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Geology 1023 – Lab #6, Winter 2014
Introduction to fossils and fossilisation
What is a fossil?
A fossil is any evidence of ancient life preserved in sediments or rocks.
Why are fossils important?
Fossils are important for several different reasons:
1.
2.
3.
4.
5.
They are used to trace the evolution of life from its beginnings to the present time.
They are used to determine the (relative) age of sedimentary rocks.
They are used to correlate sedimentary rocks.
They indicate environment of deposition of the rocks in which they occur.
They can be the main constituent of rock (limestones).
Kinds of fossils
There are trace and body fossils. A trace fossil is evidence of life activity such as tracks, trails,
burrows, borings, coprolites, gastroliths, etc. A body fossil tells us something of the shape
(morphology) of the organism itself (usually shows only a part of the organism). A body fossil
may be preserved unchanged, or “replaced” to varying degrees, or as a mold or cast. A mold is
an impression of a body part (reversed symmetry to original organism). A cast is formed when a
mold is filled by new material (same symmetry as original). Organic tissue (“soft parts”) is rarely
preserved because of microbial decay, predation, scavengers, and oxidation. Most body fossils
are “hard parts” – altered shells, teeth, bones, scales, etc.
Fossilisation
The likelihood of fossilisation is increased by:
1.
2.
3.
4.
Presence of “hard parts”. Likelihood increased if the organism moults its skeleton.
Abundance of individuals (influencing factors: size, geographic and temporal range).
Rapid burial following death (i.e., burial protects the organism).
Life (and death) in a depositional environment.
The above criteria mean that small, shelly, marine invertebrates have a much greater likelihood
of fossilisation than do terrestrial or airborne organisms. Also, life originated in the oceans and
only recently (in geological terms) moved onto the land and into the air, which also biases the
fossil record towards marine organisms.
Introduction to fossils & fossilization — Winter 2014
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Types of preservation
Organisms are preserved in a variety of ways.
1
2
3
4
5
6
7
8
Mode of preservation
Unaltered (almost) remains
a) soft parts
Example
b) hard parts
Carbonisation
distillation from organic tissues, most
commonly in plant material
Replacement
substitution of organic material (soft tissue
and hard parts) by inorganic chemicals not
normally part of the organism
Permineralisation
precipitation within pores and cavities
Recrystallisation
crystal growth without change in
composition
Solution
inorganic dissolution of material leaving a
cavity with the shape of the fossil (mold)
Casting
mold fills with material to form a replica of
the original organism (or part of organism)
Trace fossils
mammoths frozen in tundra, mummification,
pickled or “bog” people, organisms in tar pits
shells, bones, teeth, scales
coal, black leaf impressions in shales,
graptolites, graphitic worm in shale.
silicification (now SiO2)
pyritisation (now FeS2)
“solid” corals
“petrified” wood
difficult to determine, particularly in hand
sample, probably less common than was
once thought
shell molds are most common
shell casts
log casts
trace fossil casts
tracks, trails, footprints, borings, etc.
Naming and classifying fossils (taxonomy)
All organisms (living and fossil) are classified using the “Linnean” hierarchical system shown
below. Four examples are given to show evolutionary relatedness. You can see that humans are
more closely related to chimps than they are to wolves than they are to dinosaurs.
Kingdom
Phylum
Class
Order
Family
Genus
Species
Common name
Example 1
Animalia
Chordata
Reptilia
Saurischia
Tyrannosauridae
Tyrannosaurus
Tyrannosaurus
rex
“T. rex”
Example 2
Animalia
Chordata
Mammalia
Carnivora
Canidae
Canis
Canis lupus
Wolf
Example 3
Animalia
Chordata
Mammalia
Primates
Pongidae
Pan
Pan troglodytes
troglodytes
Chimpanzee
Example 4
Animalia
Chordata
Mammalia
Primates
Hominidae
Homo
Homo sapiens
sapiens
Human
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Note that when giving the specific (i.e., species) name of an organism, the generic name is often
abbreviated to its initial letter, e.g., T. rex or H. sapiens sapiens.
Paleoecology
The paleoecology of a set of fossils is the environment in which the organisms lived as well as
their relationships to each other. We can interpret some of this from the lithology (e.g., shale
implies relatively quiet conditions). We can determine the ecology by understanding the
morphology (shape) of the organisms, by the species associations, and by comparing fossil forms
with living organisms. Fossils can help determine such features as the nature of the substrate, the
clarity, turbulence, salinity, and temperature of the water as well as sedimentation rate. E.g.,
fossil corals (like modern ones) preferred clear, shallow, warm, water of normal salinity.
Organisms that live in the water column are pelagic and are either planktic (float/drift) or nektic
(actively swim). Bottom dwellers are benthic. Benthic organisms that are attached to their
substrate (encrusted, cemented, rooted) or that bore into it are sessile. If they just recline on the
substrate they are sedentary. If they crawl on or burrow actively into the substrate they are
mobile. Epifauna live on the seafloor, infauna live beneath it.
Most of the fossil record consists of benthic marine invertebrate organisms that filtered food
particles from the water (filter, or suspension feeders) or ingested sediment and extracted the
food from it (detritus feeders).
Biostratigraphy
Individual species tend to evolve and die out in geologically short time spans (a few millions of
years at most). And once they’ve died out they NEVER came back. So fossils tell us age (at least
in relative terms) of the rocks that enclose them.
An index fossil is one that has a wide geographic distribution and a short time range. Good index
fossils are those that are readily identifiable (even by non-experts) and abundant. Most fossils,
however, have a relatively wide age-range and time zones are usually established using the
overlapping time ranges of two or more species.
1. Look at the taxonomic (naming classification) table above.
a) Are both the generic and specific names of an organism capitalised?
Y
N
b) Are both the generic and specific names underlined?
Y
N
c) Tyrannosaurus is to genus as Tyrannosaurus rex is to
d) Tyrannosaurus is to genus as
saurischia
species
is to order.
Introduction to fossils & fossilization — Winter 2014
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2. Circle all the correct choice(s).
a) Each order contains one or more:
classes
families
genera
kingdoms
orders
phyla
species
kingdoms
orders
phyla
species
order
phylum
species
order
phylum
species
b) Each phylum contains one or more:
classes
families
genera
c) A particular genus will belong to a particular:
class
family
genus
kingdom
d) A particular phylum will belong to a particular:
class
family
genus
kingdom
3. Rank the following organisms by numbering them 1 to 4 in order of increasing likelihood of
preservation in the fossil record (# 1 is least, #4 is most likely).
a) jellyfish
1/2
3/2
b) fox
c) plankton
4*
fish
3
clam
4*
human
2/1
moth
1/2
spider
2/3
coral
4*
T. Rex
2/3
eagle
1
mammoth 3/2
4. Rank the following environments (locations) in order of increasing likelihood of preserving
fossils.
a) alluvial fan 1*
reef
3/2
beach
2/3
lagoon
4
A wide range of fossils is on display at the back of the lab (collection #1). Examine them before
and during the following exercises to help you answer the questions.
5. Determine the type of preservation in collection #2 (specimens 227–235) by comparing with
the samples in collection #1. Note that two sets of collection #2 are provided.
#227
unaltered
#228
unaltered
#229
carbonized
#230
replacement
#231
replacement
#232
#233
molds
#234
permineralized
unaltered
cast
#235
trace
Introduction to fossils & fossilization — Winter 2014
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6. Collection #3 (specimens 244–247) is a set of fossils from the Horton Bluff Formation
(HBF), collection #4 (specimens 248–255) is from the Windsor Group (WG).
a) What is the likely environment of deposition of each (marine/non-marine)?
HBF
non-marine
marine
WG
b) Why?
HBF
Land plant fossils
Marine fossils (shells)
WG
The following exercise uses a modern analogy to help you think about fossil assemblages. Fossil
assemblages in a rock can be thought of as photographs. If you know enough about the things in
the photo you can tell when it was taken. Similarly if you know enough about the various fossil
species in a rock you can tell how old the rock is.
“A”
“B”
“C”
Post ’99
1999
1998
1997
1996
1995
1994
1993
1992
1991
1990
1989
1988
1987
1986
Pre ‘86
7. The diagram below show a “photograph” taken of a particular location. Here are some facts.
1. Tree “A” (of unknown age) was cut down in 1994. 2. Person “B” is a member of a family
that lived in the area between 1987 and 1998. 3. Building “C” was built in 1991 (and is still
standing). Fill in the following chart by putting “x” in each year possible for each item.
X X X X X X X X X X
X X X X X X X X X X X X
X X X X X X X X X X
Introduction to fossils & fossilization — Winter 2014
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91–94
a) In what time period was the “photo” taken?
b) What item or items “bracket” the possible time best? A and C (tree, house)
Note there is a period of several years when the photo could have been taken. That does not
mean that the photo took that length of time to take. This is an important point of comparison
with rocks. A Devonian (for example) fossil age for a rock does not mean that the rock took
the entire Devonian to be deposited. The photo represents an instant within its potential range
and the rock represents a geological “instant” sometime within its potential depositional time.
Now let’s try some fossils.
8. Examine the fossils (from the fossil set in the drawers) listed in the table below.
X
Quat.
X
Tert.
X
Cret.
X
X
Juras.
X
Trias.
Bellerophon
F19
Perm.
Penn.
X
F10
Dev.
X
Mucrospirifer
Sil.
Favosites
X
X X
F2
Ord
Miss.
Genus
Camb.
a) Using the identification sheets, identify each genus and enter it in the following table.
b) Using the identification sheets enter the age range of each genus in the above table by
marking an “X” in the appropriate boxes.
c) Is there an excellent index fossil in this assemblage?
d) Which genus cannot be used as an index fossil?
e) Why?
yes
Bellerophon or Favosites
Too long time range
f) Assuming that these specimens came from a single rock unit (Layer A), what genus or
genera constrain the age of that rock unit best?
F2, Mucrospirifer
g) What would be the age of that rock layer “A”?
h) Is this a marine or non-marine assemblage?
Devonian
marine
Introduction to fossils & fossilization — Winter 2014
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9. Examine the fossils (from the fossil set in the drawers) listed in the table below.
F14
Pentremites
F21
Girtyocoelia
Quat.
X
X
Tert.
X
X
Cret.
X
X
X
X
X
X
Juras.
Trias.
Archimedes
Perm.
F7
Penn.
Chonetes
Miss.
F3
Dev.
Athyris
Sil.
F1
Ord
Genus
Camb.
a) Use the identification sheets to identify each genus and enter the genus name and age
range in the table.
X X X
X X
X X
b) Assuming that these specimens came from a single rock unit (Layer “C”), what genus or
genera constrain the age of layer “C” best?
F21, Girtyocoelia and F14, Pentremites
c) What is the age of this assemblage?
Pennsylvanian
d) Is this a marine or non-marine assemblage?
marine
10. Given the fossil age of Layer “A” (from Q. 8, above) and the fossil age of layer “C” (from Q.
9, above). What is the possible age of a layer “B” that is sandwiched between layers “A” and
“C? It will help to enter all the ages (age ranges) in the spaces below.
Layer “C” (Q. 9)
Layer “B”
Layer “A” (Q. 8)
Pennsylvanian
Devonian – Pennsylvanian
Devonian