decay of different elements or
changes in amino acids. Potassium-argon and radiocarbon dating are among
the most precise, and most commonly used, dating methods.
 Chemists often use more than one method to establish a date in time for
a specimen, to ensure accuracy.