Evolution and the Fossil Record

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					Evolution and the Fossil
  The Cambrian and Beyond
The nature of the fossil record

    How organic remains fossilize
      Four categories of fossils

•    defined by method of formation
1.   Compression and impression fossils
2.   Permineralized fossils
3.   Casts and Molds
4.   Unaltered Remains
Compression and Impression
• Made when organic material is buried in
  water or wind-borne sediment before it
• The weight of the sediment causes the
  structure to leave an impression in the
  material it is resting on
• Analogous to footprints in mud or leaves
  in wet concrete
• Fig 17.1
       Permineralized fossils

• Form when structures are buried in
  sediments and dissolved minerals
  precipitate in the cells
• Can preserve details of internal structure
• Fig 17.2
          Casts and molds
• Molds are unfilled spaces left behind as
  organic material decays or dissolves away
• Casts are made when the molds are filled
  in with new material which then hardens
  into rock
• Provide information about external and
  internal surfaces.
• Fig 17.3
         Unaltered remains
• mummified remains that are protected
  from weathering, animals and
  decomposition by bacteria and fungi
• Found in peat bogs, permafrost very dry
  desiccating environments (dessert caves).
  Preserved in plant resins (amber) Fig 17.4
• Saturated tar sands
         Trace Fossils

• Basically these are signs left behind by
  living organisms rather than parts of the
  organisms themselves
• Includes tracks, burrows, fecal material
• Can be used to get a general idea of the
  type of life in some areas
Features of Objects Which
• Durable
• Buried before or shortly after death (usually in
  water-saturated sediment)
• Located in areas devoid of oxygen
• Therefore……
  Most fossils are of hard materials left in areas
  of deposition such as river deltas, flood plains,
  marshes, beaches, ocean bottoms and river
• There is an abundant fossil record of organisms
  that normally burrow in sediments, such as
Strengths and Weaknesses of
       the fossil record

•   Bias - a potential weakness
•   3 types of sampling bias

• Most fossils come from lowland and
  marine habitats where the conditions for
  fossilization are most prevalent
• Marine fossils dominate the fossil record
  but only 10% of extant species are marine
• 2/3 of extant animal species have no hard
  parts which would lend themselves to
  being easily fossilized
• Critical parts of plants, like flowers, are
  seldom fossilized

• Old rocks are more rare than new rocks
  because when tectonic plates subduct
  or mountains erode they take their
  fossils with them
• Therefore our sampling of ancient life
  forms is poor
 Biases must be accounted for

• Therefore….
• Paleontologists need to be aware of
  limitations in what the fossil record can
  tell us
• We need to remember that bias is not,
  however, unique to paleontology
• There are many other areas of research
  which are biased
• can work with only a few model systems
  which by no means represent all living
• Examples are roundworms, fruit flies, and
  zebra fish for animals
• E. coli and Saccharomyce cerevisieae are
  models that are used for molecular and
  cell biology
• Ecology focuses on the upland havitats in
  North America and Europe.
The Geologic time scale

  a look at life through time
      Geologic time scale
• Is divided into Eons, Eras, Periods, Epochs,
  and Stages
• First formulated as a relative time scale in the
  early 1800’s
• Absolute times were added later as more
  accurate dating techniques were developed
• The time scale is constantly being refined as
  more rocks are sampled and dating
  techniques get more sophisticated
Please become familiar with the Phanerozoic Eras periods as
shown below.

                          Cenozoic Era        Quaternary (1.8 mya to today)
                          (65 mya to today)      Holocene (11,000 years to today)
                                                 Pleistocene (1.8 mya to 11,000 yrs)
                                              Tertiary (65 to 1.8 mya)
                                                 Pliocene (5 to 1.8 mya)
                                                 Miocene (23 to 5 mya)
                                                 Oligocene (38 to 23 mya)
                                                 Eocene (54 to 38 mya)
                                                 Paleocene (65 to 54 mya)

     Phanerozoic Eon      Mesozoic Era        Cretaceous (146 to 65 mya)
   (544 mya to present)   (245 to 65 mya)     Jurassic (208 to 146 mya)
                                              Triassic (245 to 208 mya)

                          Paleozoic Era       Permian (286 to 245 mya)
                          (544 to 245 mya)    Carboniferous (360 to 286 mya)
                                                 Pennsylvanian (325 to 286 mya)
                                                 Mississippian (360 to 325 mya)
                                              Devonian (410 to 360 mya)
                                              Silurian (440 to 410 mya)
                                              Ordovician (505 to 440 mya)
                                              Cambrian (544 to 505 mya)
                                                  Tommotian (530 to 527 mya)

       Entire timeline
 The Cambrian “Explosion”

• Called such because almost all of the
  currently recognized animal phyla first make
  their appearance in the fossil record in the
• The Cambrian spanned “just” 40 million years
• When the fossil record is scrutinized closely,
  it turns out that the fastest growth in the
  number of major new animal groups took
  place during the Tommotian
    Important fossil records



         EDIACARAN Fauna
• South Australia
• first fossil evidence of multicellular animals
• Pre-Cambrian - 565 mya, late Proterozoic
• mostly compression and impression
• entirely soft-bodied examples, sponges,
  jellyfish etc
• many are trace fossils
           BURGESS SHALE
• Slightly younger than Ediacaran shale, 520-515
• British Columbia
• Primarily impression and compression
• Have extraordinary detail
• Wide variety of arthropods (including trilobites),
  segmented worms, molluscs, several
  chordates, including jawless vertebrates
  Not much overlap between the two
except for a few Cnidarians (sea pens)

• Therefore it appears
  from these important
  fossil records that
  there was an
  “explosion” of animals
  in the Cambrian.

•   From Yunnan Province in China
•   veryimportant area
•   recently made accessible again
•   very rich in fossils
•   Found Zygotes and blastocyst that
    indicate bilateral symmetry
Was there really a Cambrian
       “Explosion” ?
• Using molecular clock data from DNA
  and protein sequences estimates have
  been made on the order of branching in
  the animal phylogeny Fig 17.12
           Cambrian Explosion
• Estimates show that the
  earliest branches
  occurred somewhere
  between 1200 and 900
• This is hundreds of
  millions of years before
  they are found in any
  fossil record
• This is a highly
  controversial area and
  implies a long history of
  animal evolution for which
  we have no fossil record
   Evidence From Proterozoic
• If these projections are correct we should
  eventually find fossils of these animals in
  the Proterozoic rock
• Some jawless fishes (vertebrates) have
  been found in China in the Chengjiang
  fauna that are 530 million years old
• This would be indirect evidence that
  chordates arose much earlier than this
• The Cambrian explosion is an explosion of
  morphological forms but not necessarily of
• The evolution of these lineages may have been
  occurring gradually during the Proterozoic but
  existed as small and larva-like organisms which
  left no fossils
• However, there is still no explanation for the
  dramatic changes in body size in the brief period
  of the Cambrian where these fossils are found
  What caused the Cambrian

• Changes in the ecology of the earth most
  likely led to these changes.
• Organisms were filling new niches due to
  changes in
  – FROM: predominantly either sessile (attached) filter
    feeding organisms or those floating high in the water
    column living off of plankton
  – TO: to a huge variety of feeding mechanisms
  – FROM:Sessile or free floating organisms
  – TO: swimmers, walking, burrowing, both benthic and
    pelagic predators, scavengers and on and on
  What Factors Led to These
• Locomotion changes?
  – Rising O2 levels 
  – allowed larger body size 
  – allows evolution of tissues and higher metabolic rates
    needed for powered movement
• Shells formation?
  – Probably as a result of predator selection pressure
  – Have found shells that have holes drilled by predators
  – Evidence from the types of holes drilled that predators
    were selecting their prey by size.
What other ecological interactions may
  have led to selection pressures?

• New types of food such as diversification
  in the plankton, may have favored novel
  feeding mechanisms
• Anatomy that favors swimming or grasping
  (for example) may have been favored as a
  way to obtain prey
   All of these changes require

• genetic variation to be present

• Would require changes in the genes that
  control embryonic development
  Macroevolutionary patterns

• An important part of evolutionary research
  is looking for broad patterns in the fossil
• Can give insight into how macroevolution
  may occur
• A common pattern seen in the fossil
  record is Adaptive Radiation
         Adaptive Radiation
• A single ancestral species diversifies into
  a large number of species which occupy a
  wide variety of ecological niches
• Where have we seen and talked about
  examples of adaptive radiation?
• Darwin’s Finches
• Hawaiian drosophilids
  Factors That Trigger Radiation of
• What factors were
  responsible for the
  radiation in the finches?
• ecological opportunity
   – Colonized a habitat that
     had few competitors and
     wide variety of resourcese
• Leads to morphological
  innovations like the beak
      Ecological Opportunities
• Ecological opportunity
  is not created solely
  through colonization

• Mass extinction
  – Mammals diversified
    rapidly after the
    dinosaurs became
           Adaptive radiation
• Morphological innovations lead to
• Example: arthropods ( insects, spiders,
  –   inhabit a wide variety of niches based on
      modification of their jointed limbs
  –    swimming, flying, running, jumping,
      grasping, walking
        Examples from plants

• as plants moved from
  aquatic to terrestrial
  habitat in the early
  Devonian ( 400mya)
• developed leaves and
  vascular tissue
Explosion of flowering plants in
       the Cretaceous

  • The flower structure allowed a rapid
    expansion into new niches
  • Pollination strategies
  • including co-evolution with insects
  • dispersal mechanisms for seeds
Stasis vs. Gradualism
• The Darwinian approach
• In this pattern organisms are continually
  changing gradually form one from to
• Occurs by a progressive accumulation of
  micromutations which leads to the
  formation of a new species.

• New morphologies appear in the fossil record
  and then remain unchanged for millions of
• Often, evolutionary innovations appear at the
  same time as new species
• This results in morphological evolution that
  consists of long periods of no change (stasis)
  occasionally punctuated by speciation events
  that appear instantly in the geologic record
   Gradual changes are rarely
   seen in the geologic record

• What do you suppose Darwin would say
  when confronted with today’s fossil
• He predicted that gaps in the fossil record
  would be filled in over time, with gradual
           WHY STASIS
• Possibly a lack of genetic variation to work
  – There is strong evidence that lack of genetic
    variation is not the cause of stasis
  – One living fossil, the horseshoe crab does not
    have any less genetic variation than groups
    that have evolved significantly
• May be in dynamic stasis. Think of the
  finches and how they change with drought
  vs. flood years. there is an oscillation back
  and forth, fluctuating about a mean, but in
Theory of Punctuated Equilibrium

 • Proposed in 1972 by Eldredge and Gould (we
   will be reading this paper later.)
 • Led to some very heated debates for over 20
 • Debate revolved around differences in
   observing patterns of speciation and change on
   a biological time scale of years or decades vs.
   a geological time scale of millions of years
 • On a biological time scale gradual change and
   natural selection are important (and

•   The ultimate fate for all species
•   Several clear patterns of extinction
•   Global extinction rates are not constant
•   Two basic categories of extinctions
    – Mass extinctions
    – Background extinctions
    To be a mass extinction

1. A broad range of organisms being
2. Global extinction
3. Rapid relative to the expected life span
   of the taxa that are lost
4. A mass extinction leads to the loss of
   over 60% of the species in a period of
   a million years
          “ The Big Five”

•    During the Phanerozoic there have been
     5 mass extinctions
•    Together these account for 4% of all
1.   At the terminal Ordovician 440 mya
2.   Late Devonian 365 mya
3.   End-Permian 250 mya
4.   end Triassic 215 mya
5.   Cretaceous-Tertiary (K-T) 65 mya
      Background Extinctions
• occurred at constant rates
• make up 96% of all extinctions
• The likelihood of subclades becoming extinct is constant
  and independent on how long the taxa have been in
   – The probability of a subgroup becoming extinct is
     constant over the lifespan of the larger clade
   – Rates of extinction are constant within clades but
     highly variable across clades
• The extinction rate of marine organisms vary depending
  on how far the larvae disperse after the egg is fertilized
   – Greater distance leads to greater colonizing ability which might
     reduce extinction rate
           The K-T Extinction
•   Cause is purported to be impact from a
    huge meteorite
    1. iridium sediments
    2. unusual elements
       1. shocked quartz particles
       2. microtektites
Iridium sediments laid down
     at the K-T boundary

• Iridium is rare on earth but abundant in
  extra-terrestrial objects
• The amount of iridium found from 95
  different samples taken in various K-T
  boundary sites indicates a likely 10 km
  wide meteorite impact
Two unusual elements found
  in K-T boundary layers

 1. shocked quartz particles
      have only been found at the site of
      meteorite impact craters
      quartz grains that have parallel planes
      called lamellae. The deformation is
      thought to be due to the shock of impact,
      thus “ shocked quartz”
  Unusual elements (cont)

2. Microtektites
  – tiny glass particles which may be
    composed of a variety of source rock
    types but all of which originate as
    grains melted by the heat of impact
  – May be melted in place or ejected by
    the impact
  – If ejected will take on a teardrop or
    dumbbell shape as a result of
    solidifying in flight
     Locating the crater
• abundant shocked quartz and
  microtektites were found in Haiti and
  throughout the rest of the Caribbean
• Then in early 1900s evidence from
  magnetic and gravitational anomalies
  confirmed the existence of a crater in the
  Yucatan peninsula. Fig 17.26
• The impact of the meteorite is now
  accepted universally but the actual
  consequences of the impact are still in
what are the possible consequences of
           such an impact?
1. Vaporization of anhydrite and
   seawater cause an influx of SO2 and
   water vapor to the atmosphere
  – this leads to acid rain
  – and scatter of solar radiation which could
    cause global cooling
2. Dust-sized particles in the atmosphere
   could compound the cooling as well,
   by blocking incoming solar radiation
What are the possible consequences of
        such an impact (cont)

3. Widespread wildfires are suggested by
   soot deposits at many K-T sites
4. The soot could increase smog and
   increase cooling
5. Massive earthquakes may have been
6. Volcanoes
  – If volcanoes put ash and sulfur dioxide into
    the atmosphere could have caused global
    cooling while increase in CO2 may have led
What are the possible consequences of
        such an impact (cont)

7. There is much evidence that there was an
   enormous tidal wave or tsunami caused by
   the impact. Perhaps as high as 4 km
   • Sandstone deposits in several areas and
   along a 300km strip are interpreted as typical
   of what a tsunami might do
   • Evidence that an initial splash of
   microtektites formed a layer which was then
   covered by tsunami-induced deposits and
   then a final layer of iridium enriched
   particulates that settled down out of the
The decline of many groups of
     organisms was not
 Many extinctions were probably caused by
 interactions between organisms and the
 traumatized environment.
 •   specifically due to disruptions in …
 •   ecological processes
 •   species interactions
 •   biogeochemical cycles

• primary productivity of phytoplankton would
  be dramatically reduced
• Local temperature and chemical gradients in
  the water would be disrupted
• This would lead to the decimation of marine
  and terrestrial biota in the time frame
  immediately after the impact
• The decline of many was not immediate and
  was drawn out over 500,000 years
Some important characteristics of the
         K_T extinction

 • 60% TO 80% of all species became
   extinct at the end of the Cretaceous
 • Losses were not distributed evenly
 • Dinosaurs and pterosaurs were wiped
 • Large bodied mammals disappeared
 • Only one order of birds, a shorebird,
 • Amphibians, crocodilians, mammals,
 Characteristics of K-T extinction(cont)

• Insects virtually unscathed
• Some marine invertebrates were
• marine plankton became very scarce
• In North America 35% of land plants were
• Forest communities were replaced by
Differential survival has not yet
 been adequately explained.
 • One interesting pattern is emerging
 • losses are much more severe in North
   America Perhaps because these species
   were in the splash zone when heated
   material was sent to the north and west
   from the impact site
 • One outstanding pattern shows that for
   bivalves and gastropods, genera with wide
   geographic ranges were less likely to get
   wiped out than those with narrower ranges
originally thought to be the impressions of annelid worms
 (earthworms), is now interpreted as the feeding traces of
Feed trails
Dwelling traces