Fossils and their preservation

1
Fossils and their preservation
2
Why are people fascinated with fossils ?

• A few reasons
• 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 Earths biosphere long
  before we (individually, and as a species)
  existed.
• 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.
• 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.

3
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.
4
Why do fossils look the way they do ?
Obviously, a fossils 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.
5
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! For instance, take a close look at road
kill next summer! 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)
6
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. 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.
Corpse bacteria
Physical and chemical Weathering, and erosion
7
Such remains (particularly cattle skulls) are
commonly encountered in the American southwest,
and have become icons of this area, strongly
influencing native and non-native art of the
region. Various skulls collected form the New
Mexico desert were featured in several paintings
by Georgia OKeeffe.
Cows Skull with Calico Roses (1932)
Georgia OKeeffe (1887-1986)
8
These, however, are not fossils! Their age (lt500
years) and lack of burial (they havent 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 what
is normally considered to constitute
fossilization.
9
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.
10
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 hardparts 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
hardparts via dissolution of minerals in
hardparts. 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.
11
In some cases, the orientations of fossil remains
can indicate aspects of the environment in which
they were deposited.
Current-aligned shells of the nautiloid
Orthoceras (indicates unidirectional current)
Convex-up bivalve shells with bidirectional
orientation of umbos (bidirectional currents
related to wave action)
12
Factors That Favour Fossilization

• Special conditions are therefore required to
  preserve dead tissues (most organisms have lt 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
• Rapid burial/entombment This isolates remains
  from the work of scavengers and long-term
  physical disturbance.
• 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.

13
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. Si
lica 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 well-preserved). Principal mineral
component of bones, teeth and some shells.
14
Post-Burial Processes Modes of Preservation
15
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.
16
Unaltered/ Actual Remains
Hardpart 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.
17
Although less stable than calcite, aragonite is
occasionally preserved (at least over shorter
periods of geological time). 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
Hardpart Preservation
18
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!
19
Unaltered/Actual Remains
Soft Tissue Preservation Amber Entombment
scorpion
ant
20
Unaltered/Actual Remains
Soft Tissue Preservation Tar Immersion/Impregnati
on
Beetle encased in solidified tar
21
Altered Remains

• More often than not, fossil remains are
  physically and/or chemically altered in some way.
  
• Four main types of alteration processes are
• Recrystallization
• Petrification/Permineralization
• Replacement
• Carbonization

22
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 Earths
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.
23
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
24
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.
25
Replacement Silicification (replacement by
silica)
Fossil brachiopod (calcite replaced by
microcrystalline quartz).
Fossil calcareous sponge (originally calcitic,
now siliceous)
26
Replacement Pyritization (replacement by pyrite)
Original calcitic brachiopod
Pyritized brachiopod
27
Replacement Phosphatization (replacement by
calcium phosphate minerals)
shark vertebrae (originally cartilage (organic),
now phosphatic)
28
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.
29
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. The characteristics (variety) of coal
varies based upon the amount of cooking and
degassing that has occurred.
Carbonization
Carbonized fern leaves and coal
30
Moulds and Casts
31
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.
External mould
Internal mould
Note the original shell of this ammonite has
completely dissolved away, but its former
presence is indicated by external and internal
moulds.
32
Moulds
1. Sediment surrounding the shell and filling the
shell cavity hardens
Fossil clam
Clam
Internal mould
2.Shell is dissolved
External mould
Snail
Fossil snail
1. Sediment surrounding the shell and filling the
shell cavity hardens
Fossil snail
External mould
2.Shell is dissolved
Internal mould
33
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
34
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).
35
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.
36
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 arent 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.
37
Common Fossil Behaviours
walking
resting
escape
Fossil Behaviour
grazing
farming
dwelling
feeding
38
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
ploughing
walking
39
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.
40
Behaviours leading to the formation of different
ichnogenera (as produced by the same organism)
41
Trace fossils that dont fit the mould (no pun
intended).
Fossils which qualify as trace fossils based on
the definition of the term, but which dont
correspond to the normal notion of trace fossils,
and are not named or classified in the same
manner, include the following.
42
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?)
Ancient stromatolites
Modern stromatolites
43
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.
Fish coprolites
Dinosaur coprolite
44
END OF LECTURE