Transitional Fossils

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)


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)


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


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


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




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.


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.

http://www.primitivefish.com/Marbled%20African%20Lungfish.htm

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

Documents

Transitional Fossils