LIFE on EARTH: What Do Fossils Reveal?

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LIFE on EARTH What Do Fossils Reveal?

• CHAPTERS 3 and 7

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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.

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Fossils

• Fossils include both
• The remains of organisms.
• The traces of organisms.

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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.

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Trace Fossils

• Indications or traces of organic activity
  including tracks, trails, burrows, and nests are
  called trace fossils.

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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.

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How Do Fossils Form?

• To become preserved as a fossil, an
  organism typically must
• Have preservable parts
• Hard parts (bones, shells, teeth, wood)
• Only rare preservation of soft parts (muscle,
  skin, internal organs)
• Be buried rapidly by sediment
• Protects the organism from decay
• Escape physical, chemical, and biological
  destruction after burial
• 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.

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Types of Fossil Preservation

1. Preservation of Unaltered Hard Parts
2. Chemical Alteration of Hard Parts
3. Imprints of Hard Parts in Sediment
4. Preservation of Unaltered Soft Parts
5. Trace Fossils or Ichnofossils

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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
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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.

Permineralized bone in which the porous marrow
cavities have been filled with mineral matter.
Unaltered bone with porous marrow cavities.
www.geol.umd.edu/.../lectures/104fossils.html
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Chemical Alteration of Hard Parts Petrification
by Replacement
Replacement Molecule-by-molecule substitution
of another mineral of different composition for
the original material.
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.
www.stat.wisc.edu/.../Fossils/fossils.html
www.geol.umd.edu/jmerck/ASTR380/evolution.html
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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
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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
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Amminoid Casts
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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

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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.
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Preservation of Soft Parts by Amber
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Preservation of Soft Parts by Tar
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Preservation of Soft Parts by Peat Bogs
Preserved torso, arms, and head of the
2000-year-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.
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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 Wasp Victim of a ash showers from the
Colorado volcano eruption about 30 mya (Fossil
Beds National Monument, Florissant, Colorado).
Fossil Seed Fern from rocks of Pennsylvanian age
(300 mya).
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Preservation of Soft Parts by Carbonized Imprints
Fossilized fish with fleshy parts USGS (public
domain)
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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

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Dinosaur Tracks, Morrison Formation
Dinosaur Footprint in Limestone
Burrows probably made by worms
Trails in Red Triassic Siltstone, Virginia
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Trace Fossils

• A land-dwelling beaver, Paleocastor, made this
  spiral burrow in Nebraska.

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Trace Fossils or Ichnofossils

• Trace fossils provide information about
• Ancient Water Depths
• Paleocurrents
• Availability of Food
• Sediment Deposition Rates
• Tracks can provide information about
• Diet
• Foot Structure
• Number of Legs
• Leg Length
• Speed
• Herding Behavior
• Reproductive Behavior
• Interactions

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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.

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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?
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Fossils Indicate Past Environments
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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.

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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.

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Index Fossils

• Index Fossils (or guide fossils) are useful in
  identifying time-rock units and in correlation.
• Characteristics of an Index Fossil
• Abundant
• Easily Identified
• Widely Distributed (cosmopolitan)
• Short Geologic Range (rapid evolution or
  extinction rates)

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Use of Fossils in Reconstructing Ancient
Geography

• Environmental limitations that control the
  distribution of modern plants and animals
  include
• 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

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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

• 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

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Marine Ecosystem Planktonic/Nektonic

• Where and how animals and plants live in the
  marine ecosystem

Floaters Plankton Jelly Fish
Swimmers (Nekton) Fish Cephalopod
Sessile Epiflora Seaweed
Sessile Epifauna Bivalve
Benthos d-k
Crinoid
Coral
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Marine Ecosystem Benthos
Infauna Worm Bivalve
Mobile Epifauna Gastropod Starfish
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Ancient Marine Environment
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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.

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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.

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Interdependence of Photosynthesis and Respiration
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 near-surface waters.
• Clarity of the Water (or conversely, the amount
  of suspended sediment in the water).

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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.

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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)

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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.
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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.

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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.

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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.

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Species Diversity and GeographyLand 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.

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Use of Fossils in the Interpretation of Ancient
Climatic Conditions

• Fossils can be used to interpret paleoclimates
  (ancient climates)
• Fossil spore and pollen grains can tell about the
  types of plants that lived, which is an
  indication of the paleoclimate.
• Plant fossils showing aerial roots, lack of
  yearly rings, and large wood cell structure
  indicate tropical climates.
• Presence of corals indicates tropical climates.

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Use of Fossils in the Interpretation of Ancient
Climatic Conditions

1. Marine molluscs (clams, snails, etc.) with spines
   and thick shells inhabit warm seas.
2. Planktonic organisms vary in size and coiling
   direction with temperature, for example the
   foraminifer Globorotalia.
3. Compositions of the skeletons, for example
   shells, have higher magnesium contents in warmer
   waters.
4. 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.

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Fossils and Stratigraphy
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• 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.

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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.)
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.

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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
Assilina-Heterostegina concurrent range zone.
Biostratigraphic Zones
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Figure 6-30 (p. 140)Use of geologic ranges of
fossils to identify time-rock units.

• 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.

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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).

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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.
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Paleogeographic Maps

• Constructing a Paleogeographic Map
• For the selected area, collect all available data
  that show the occurrence of the selected
  time-rock unit.
• 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.

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Paleogeographic Maps

• Constructing a Paleogeographic Map
• 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 non-marine environments.
• Plot the environments to produce a
  paleogeographic map for that time interval.

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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) 1341-1354.)