LIFE on EARTH:
What Do Fossils Reveal?
CHAPTERS 3 and 7
Fossils
• Fossils are the remains or traces of prehistoric
organisms which have been preserved by natural
causes in the Earth's crust.

Evidence of past life.
• Most common in sedimentary rocks.

Some in pyroclastic materials, especially ash.
• Useful for determining relative ages of strata.

Also environments of deposition.
• Fossils provide some of the evidence for organic
evolution.

Many fossils are of organisms now extinct.
Fossils
• Fossils include both:


The remains of
organisms.
The traces of
organisms.
Body Fossils
• Remains of organisms are called body fossils.

Mostly durable skeletal elements such as bones,
teeth and shells.

Rarely we might find entire animals
preserved by freezing or
mummification.
Trace Fossils
• Indications or traces of organic activity
including tracks, trails, burrows, and nests
are called trace fossils.
Fossil Record
• The fossil record is the record of ancient life
preserved as fossils in rocks.
• The fossil record is incomplete because of:




Bacterial Decay
Physical Processes (Weathering and Erosion)
Scavenging
Metamorphism
• In spite of this, fossils are quite common.
How Do Fossils Form?
To become preserved as a fossil,
an organism typically must:
1.
Have preservable parts:


2.
Hard parts (bones, shells, teeth, wood)
Only rare preservation of soft parts
(muscle, skin, internal organs)
Be buried rapidly by sediment:

3.
Protects the organism from decay
Escape physical, chemical, and
biological destruction after burial:

4.
Escape destruction by burrowing
(bioturbation), dissolution,
metamorphism, or erosion.
Live in a suitable environment:

Marine and transitional environments
with higher rates of sediment
deposition are more favorable for fossil
preservation.
Types of Fossil Preservation
1.
2.
3.
4.
5.
Preservation of Unaltered Hard Parts
Chemical Alteration of Hard Parts
Imprints of Hard Parts in Sediment
Preservation of Unaltered Soft Parts
Trace Fossils or Ichnofossils
Preservation of Unaltered Hard Parts
• Hard Parts may be preserved unaltered:


Shells of invertebrates and single-celled organisms
Vertebrate bones and teeth
www.clas.ufl.edu/.../gly3603c/fossils.html
paleo.cortland.edu/.../preservation.htm
Chemical Alteration of Hard Parts:
Petrification by Permineralization
• Permineralization: Filling of pores (tiny holes) in bone or
shell by the deposition of minerals from solution.
Unaltered bone with Permineralized bone in
which the porous
porous marrow
marrow cavities have
cavities.
been filled with mineral
matter.
www.geol.umd.edu/.../lectures/104fossils.html
Chemical Alteration of Hard Parts:
Petrification by Replacement
Replacement:
Molecule-by-molecule substitution
of another mineral of different
composition for the original
material.
www.stat.wisc.edu/.../F
ossils/fossils.html
www.geol.umd.edu/~j
merck/ASTR380/evolut
ion.html
The shell of an extinct
marine organism known
as an amminoid. In this
160-million-year-old
Jurassic fossil, the
original calcium
carbonate skeleton has
been completely
replaced with the
mineral pyrite.
Chemical Alteration of Hard Parts:
Petrification by Recrystallization
• Recrystallization:
• The original crystals forming
the shell undergo a change in
form and size, but the
composition remains
unchanged.
• Many modern shells are
made of aragonite, a
metastable form of calcium
carbonate (CaCO3) that will
alter or recrystallize to
calcite.
www.nasmus.co.za/PALAEO/jbotha/paleo101.htm
Imprints of Hard Parts in Sediment
Cast
• Impressions or molds are
the imprints of an
organism (or part of an
organism) in the sediment.


External Molds are imprints
of the outside of a shell.
Internal Molds are imprints
of the inside of the shell.
• A cast may be produced if
a mold is filled with
sediment or mineral
matter.
Mold
Amminoid Casts
Preservation of Soft Parts
• Soft parts may be preserved as fossils by:



Freezing (wooly mammoths in Siberian tundra)
Desiccation (drying or mummification)
Preservation in tree sap (amber – Preserves delicate
organisms and insects – like in Jurassic Park)


Preservation in tar (LaBrea tar pits)
Preservation in peat bogs (Lindow Man – England;
Tollund Man – Denmark)

Carbonized Imprints in fine-grained sediment
Preservation of Soft Parts by Freezing
Baby mammoth dug
from permafrost
(permanently frozen
soil) in northeast
Siberia. The
mammoth stood
about 3 feet tall at
the shoulders, was
covered with reddish
hair, and was
probably only
several months old.
Radiocarbon dating
indicates it died
44,000 years ago.
Preservation of Soft Parts by Amber
Preservation of Soft Parts by Tar
Preservation of Soft Parts by Peat Bogs
Preserved torso, arms,
and head of the 2000year-old Lindow Man.
This example of
preservation of soft
tissue was found in a peat
bog in 1984 at Lindow
Moss, England. The lower
half of the body was
destroyed by an
excavation machine.
Preservation of Soft Parts by
Carbonized Imprints
• Carbonization: Preserves soft tissues of plants or animals as a thin carbon film
after pressure from overlying sediments squeezes out the liquid and gases.
Usually in fine-grained sediments (shales). If the carbon is removed, a fossil
impression remains that replicates the surface of the organism.
Fossil Seed
Fern from
rocks of
Pennsylvanian
age (~300 mya).
Fossil Wasp – Victim of a ash
showers from the Colorado
volcano eruption about 30 mya
(Fossil Beds National Monument,
Florissant, Colorado).
Preservation of Soft Parts by
Carbonized Imprints
Fossilized fish with fleshy parts – USGS (public domain)
Trace Fossils or Ichnofossils
Markings in the sediment made by
the activities of organisms
• Tracks
• Trails
• Burrows –
in soft sediment
• Borings –
in hard material
• Root marks
•
•
•
•
Nests
Eggs
Coprolites
Bite marks
Dinosaur Tracks, Morrison Formation
Dinosaur Footprint in Limestone
Burrows probably made by worms
Trails in Red Triassic Siltstone, Virginia
Trace Fossils
• A land-dwelling
beaver, Paleocastor,
made this spiral
burrow in Nebraska.
Trace Fossils or Ichnofossils
• Trace fossils provide information about:




Ancient Water Depths
Paleocurrents
Availability of Food
Sediment Deposition Rates
• Tracks can provide information about:

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

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


Diet
Foot Structure
Number of Legs
Leg Length
Speed
Herding Behavior
Reproductive Behavior
Interactions
Trace Fossils
• A coprolite is a type of trace fossil consisting
of fossilized feces that may provide
information about the size and diet of the
animal that produced it.
• Fossilized feces (coprolite)
of a carnivorous
mammal.

Specimen measures about 5 cm
long and contains small
fragments of bones.
Figure 6-10 (p. 110)
Dinosaur trackways arranged
to indicate the passage of a
biped (identical three-toed
imprints) whose tracks were
crossed by a quadruped
having larger rear than front
feet (typical of many
quadruped dinosaurs). Claw
imprints on the biped suggest
that it was a predator.
What indicates the quadruped
crossed the area after the
biped?
Fossils Indicate
Past Environments
Use of Fossils in Reconstructing
Ancient Geography
•
Environmental limitations control the
distribution of modern plants and
animals:


Each species has a definite range of conditions
for living and breeding, and generally, it is not
found outside that range.
Ancient organisms had similar restrictions on
where they could survive.
Paleontologic Correlation
• Cosmopolitan Species are found almost
everywhere; they are not restricted to a
single geographic location in their
environment.

Cosmopolitan species have been especially
useful in establishing the contemporaneity of
strata (correlation).
• Endemic Species are confined to a restricted
area in the environment in which they live.

Endemic species are generally good indicators
of the environment where the strata were
deposited.
Index Fossils
•
Index Fossils (or guide fossils) are useful
in identifying time-rock units and in
correlation.
Characteristics of an Index Fossil:
1.
2.
3.
4.
Abundant
Easily Identified
Widely Distributed (cosmopolitan)
Short Geologic Range (rapid evolution or
extinction rates)
Use of Fossils in Reconstructing
Ancient Geography
•
Environmental limitations that control the
distribution of modern plants and animals
include:
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Marine Ecosystem Realm
Chemistry of the Water
Movement of Ocean Water
Water Temperature
Depth
Light
Carbonate Compensation Depth (CCD)
Land Bridges and Barriers (Mountains and Oceans)
Latitude
Marine
Ecosystem
• The ocean may
be divided into
two realms:


Pelagic Realm
– The water
mass lying
above the ocean
floor
Benthic Realm
– The bottom of
the sea
Classification of marine environments
(After Hedgspeth, UJ. W., ed. 1957. Treatise of Marine Ecology and
Paleoecology. Geological Society of America Memoirs 67(1): 18.)
Marine Ecosystem – Planktonic/Nektonic
• Where and how animals and plants live in the
marine ecosystem
Floaters:
Plankton
Jelly
Fish
Sessile Epiflora:
Swimmers (Nekton): Fish
Cephalopod
Seaweed
Sessile Epifauna:
Benthos: d-k
Bivalve
Coral
Crinoid
Marine Ecosystem – Benthos
Infauna:
Worm
Bivalve
Mobile Epifauna: Gastropod Starfish
Ancient Marine Environment
The Chemistry of Sea Water
• Salinity – A measure of the total dissolved solids in water

Salinity is measured in parts per thousand (ppth or o/oo) by weight
• Salinity terms for various types of water:
• Normal Ocean Water = 35 ppth or 3.5%

A salinity of 35 ppth means that there are 35 pounds of salt per
1000 pounds of sea water.
• Freshwater = about 5 ppth to less than 1 ppth.
• Brackish Water = sea water with less than about 30 ppth.
• Hypersaline Water = more than 250 ppth.

Typically in lakes in arid areas, or in enclosed areas like lagoons or
isolated seas in arid areas.
Water Temperature and Depth
• Water temperature varies with latitude
and depth:





Near the poles, water may be at or near
freezing.
Near the equator, it may be as much as 28 Cº.
Surface waters are generally the warmest,
because they are warmed by the Sun.
Temperature decreases with depth.
At great ocean depths, temperatures may be
just above freezing.
Light
• The well-illuminated
water near the surface
of the ocean is called
the photic zone.
• Light is used by certain
organisms in the water
for photosynthesis.
• Therefore,
photosynthetic
organisms are
restricted to the nearsurface waters.
• Clarity of the Water (or
conversely, the amount
of suspended sediment
in the water).
Interdependence of Photosynthesis
and Respiration
Carbonate Compensation Depth or CCD
• The Carbonate Compensation Depth or
CCD is a particular depth in the oceans,
which particles of calcium carbonate from
micro-organisms are dissolved as fast as
they descent through the water column.
• 4-5 km, varying from place to place
• The CCD affects where calcareous
sediments/oozes may or may not
accumulate.
Carbonate Compensation Depth or CCD
• Above the CCD, water is warmer, and precipitation
of CaCO3 is greater than dissolution.


Calcarous plankton can be found in the water column, and on the
bottom.
Bottom sediments can consist of calcareous sediments forming chalk or
limestone.
• Below the CCD, water is colder, and CaCO3 tends to
dissolve (dissolution is greater than precipitation).

Tiny shells of CaCO3 dissolve, and do not accumulate on the bottom if
water is deeper than the CCD.
• Below the CCD, the bottom sediments consist of:


Clay
Silica shells of plankton (diatoms, radiolarians)
Carbonate Compensation Depth (CCD):
(1) Calcium carbonate accumulates along parts of the midoceanic
ridge that are above the CCD.
(2) The accumulated layer is then carried away as the lithospheric
plates diverge from the ridge.
(3) When a given region of the seafloor has reached depths below
the CCD, calcium carbonate no longer is deposited, but clay and
siliceous remains of radiolaria and diatoms may accumulate.
Species Diversity and Geography
Species diversity is related to geographic
location, particularly latitude:
• High Latitudes:


Low species diversity.
Large numbers of
individuals.
• Low Latitudes:


High species diversity.
Fewer individuals
within each.
Species Diversity and Geography
As a general rule, species diversity
increases toward the equator:
• Likely because
relatively fewer
species can adapt
to the rigors of
polar climate.
Species Diversity and Geography
Warm areas place less stress on organisms and
provide opportunities for continuous
uninterrupted evolution, encouraging more
variety.
• The equator offers:



A stable input of
solar energy.
Less duress from
changing seasons.
More stable food
supply.
Species Diversity and Geography:
Land Bridges, Isolation, and Migration
Migration and dispersal patterns of land
animals can indicate the existence of:
• Former Land Bridges
(Bering Strait)
• Mountain Barriers
• Ocean Barriers
• Closing and
Opening of
Ocean Basins.
Use of Fossils in the Interpretation of
Ancient Climatic Conditions
Fossils can be used to interpret paleoclimates
(ancient climates):
1. Fossil spore and pollen grains can tell about the
types of plants that lived, which is an indication
of the paleoclimate.
2. Plant fossils showing aerial roots, lack of yearly
rings, and large wood cell structure indicate
tropical climates.
3. Presence of corals indicates tropical climates.
Use of Fossils in the Interpretation of
Ancient Climatic Conditions
4. Marine molluscs (clams, snails, etc.) with spines
and thick shells inhabit warm seas.
5. Planktonic organisms vary in size and coiling
direction with temperature, for example the
foraminifer Globorotalia.
6. Compositions of the skeletons, for example shells,
have higher magnesium contents in warmer
waters.
7. Oxygen isotope ratios in shells. Oxygen16
evaporates easier than Oxygen18 because it is
lighter. O16 falls as precipitation and gets locked
up in glaciers, leaving sea water enriched in O18
during glaciations. Shells that are enriched in O18
indicate times of glaciation.
Fossils and Stratigraphy
•
•
•
•
•
•
Principle of Fossil Succession:
Time periods and certain ROCK units can be
recognized by its fossil content.
Fossil species appear and disappear throughout
the stratigraphic record.
The Geologic Time Scale is based on these
appearances and disappearances.
Each of the Eras ends with a mass extinction.
Period boundaries coincide with smaller
extinction events, followed by appearances of
new species.
The Geologic Range
• Geologic Range –
The interval between the first
and last occurrence of a fossil
species in the geologic record.

Determined by recording the
occurrence of the fossils in
numerous stratigraphic
sequences from hundreds of
locations.
Example of a range chart showing the ranges of
late Cretaceous ammonite cephalopods
(chambered mollusks) from the Lopez de
Bertodano Formation, Seymour Island, Antarctic
Peninsula.
(From Macellari, E. E. 1986. J. Paleontol. Mem. 18, Part 2.)
Biostratigraphic Zones
Figure 6-34 (p. 143)
Geologic ranges of three genera
of foraminifera.
The interval between A and B is the
total range zone of Assilina.
The interval between X and Y
could be designated the AssilinaHeterostegina concurrent range
zone.
•
•
•
•
•
•
This diagram illustrates how
geologists use geologic
ranges of fossils to identify
time-rock units.
In Region 1, geologists
identify time-rock systems of
strats O, D, and M
(Ordovician, Devonian, and
Mississippian).
In Region 2, they identify
units O and D, and an older
unit C (Cambrian).
In Region 3, they locate unit S
(Silurian) between units O and
D.
They can put the information
together to come up with a
composite geologic column
and time-rock units C, O, S,
D, and M.
Note the geologic ranges of
the three fossils plotted
beside the composite section.
Figure 6-30 (p. 140)
Use of geologic ranges of fossils to identify time-rock units.
Limiting Factors on
Correlating with Fossils
• Appearances and disappearances of fossils
may indicate:




Evolution
Extinction
Changing environmental conditions that cause
organisms to migrate into or out of an area.
Reworked Fossils (weathering, erosion,
transport, re-lithification).
Use of Fossils in Reconstructing Ancient
Geography & Paleogeographic Maps
Paleogeographic Maps
are interpretive maps
which depict the
geography of an area
at some time in the
past, for example,
maps showing the
distribution of land
and sea in the past.
Paleogeographic Maps
• Constructing a
Paleogeographic Map:
1. For the selected area, collect all
available data that show the
occurrence of the selected timerock unit.
2.
3.
4.
Note locations of fossil species of the
same age on a map.
Interpret paleoenvironment for each
region using rock types, sedimentary
structures, and fossils.
Modern coral reefs occur in the tropics,
within 30o north and south of the
equator. Ancient coral reefs likely had
similar distributions.
Paleogeographic Maps
• Constructing a
Paleogeographic Map:
5.
Plot locations of non-marine (terrestrial)
deposits using locations of land-dwelling
organisms such as dinosaurs or
mastodons, fossilized tracks of land
animals, and fossils of land plants. It may
also be possible to recognize various nonmarine environments.
6. Plot the environments to produce a
paleogeographic map for that time
interval.
Figure 6-46 (p. 150)
Major land and sea
regions in North and
South America during
the Carboniferous
Period.
Fossils allow use to infer
the locations of ancient
seafloors, land areas, and
coastlines
(Adapted from Ross, C. A.
1967. J. Paleontol. 41(6): 13411354.)