Petroleum Geology Course
By: Charles Dekori
Palaeontology/Dept/Geology/UofJ/sem/4
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Course Outline
• Part One:
General Palaeontological
Concepts
 Principles of Palaeontology,
 Evolution and fossil Records,
 Major Events in the Life’s
History,
• Part Two:
Invertebrate Phyla
 Sponges,
 Cnidarians,
 Bryozaons,





Brachiopods,
Molluscs,
Echinoderms,
Graptolites and
Arthopods
Part Three (optional):
Application of Palaeontological
knowledge in solving some
exploration, Exploitation and
Production problem.
Palaeontology/Dept/Geology/UofJ/sem/4
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Introduction to Paleontology
Paleontology or palaeontology is the study of the history of life on Earth as
based on fossils. Fossils are the remains of plants, animals, fungi, bacteria, and
single-celled living things that have been replaced by rock material or
impressions of organisms preserved in rock. Paleontological observations have
been documented as far back as the 5th century BC. The science became
established in the 18th century as a result of Georges Cuvier's work on
comparative anatomy, and developed rapidly in the 19th century. The term
itself originates from Greek παλαιός, palaios, i.e. "old, ancient", ὄν, on (gen.
ontos), i.e. "being, creature" and λόγος, logos, i.e. "speech, thought, study".[1]
Palaeontology/Dept/Geology/UofJ/sem/4
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Why are people fascinated with fossils ?
A few reasons
1. Fossils provide our only tangible link with lifeforms of the distant
past and are our only direct means of envisioning and
understanding the appearance and nature of Earth’s biosphere long
before we (individually, and as a species) existed.
2. They remind us that all species (even the greatest and most
successful organisms) are ephemeral. Consequently, they make us
realize how insignificant we really are in the big picture.
3. Fossils are beautiful objects in their own right and are highly sought
after by collectors of various types for various reasons. Usually the
fossils that are preserved to the greatest extent (i.e. those that are
least altered from their original condition) are those that carry the
greatest value (aesthetic and monetary), though these are not
necessarily of the highest value to science.
What exactly are fossils?
“Fossil” (Latin prefix “Foss” refers to digging/excavation) in its most
general sense refers to any preserved evidence of ancient
biological organisms in the rock record that is obtained through
digging/extraction from the host rock.
Three general types:
1) Body fossils: These represent the actual physical remains of
ancient organisms as preserved in the rock record.
2) Trace fossils: These represent the activities of ancient
organisms as preserved in the rock record. Usually these reflect
interactions between the organisms and the sediment in/on which
they lived, crawled, fed etc.
3) Chemical fossils: Organisms may secrete chemical compounds
(biomarkers), preservable in the rock record, which are unique to
particular group and therefore may be used as evidence of the
existence of that group in the corresponding ancient environment.
Why do fossils look the way they do ?
Obviously, a fossil’s appearance will be primarily dictated by the shape and
structure of the living organism which produced it. In body fossils this will relate
most particularly to the characteristics of the preservable (hard) parts. For
example, this coiled shell has a chambered interior that was used as a
buoyancy control device by the animal that produced it.
Fossil nautiloid
Modern Nautilus
The appearance of a fossil, however, is also governed by processes which
occurred after its maker died.
It is important to realize that only a tiny proportion of the total number of living
organism to have lived on the Earth have become fossils. Even organisms
which have ‘high preservation potential’ are only scarcely represented in the
fossil record compared to the total number of individuals of these species which
existed in ancient ecosystems. Thus, fossils are typically also a representation
of particular post-mortem conditions which were conducive to preservation.
Information loss in the fossil record
After an organism dies, its tissues are
commonly subjected to destructive
processes occurring through several
means, and at several scales.
The average person can see these
processes in action!
At the largest scale, scavenging
organisms descend upon the carcass
to eat the choicest parts of the soft
tissues. The carcass may remain
largely intact or may be dismembered
and scattered over a large area.
Large scavengers
Small scavengers
(e.g. insect larvae; maggots)
Smaller scavengers (e.g. insect larvae)
focus on removing the remainder of the
soft tissues.
At the smallest scale, bacteria break
down the dead organic
matter at the molecular level.
Corpse bacteria
Once denuded of all soft tissues,
physical and chemical weathering
processes will begin to break down the
remaining hard, mineralized tissues (e.g.
shells, bone, teeth).
As chemical weathering is less of a
factor in dryer climates, bones may
remain intact for considerable lengths of
time (decades, centuries?) in deserts.
Physical and chemical
Weathering, and erosion
These, however, are not fossils!
Their age (<500 years) and lack of burial (they haven’t been dug
up!) generally rules them out as such.
Although there is no absolute limit in terms of the age of remains
considered to be fossils, it is generally assumed that the process
of fossilization (usually accompanied by lithification of the
sediment) requires, at minimum, thousands of years to complete.
One might argue, however, that fossils can be formed on much
shorter timescales through burial of organisms in volcanic ash
(e.g. Pompeii) and through immersion in sticky hydrocarbons (e.g.
the LaBrea Tar pits), and freezing (e.g. Siberian Wooly
Mammoths).
These modes of preservation are quite uncommon and therefore
may be regarded to constitute “grey areas” at the periphery of
As a general rule, hard parts have a greater chance of
preservation in the fossil record than do soft tissues.
This is because hard parts are more physically more
robust, more chemically stable and are less prone to
destruction via decay.
Major pre-burial processes affecting fossil preservation include:
Soft Tissue Decay: Destruction of soft parts (e.g. muscle, skin, nerve tissue)this also occurs in soft tissues (e.g. blood vessels) within hard parts such as
bone. These processes will occur rapidly wherever sufficient oxygen is present
for oxidizing aerobic bacteria to catalyze the decay process.
Disarticulation: Dissociation of hard parts (e.g. limb bones).
Fragmentation: Breakage and dissociation of fragments thus formed.
Dissolution: Breakdown of hard parts via dissolution of minerals in hard parts.
Abrasion: Erosion of hard tissues due to “sandblasting” effects of suspended
sediment particles.
Note: Some of these processes affect others.
For example, decay of organic components of hard parts commonly produces
acids and other corrosives which through dissolution, structurally weaken the
hard parts, rendering them more prone to fragmentation and abrasion.
Factors That Favour Fossilization
Special conditions are therefore required to preserve dead tissues (most
organisms have < 5 % chance of leaving any trace of their existence in
the fossil record).
Two of the most important factors that promote the preservation of remains
are:
1. Rapid burial/entombment – This isolates remains from the work of
scavengers and long-term physical disturbance.
2. Low oxygen – This also allows remains to be isolated from scavengers
(most of which need oxygen to survive) and slows down bacterial
decay. The precipitation of certain minerals that enhance preservation
also tends to occur in oxygen-free environments.
In many cases, rapid burial and low-oxygen conditions go hand-in-hand.
Hard Parts: Common mineral components
Calcium carbonate CaCO3 Principal mineral components of most seashells.
Two common calcareous minerals in seashells:
Aragonite – unstable over geological time.
Aragonitic shells commonly are dissolved (preserved as moulds) or
transform to calcite (poor preservation of primary textures).
Calcite – stable over geological time. Consequently calcitic (e.g.
brachiopod) shells tends to be well-preserved.
Silica SiO2 – Usually amorphous hydrated silica (Opal A) which commonly
transforms to quartz and other silica minerals following death.
Opal may be well preserved in pelagic sediments, if deeply buried.
This is the principal “mineral” component of the skeletons of some sponges,
and micro-organisms such as diatoms and radiolarians.
Skeletons may be lost or degraded through opal dissolution and obliteration
of the original amorphous structure through quartz crystallization.
Calcium phosphate (apatite)
Ca3(F.Cl.OH)(PO4)3 –stable over geological time (tends to be wellpreserved). Principal mineral component of bones, teeth and some shells.
Post-Burial Processes:
Modes of Preservation
Unaltered/Actual Remains
Skeletal remains that are composed of stable minerals
(e.g. calcite, calcium phosphate) can be preserved
without significant change in chemical makeup or internal
structure.
Under rare circumstances, soft tissues can also be
preserved without significant alteration.
Unaltered/ Actual Remains
Hard part Preservation
These brachiopod shells are made
of calcite. This is the same calcite
that was present in the shells when
the animals died.
Even though these shells are about
375 million years old, they have
undergone no significant change in
their chemical/mineralogical and
physical character.
Although less stable than calcite, aragonite is
Unaltered/ Actual Remains occasionally preserved (at least over shorter
periods of geological time).
Hard part Preservation
This ammonite (the shell of a squid-like
mollusc) preserves its original nacreous layer
(mother of pearl), composed of sheets of
aragonite crystals.
These fossils are only of Late Cretaceous
age (~70 million years) and therefore may still
be within the window of common aragonite
preservation, though other ammonites of this
age show incipient, or more advanced stages
of alteration to calcite. This suggests that
these well-preserved aragonitic fossils
represent depositional environments
conducive to long-term aragonite stability.
The oldest known aragonite in the rock record
is of Late Palaeozoic age (~350 Ma).
Unaltered/Actual Remains
Soft Tissue Preservation: Refrigeration
20,000 year old Siberian Wooly Mammoth preserving the original hair
and flesh. The flesh is so well preserved that it is still edible, and has
apparently been served to various people on several occasions in the
last century.
This is certainly a highly exceptional case of unaltered preservation!
Unaltered/Actual Remains
Soft Tissue Preservation: Amber Entombment
scorpion
ant
Unaltered/Actual Remains
Soft Tissue Preservation: Tar Immersion/Impregnation
Beetle encased in solidified tar
Altered Remains
More often than not, fossil remains are physically and/or
chemically altered in some way.
Four main types of alteration processes are
1.
2.
3.
4.
Recrystallization
Petrification/Permineralization
Replacement
Carbonization
Recrystallization
Although some hard parts can be preserved with minimal change, most
experience at least some degree of recrystallization after burial (crystals
tend to increase in size due to the higher temperatures encountered below
Earth’s surface).
With burial, and over time, the very fine crystals of the original skeleton tend to
increase in size, and/or change to more stable crystal morphologies.
Increased size of crystals
Due to the coarser overall texture of the skeleton, finer textural details
(e.g. ribs in this case) will be lost or obscured.
Petrification/Permineralization
This occurs when mineral matter (carried in solution) in percolating
ground waters precipitates in voids and pores in the remains of an
organism. This does not necessarily involve replacement of the original
skeletal material which may remain entombed in the new mineral matrix.
Common petrifying minerals: silica (quartz, chalcedony), calcite.
Petrified dinosaur bone
Petrified wood
Replacement
In some cases, organic matter or minerals of an
organism can be replaced by different mineral
substances. This replacement occurs at a microscopic
(molecule for molecule) level.
Depending on the chemistry of pore waters within
sediment, a number of minerals can replace the
original material. These transformations may occur at
earlier (before or during lithification), or later (after
lithification) stages of fossilization.
Calcareous (calcitic and aragonitic) shells are
commonly replaced by silica minerals (silicon dioxide),
pyrite (iron sulphide), or calcium phosphate minerals.
Replacement: Silicification
(replacement by silica)
Fossil brachiopod
(calcite replaced by
microcrystalline quartz).
Fossil calcareous sponge
(originally calcitic, now siliceous)
Replacement: Pyritization
(replacement by pyrite)
Pyritized brachiopod
Original calcitic brachiopod
Replacement: Phosphatization
(replacement by calcium phosphate minerals)
shark vertebrae
(originally cartilage (organic), now phosphatic)
Replacement: Phosphatization
(replacement by apatite)
As suggested by the previous
example, replacement is not
restricted to mineralized hard
parts.
In rare instances, relatively
durable organic tissues can
be replaced by minerals,
thereby enhancing their
preservation potential.
Phosphatized embryo of an arthropod
(crustacean?) in an egg capsule
over 500 million years old.
Carbonization
Carbonization refers to the process of
carbon enrichment of organic-rich
remains through their burial and heating.
Organic remains, when buried to
relatively shallow (few kms) depths, are
lightly heated due to higher temperatures
closer to the centre of the Earth.
During this low-grade cooking, the
volatile elements in the original organic
molecules such as oxygen, hydrogen,
and nitrogen are released as gasses,
while carbon (non-volatile) is left behind.
As a result, the remains are increasingly
enriched in carbon.
Coal is the typical product of this process.
Carbonized fern leaves and coal The characteristics (variety) of coal varies
based upon the amount of cooking and
degassing that has occurred.
Moulds and Casts
Moulds
When remains are buried, they are
surrounded with sediment.
The impression that the buried object
made in the surrounding sediment is
called an external mould.
(impression of shell exterior surface)
If the buried object is hollow, it can also
be infilled with sediment. The impression
of the interior of the buried object is called
an internal mould (also known as a
“steinkern”).
(impression of shell interior surface)
In many cases, the actual buried object
(in this case a shell) decays or is
dissolved, leaving only internal and
external moulds.
Internal
mould
External
mould
Note: the original shell of this
ammonite has completely
dissolved away, but its former
presence is indicated by
external and internal moulds.
Moulds
Clam
1. Sediment
Fossil clam
surrounding the shell
and filling the shell
cavity hardens
Internal mould
2.Shell is dissolved
External mould
Snail
Fossil snail
1. Sediment
surrounding the shell
and filling the shell
cavity hardens
2.Shell is dissolved
Fossil snail
External mould
Internal mould
Casts
Casts are formed when the void within an
exterior mould is filled in by siliciclastic
sediment or minerals precipitated from ground
water. The product of this infilling (a cast) will
consequently take the form of the original
remains.
In this case, the sediment surrounding a tree
trunk hardened sufficiently to hold its shape
after the tree trunk completely decayed.
Sediment was later washed into the resulting
hole and hardened, producing a natural cast.
In this case, the cast is made entirely of
sandstone (none of the original tree tissue has
been preserved).
Casts, primarily replicate only the shape of the
original remains, though textural impressions of
the exterior of the remains may preserved.
Cast of
Tree Trunk
Post-burial processes
Effects of tectonic stress:
The shape of fossils can become distorted due to differential pressure
associated with tectonic activity. Such changes are normally observed in low
grade regional metamorphic rocks (e.g. slates and low grade marbles).
Biological Classification of Fossils
As fossils clearly represent the the remains of ancient biological organisms, it
only makes sense that they should also be classified in the same manner as
living organisms.
The fundamental unit of biological classification is the species. Members of a
species are able to interbreed and give rise to fertile offspring.
Palaeontologists, lacking evidence of reproductive isolation of ancient
“species”, focus on morphological definitions of species. Above the species
level are increasingly more inclusive groups which are defined by certain
characteristics possessed by all their members. These various groupings are
as follows:
Kingdom: (e.g. Animalia)
Phylum: (e.g. Chordata)
Class: (e.g. Mammalia)
Order: (e.g. Primates)
Family: (e.g. Hominidae)
Genus (e.g. Homo)
Species: (e.g. Homo sapiens)
This classification heirarchy applies mainly to body fossils. As you will see,
trace fossils classification is more limited in this respect.
Body Fossils vs. Trace Fossils
Thus far, we have concentrated on the physical remains of
organisms. As these fossils represent remains of the actual body
tissues of ancient organisms, we call them body fossils.
As we already know, body fossils aren’t the only types of fossils
that we have to work with.
Palaeontologist also commonly concerned themselves with trace
fossils.
Trace fossils record the activities of ancient organisms.
Whereas body fossils tell us things about the anatomy of ancient
organisms, trace fossils provide evidence of their behaviour.
Common Fossil Behaviours
Fossil
Behaviour
grazing
Identification of Trace Fossils
Note: a single organism may produce multiple types of traces (e.g. trilobites
produce resting, ploughing (feeding), and walking traces, all of which look very
different). Similarly, different types of organisms may produce very similar
traces which reflect similar activities. Consequently, identification of the trace
maker is not easily established and is not the primary goal of trace fossil
nomenclature and classification.
resting
walking
ploughing
Naming Trace fossils
Trace fossils are named and classified under a different set of rules
from those governing body fossils, though the hierarchy of
classification still holds to the generic level and the species have
binomial names consisting of the generic name, followed by the
species name.
Using the example of Homo sapiens (clearly not a trace fossil
species!), Homo is the generic name (tells us the genus) and sapiens
is the specific name (tells us the species of that particular genus).
Trace fossils are also known as “Ichnofossils”; consequently, the prefix
“ichno” is used in reference to the two categories of classificaton, i.e
ichnogenus and ichnospecies.
The names of trace fossils reflect the morphology or other key
characteristics of the impressions themselves, and have no necessary
relation to the organism(s) believed to have produced them.
Behaviours leading to the formation of different ichnogenera
(as produced by the same organism)
Trace fossils that don’t fit the mould (no pun intended).
Fossils which qualify as trace fossils based on the
definition of the term, but which don’t correspond to
the normal notion of trace fossils, and are not named
or classified in the same manner, include the
following.
Stromatolites
Trace fossils formed by daily
sediment accretion and
cementation on successive
layers of bacterial mats.
Some bacterial may be
preserved within the layers.
(composite body-trace fossils?)
Modern stromatolites
Ancient stromatolites
Coprolites: Fossil excrement
Coprolites represent the solid end-products of digestion of food by
ancient organisms. Contrary to the normal procedure for trace
fossils, they are classified primarily on the basis of the organism
interpreted to have produced them. These may be very revealing
of the diet and food preferences of the producing organisms.
Dinosaur coprolite
Fish coprolites
Questions & Answers
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