Transitional Fossils
Evidence for Evolution Seminar
Talline Martins and Heidi Hillhouse
02-14-06
Outline
Introduction to transitional fossils
Case studies
Vascular 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 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).
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 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)
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)
Vascular plant evolution
Two main branches of vascular plant
development
Clubmosses
All other land plants
Kendrick and Crane 1997
Likely sister groups
to land plants
Kenrick and Crane (1997)
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)
Cooksonia pertonii apiculispora
(Non-clubmoss vascular plant
lineage)
Tortilicaulis offaeus
(clubmoss lineage)
Kendrick and Crane 1997
Case study
Evolution of Tetrapods
legs
mammals
birds
reptiles
amphibians
lobe-finned fishes
ray-finned fishes
Tetrapod evolution
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 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
Relationships
among early
tetrapods
Long and Gordon. 2004.
Physiological and Biochemical
Zoology (77).
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
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
Pederpes (354-344 mya)
Found in 1971, but only described in 2002, western Scotland
Limbs: forward-facing hind feet, non-paddlelike, 5 digits!
Skeleton: resembles that of Ichthyostega
Lungs: present/gills
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
Arguments against transitional
fossils as evidence for evolution
Reconstructions are based often on
incomplete fossils
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
References
Freeman and Herron 2001. Evolutionary analysis 2nd edition. PrenticeHall, 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
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