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Transitional Fossils
Evidence for Evolution Seminar
Talline Martins and Heidi Hillhouse
02-14-06
Outline
Introduction to transitional fossils
Case studiesVascular plant evolution
Tetrapod evolution
Limitations
Arguments against transitional fossils
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)
Defining the issueA 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).
Dinosauria cladogram
http://www.dinosauria.com/pics/clados/clado.gif
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
Case study
Evolution of vascular plants
Land plant evolution
Raven (1977) predicted that adaptations should have arisen in a specific order:
1. Alternation of generations, dispersal of spores by erect sporophyte
2. Cuticle
3. Xylem, later reinforced by lignin
4. Intercellular gas spaces and epidermal pores
5. Stomata with guard cells(as reported in Thomas 1984)
Land plant evolution
Spore developmentCombination of decay resistant spore walls (suggesting the presence of sporopollenin) and tetrahedral arrangement of spores (from haploid meiotic division) is considered diagnostic for land plantsSpore fossils suggest the presence of land plants 50 million years before the first unequivocal land plant megafossils
(Kendrick and Crane 1997)
Land plant evolution
Xylem developmentStructure 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)
Vascular plant evolution
Two main branches of vascular plant development
Clubmosses
All other land plants
Kendrick and Crane 1997
Kenrick and Crane (1997)
Likely sister groups to land plants
Vascular plant evolution
Two main branches of vascular plant development
ClubmossesAll other land plants
Transitional fossils exist for both branchesFor clubmosses: Rhyniopsids, including Tortilicaulis For other land plants: Cooksonia (actually several species, some more similar to clubmosses)
Cooksonia pertonii apiculispora (Non-clubmoss vascular plant
lineage)
Tortilicaulis offaeus (clubmoss
lineage)
Kendrick and Crane 1997
Case study
Evolution of Tetrapods
Tetrapod evolutionra
y-fi
nned
fis
hes
lobe
-fin
ned
fish
es
amph
ibia
ns
rept
iles
bird
s
mam
mal
s
legs
Backboned animals with four limbs
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
Tetrapod evolution
?
Some structures associated with tetrapod evolution
Limbs and digitsAbility to move on terrestrial environment
Interlocking vertebrae/connecting ribsProvide support on land—necessary due to gravity
Ear ability to hear on land
LungsBreathing
Long and Gordon. 2004. Physiological and Biochemical Zoology (77).
Relationships among early tetrapods
Acanthostega (~360 mya)
Limbs: no ankles, paddle-like, 8 digits!
Skeleton: ribs too short to support weight out of the water
Lungs: present, internal gills
Found in 1952, eastern Greenland
First fossil evidence that feet did not evolve for walking!
Ahlberg et al. (2005) Nature
Ichthyostega (~363 mya)
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!
Found in 1955, eastern Greenland
Ahlberg et al. (2005) Nature
Pederpes (354-344 mya)
Limbs: forward-facing hind feet, non-paddle-like, 5 digits!
Skeleton: resembles that of Ichthyostega
Lungs: present/gills
Found in 1971, but only described in 2002, western Scotland
First fossil evidence for terrestrial locomotion!
Clack (2002) Nature
“Limitations” of transitional fossils
Some organisms don’t fossilize well: terrestrial animals, invertebrates
Some strata do not produce many fossils
Fossils are hard to find!
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
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
Reconstructions are based often on incomplete fossils
Arguments against transitional fossils as evidence for evolution
Ahlberg et al. (2005) Nature
Arguments against transitional fossils as evidence for evolution
Some arguments are dead ends:
The earth isn’t really that old, therefore any dating arguments are obviously invalid
Fossils were placed by the devil to lure us away from proper faith.
Arguments against transitional fossils as evidence for evolution
Some arguments are dead ends:
The earth isn’t really that old, therefore any dating arguments are obviously invalid
Fossils were placed by the devil to lure us away from proper faith.
Simpson tree of life
ReferencesFreeman 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-bielefeld.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
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 437:137-140.
Clack, J. A. 2002. An early tetrapod from ‘Romer’s Gap’. Nature 418:72-76.
Clack, J. A. et al. 2003. A uniquely specialized ear in a very early tetrapod. Nature 425:65-69.
Coates, M. I. And Clack, J. A. 1991. Fish-like gills and breathing in the earliest known tetrapod. Nature 352:234-236.
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.
A: mechanical support/environment/structures
C: respiratory structuresD: water balance/osmoregulationE: sensory systems: cutaneous/visualF: probable prey-capture and feeding mechanismsG: probable mode of reproduction
B: probable locomotion