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 ColumnEarth’s history astold by rock strata

The Geologic Time Scale

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

(1) Compression/impression/casts/molds:impressions or casts made before decomposition (like footprints).

(2) Permineralized fossils: precipitation of minerals incells before decomposition.

Or Occasionally:

(1) Unaltered remains: Frozen, amber embedded, peat bogs.

After remains are buried by sediments:

amber

cast

Impression fossil

permineralized fossils

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 dependson the geographical area and historical time.

Fossils and associated rocks containreactive elements that decay at known rates

• Uranium235 --> Lead235 = 7 x 108 years

• Carbon14 --> Nitrogen14 = 5730 years

Parent Atom Daughter Atom

Half-life

Radiometric Dating

Time (half-lives)

SurvivingParent Atoms

AccumulatingDaughter

Atoms

1 2 3

1/1

1/3

1/7

Radioisotope Decay

1. Find fossil2. Determine ratio of parent to daughter atoms3. Determine number of elapsed half-lives4. Estimate age of fossil

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?

Try this problem

17,190 yrs

There are Good Examples of Macroevolution in the Fossil Record

- see also (Figure 4.6)

Gradualism : (e.g. limbs are intermediate)

Mosaic evolution: (limbs evolvedfaster than braincase, tail fin, tooth structure

Sarcopterygii --> Ichthyostega --> Amphibia

4.6(1) Eusthenopteron, a member of the group of lobe-finned fishes from which tetrapods arose

4.6(2) Eusthenopteron, a member of the group of lobe-finned 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