Transitional Fossils

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Transitional Fossils

• Evidence for Evolution Seminar
• Talline Martins and Heidi Hillhouse
• 02-14-06

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Outline

• Introduction to transitional fossils
• Case studies
• Vascular plant evolution
• Tetrapod evolution
• Limitations
• Arguments against transitional fossils

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Defining the issue

• A transitional fossil may be defined as a fossil
  which exhibits traits common to both ancestral
  and derived groups. This is especially important
  when groups are sharply differentiated. (Freeman
  and Herron 2001)

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Defining the issue

• A transitional fossil may be defined as a fossil
  which exhibits traits common to both ancestral
  and derived groups. This is especially important
  when groups are sharply differentiated. (Freeman
  and Herron 2001)
• Transitional fossils are often used to test
  theories of species development. These theories
  include broad shifts in habitat use (aquatic to
  terrestrial organisms) or shifts in physical
  characteristics within a species or group of
  species (development of hard shells in
  trilobites).

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Dinosauria cladogram
http//www.dinosauria.com/pics/clados/clado.gif
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Examples of Transitional Fossils

• Cooksonia- emergence of vascular tissue in plants
• Kalbarria brimmellae-transition from myriapods to
  insects
• Ichthyostega- emergence of legs
• Archaeopteryx- early evolution of birds
• Pakicetus, Ambulocetus, Indocetus, Protocetus-
  land to water transition (whale)

Many fossils that bridge major transitional gaps
have been found
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Case study

• Evolution of vascular plants

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Land plant evolution

• Raven (1977) predicted that adaptations should
  have arisen in a specific order
• Alternation of generations, dispersal of spores
  by erect sporophyte
• Cuticle
• Xylem, later reinforced by lignin
• Intercellular gas spaces and epidermal pores
• Stomata with guard cells
• (as reported in Thomas 1984)

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Land plant evolution

• Spore development
• Combination of decay resistant spore walls
  (suggesting the presence of sporopollenin) and
  tetrahedral arrangement of spores (from haploid
  meiotic division) is considered diagnostic for
  land plants
• Spore fossils suggest the presence of land plants
  50 million years before the first unequivocal
  land plant megafossils
• (Kendrick and Crane 1997)

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Land plant evolution

• Xylem development
• Structure of early land plants was similar to
  that of some green algae, but the addition of
  tracheid-like tubes demonstrates vascular plant
  status.
• Example Differentially thickened walls in
  Cooksonia fossils supports status as early
  vascular plant (Edwards et al. 1992)

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Vascular plant evolution

• Two main branches of vascular plant development
• Clubmosses
• All other land plants

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Kendrick and Crane 1997
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Likely sister groups to land plants
Kenrick and Crane (1997)
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Vascular plant evolution

• Two main branches of vascular plant development
• Clubmosses
• All other land plants
• Transitional fossils exist for both branches
• For clubmosses Rhyniopsids, including
  Tortilicaulis
• For other land plants Cooksonia (actually
  several species, some more similar to clubmosses)

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Cooksonia pertonii apiculispora (Non-clubmoss
vascular plant lineage)
Tortilicaulis offaeus (clubmoss lineage)
Kendrick and Crane 1997
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Case study

• Evolution of Tetrapods

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Tetrapod evolution
Backboned animals with four limbs
legs
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How did it happen?

• Paleozoic Era late Devonian-early Carboniferous
  (365-310 mya)
• Crossopterygians (bony fish)
• Sarcopterygian fishes (lobe-finned)
• Coelacanth or lung fishes?
• Where?
• Shallow fresh water, tropical Euramerica

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Tetrapod evolution
?
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Some structures associated with tetrapod evolution

• Limbs and digits
• Ability to move on terrestrial environment
• Interlocking vertebrae/connecting ribs
• Provide support on landnecessary due to gravity
• Ear
• ability to hear on land
• Lungs
• Breathing

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Relationships among early tetrapods
Long and Gordon. 2004. Physiological and
Biochemical Zoology (77).
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Acanthostega (360 mya)
Found in 1952, eastern Greenland

• Limbs no ankles, paddle-like, 8 digits!
• Skeleton ribs too short to support weight out of
  the water
• Lungs present, internal gills

First fossil evidence that feet did not evolve
for walking!
Ahlberg et al. (2005) Nature
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Ichthyostega (363 mya)
Found in 1955, eastern Greenland

• Limbs strong enough to support itself, hind
  limbs were flipper-like, 7 digits!
• Skeleton able to support organs out of water,
  but no lateral movement
• Lungs present/gills

First fossil evidence for non-aquatic limb use!
Ahlberg et al. (2005) Nature
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Pederpes (354-344 mya)
Found in 1971, but only described in 2002,
western Scotland

• Limbs forward-facing hind feet,
  non-paddle-like, 5 digits!
• Skeleton resembles that of Ichthyostega
• Lungs present/gills

First fossil evidence for terrestrial locomotion!
Clack (2002) Nature
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Limitations of transitional fossils

• Some organisms dont fossilize well terrestrial
  animals, invertebrates
• Some strata do not produce many fossils
• Fossils are hard to find!

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Arguments against transitional fossils as
evidence for evolution

• Some arguments can be dealt with logically
• Transitional fossils are missing
• Response Only popular transitional fossils have
  gotten press (whale, Archaeopteryx). Others are
  not seen often beyond primary literature.
• Punctuated equilibrium is often misunderstood
  with scientists agreeing with the lack of
  transitional forms
• Response Need to educate the public about
  punctuated equilibrium

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Arguments against transitional fossils as
evidence for evolution

• Some arguments can be dealt with logically
• Transitional fossils are missing
• Response Only popular transitional fossils have
  gotten press (whale, Archaeopteryx). Others are
  not seen often beyond primary literature.
• Punctuated equilibrium is often misunderstood
  with scientists agreeing with the lack of
  transitional forms
• Response Need to educate the public about
  punctuated equilibrium

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Arguments against transitional fossils as
evidence for evolution

• Reconstructions are based often on incomplete
  fossils

Ahlberg et al. (2005) Nature
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Arguments against transitional fossils as
evidence for evolution

• Some arguments are dead ends
• The earth isnt really that old, therefore any
  dating arguments are obviously invalid
• Fossils were placed by the devil to lure us away
  from proper faith.

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Arguments against transitional fossils as
evidence for evolution

• Some arguments are dead ends
• The earth isnt really that old, therefore any
  dating arguments are obviously invalid
• Fossils were placed by the devil to lure us away
  from proper faith.

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Simpson tree of life
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References

• Freeman and Herron 2001. Evolutionary analysis
  2nd edition. Prentice-Hall, New Jersey.
• Kenrick, P. and P. Crane, 1997. The origin and
  early evolution of plants on land. Nature. 389
  33-39
• Raven, J. A. 1977. The evolution of vascular
  land plants in relation to supracellular
  transport processes. Pp. 153-219. In Woodhouse,
  H. W., ed. Advances in Botanical Research 5.
  Academic Press New York.
• Simpson based Tree of Life gi.cebitec.uni-bielefel
  d.de/people/boecker/
• Thomas, R. D. K. 1984. When and How did plants
  and animals take to the land? Paleobiology 10(1)
  1-8
• The Virtual Fossil Museum http//www.fossilmuseum.
  net/index.htm

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References (cont.)

• Ahlberg, P. E. and Milner, A. R. 1994. The origin
  and early diversification of tetrapods. Nature
  368 507-514.
• Ahlberg, P. E., Clack, J. A., and Blom, H. 2005.
  The axial skeleton of the Devonian tetrapod
  Ichthyostega. Nature 437137-140.
• Clack, J. A. 2002. An early tetrapod from
  Romers Gap. Nature 41872-76.
• Clack, J. A. et al. 2003. A uniquely specialized
  ear in a very early tetrapod. Nature 42565-69.
• Coates, M. I. And Clack, J. A. 1991. Fish-like
  gills and breathing in the earliest known
  tetrapod. Nature 352234-236.
• Long, J.A. and Gordon, M. S. 2004. The greatest
  step in vertebrate history a paleobiological
  review of the fish-tetrapod transition.
  Physiological and Biochemical Zoology
  77(5)700-719.

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A mechanical support/environment/structures
B probable locomotion
C respiratory structures
D water balance/osmoregulation
E sensory systems cutaneous/visual
F probable prey-capture and feeding mechanisms
G probable mode of reproduction