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Evolution in the Fossil Record
Evolution of phenotype
Evolution of diversity
The Earth is Old
The Geological Time Scale is Based Upon the
Principles of Relative Dating
•Younger rocks are deposited on top of older rocks
•Lava and sedimentary rocks were originally laid
down in a horizontal position
•Boulders or cobbles found in a body of rock are
older than their host rock
•Earlier fossil forms are simpler than more recent
forms, and more recent forms are most similar to
existing forms.
Geologic Column
Earth’s history as
told by rock strata
The Geologic Time
Chapter 4, p. 70
Why is the fossil record incomplete?
It takes the right conditions at the right time in the
right place.
- some organisms are simply not likely to yield fossils
- sedimentation does not occur in all habitats
- fossils must survive geological events and weather
the elements for millions of years before discovery
Process of Fossilization
After remains are buried by sediments:
(1) Compression/impression/casts/molds:impressions or casts
made before decomposition (like footprints).
(2) Permineralized fossils: precipitation of minerals in
cells before decomposition.
Or Occasionally:
(1) Unaltered remains: Frozen, amber embedded, peat bogs.
permineralized fossils
Impression fossil
Dinosaur National Park
Fossil Record is Biased
Fossilization is higher for organisms that are:
Durable and likely to be buried in an
anoxic environment (low land or marine habitats)
Also, there is temporal and geographic bias:
Probability that an organism will be fossilized depends
on the geographical area and historical time.
Radiometric Dating
Fossils and associated rocks contain
reactive elements that decay at known rates
• Uranium235 --> Lead235
• Carbon14
Parent Atom
= 7 x 108 years
--> Nitrogen14 = 5730 years
Daughter Atom
Radioisotope Decay
Parent Atoms
Time (half-lives)
1. Find fossil
2. Determine ratio of parent
to daughter atoms
3. Determine number of
elapsed half-lives
4. Estimate age of fossil
Try this problem
You are using C14 (half-life = 5730 years)
radioisotope dating to determine the age of
rocks associated with a fossil. The
percentage of daughter isotope atoms (N14))
is 87.5%. What is the age of the fossil?
17,190 yrs
There are Good Examples of Macroevolution
in the Fossil Record
Sarcopterygii --> Ichthyostega --> Amphibia
- see also (Figure 4.6)
Gradualism : (e.g. limbs
are intermediate)
Mosaic evolution:
(limbs evolved
faster than braincase,
tail fin, tooth structure
4.6(1) Eusthenopteron, a member of the group of lobefinned fishes from which tetrapods arose
4.6(2) Eusthenopteron, a member of the group of lobefinned fishes from which tetrapods arose
Other Examples From Book To Review
(Chapter 4)
• Origin of Birds
– Archaeopteryx is but one of many fossils
• Origin of Mammals
- Gradual evolution of “reptilian” skull and jaw
• Origin of Cetacea
– A series of fossils have recently been discovered showing
adaptation to aquatic life
• Origin of Hominins
– We are the lone survivors of an otherwise
extinct radiation of bipedal African hominids