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Transitional Fossils Evidence for Evolution Seminar Talline Martins and Heidi Hillhouse 02-14-06.

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Presentation on theme: "Transitional Fossils Evidence for Evolution Seminar Talline Martins and Heidi Hillhouse 02-14-06."— Presentation transcript:

1 Transitional Fossils Evidence for Evolution Seminar Talline Martins and Heidi Hillhouse

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

3 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)

4 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).

5 Dinosauria cladogram

6 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

7 Case study Evolution of vascular plants

8 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)

9 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)

10 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)

11 Vascular plant evolution Two main branches of vascular plant development Clubmosses All other land plants

12 Kendrick and Crane 1997

13 Kenrick and Crane (1997) Likely sister groups to land plants

14 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)

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

16 Case study Evolution of Tetrapods

17 Tetrapod evolution ray-finned fishes lobe-finned fishesamphibiansreptilesbirds mammals legs Backboned animals with four limbs

18 How did it happen? Paleozoic Era: late Devonian-early Carboniferous (~ mya) Crossopterygians (bony fish) Sarcopterygian fishes (lobe-finned) Coelacanth or lung fishes? Where? Shallow fresh water, tropical Euramerica

19 Tetrapod evolution ?

20 Some structures associated with tetrapod evolution Limbs and digits Ability to move on terrestrial environment Interlocking vertebrae/connecting ribs Provide support on land—necessary due to gravity Ear ability to hear on land Lungs Breathing

21 Long and Gordon Physiological and Biochemical Zoology (77). Relationships among early tetrapods

22 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

23 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

24 Pederpes ( 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

25 “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!

26 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

27 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

28 Reconstructions are based often on incomplete fossils Arguments against transitional fossils as evidence for evolution Ahlberg et al. (2005) Nature

29 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.

30 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.

31 Simpson tree of life

32 References Freeman and Herron Evolutionary analysis 2 nd edition. Prentice- Hall, New Jersey. Kenrick, P. and P. Crane, The origin and early evolution of plants on land. Nature. 389: Raven, J. A The evolution of vascular land plants in relation to supracellular transport processes. Pp 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 When and How did plants and animals take to the land? Paleobiology 10(1): 1-8 The Virtual Fossil Museum

33 References (cont.) Ahlberg, P. E. and Milner, A. R The origin and early diversification of tetrapods. Nature 368: Ahlberg, P. E., Clack, J. A., and Blom, H The axial skeleton of the Devonian tetrapod Ichthyostega. Nature 437: Clack, J. A An early tetrapod from ‘Romer’s Gap’. Nature 418: Clack, J. A. et al A uniquely specialized ear in a very early tetrapod. Nature 425: Coates, M. I. And Clack, J. A Fish-like gills and breathing in the earliest known tetrapod. Nature 352: Long, J.A. and Gordon, M. S The greatest step in vertebrate history: a paleobiological review of the fish-tetrapod transition. Physiological and Biochemical Zoology 77(5):

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


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