Transitional Fossils

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

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 http://www.dinosauria.com/pics/clados/clado.gif

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: • 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)

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

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. Cooksoniapertonii apiculispora (Non-clubmoss vascular plant lineage) Tortilicaulis offaeus (clubmoss lineage) Kendrick and Crane 1997

16. Case study Evolution of Tetrapods

17. lobe-finned fishes ray-finned fishes amphibians mammals reptiles birds Tetrapod evolution Backboned animals with four limbs legs

18. 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

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. Relationships among early tetrapods Long and Gordon. 2004. Physiological and Biochemical Zoology (77).

22. 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

23. 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

24. 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

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. Arguments against transitional fossils as evidence for evolution • Reconstructions are based often on incomplete fossils 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 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

33. 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.

36. 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