AnthropologyGeorgia Perimeter College

Studying the Past

- The evidence or remains of a once-living plant or animal.

A. To provide evidence of the past existence of life forms.

B. To provide information about past environmental conditions.

C. To provide evidence that populations have undergone change over time due to environmental changes (evolution).

II. Why Do We Study Fossils Found in Rocks?

I. What is a Fossil?

http://www.youtube.com/watch?v=TVwPLWOo9TE

• Taphonomy: The study of what happens to remains from to discovery.

• Stratigraphy: The study of the order of rock layers and the sequence of events they reflect.

• Geological Time Scale: Categories of time into which Earth’s history is usually divided.

Some Vocabulary Terms

Types of Fossils

A. Original Preservation1. Plant or animal remains that have

not undergone change since death.a. Examples:

• Mummified humans• Frozen organisms (Ice Man)• Mammoths & cats in La Brea Tar

Pits• Fossilized insects in tree sap

(amber)Otzi the Iceman was found in 1991 in the Otztal Alps between Austria and Italy and his remains date to 5,300 years ago.http://en.wikipedia.org/wiki/%C3%96tzi_the_Iceman

Bog People of Europe (up to 8,000 bp; http://cogitz.com/2009/09/04/bog-bodies-preserved-corpses/

La Brea tar pits near Los Angeles (up to 38,000 bp;

http://www.youtube.com/watch?v=teh90FTIKec

The Windover Bog People of Florida (up to 7,500 bp)

Bog Fossils

B. Altered Hard Parts1. All organic material has been

removed and the hard parts of the organism have been changed.a. Minerals seep in slowly and

replace the original organic tissue, forming a rock-like fossil.

b. The fossil has the same shape as the original object, but is chemically more like a rock!

c. Examples: • Petrified wood• Recrystallized shellsThe Taung Child:

Australopithecus africanus. South Africa 2.5 mya. Discovered in 1924 by Raymond Dart

C. Index Fossils – Guide Fossils1. Remains of unique species that

can be used to correlate rock layers or to date a particular rock layer.a. Must be easily recognized,

abundant, and widely distributed geographically.

b. Must have lived during a relatively short time period.

Jurassic Ammonite

D. Molds and Casts1. Fossils that do not contain

any shell or bon.ea. A mold is formed when

original shell parts are weathered and eroded, leaving an impression of the shell.

b. This cavity might later become filled with minerals or sediment to create a cast.

c. Examples: • Common with shellfish

Achaeopteryx

E. Trace Fossils1. Indirect evidence of plant and

animal lifea. Provide information about how an

organism lived, moved or obtained food

b. Examples: • worm trails• burrows• footprints

Laetoli footprints. Oldest known footprints of human ancestors (Australopithicus afarenses) dates to 3.6 mya. Found in Tanzania in 1976.

Dating Fossils

A. Relative-Age Dating: 1. Dating rocks and fossils by

placing them in chronological order without exact dates.

2. Geologic Principles (used in this dating process):a. Original Horizontality

• Sedimentary rocks are deposited in horizontal layers.

b. The Law of Superposition • in an undisturbed sequence

the oldest rocks are at the bottom and each successive layer is younger.

c. Principle of Cross-Cutting Relationships: • an intrusion or a fault is

younger than the rock it cuts across.

• Lithostratigraphy: The study of geologic deposits and their formation, stratigraphic relationships, and relative time relationships based on rock properties.

• Tephrostratigraphy: A form of lithostratigraphy in which the chemical fingerprint of volcanic ash is used to correlate across regions.

• Biostratigraphy: Comparison of fossils from different stratigraphic sequences to estimate which layers are older and which are younger.

Relative Dating Methods

Methods cont.:b. Radioactive Dating: Dating

fossils based on the amount of radioactive material remaining in a substance over time:• Radioactive substances (unstable atoms) emit

protons and neutrons at a constant rate.• The original element (parent) is converted to

a different element (daughter).• Since the rate of decay is constant, you can

measure the parent to daughter ratio to determine the age of the rock.

• The length of time it takes for one-half of the original amount to decay is called the elements half-life.

• Isotopes: Variant forms of an element that differ based on their atomic weights and numbers.

• Half-life: The time it takes for half of the original amount of an unstable isotope of an element to decay into more stable forms.

• Potassium-argon (K-AR) dating: Measures the decay of 40K to 40Ar in potassium-bearing rocks. Estimates the ages of the sediments in which fossils are found.

Radiometric Dating - Terms

Radiometric Dating - Terms

Argon/argon dating (40Ar/39Ar): Allows measurement of a smaller sample than K-AR dating.

Fission-Track dating: Dates noncrystaline matierials using the decay or 238Ur and counting the tracks that are produced by this fission.

B. Absolute-Age Dating: 1. Dating rocks and fossils by using

techniques to determine their actual age.

2. Methods:a. Tree Rings and Seasonal Climatic

Changes• Each tree ring represents 1 year

of growth• Varves are bands of sediment

that show a yearly cycle from climate change

• Although accurate, neither method can be used to date very far back in time.

Absolute Dating – RadioCarbon Dating

http://www.universetoday.com/74125/carbon-dating/

http://www.youtube.com/watch?v=8wYvKeSK1IY&feature=relmfu

http://www.youtube.com/watch?v=ErgdpG_N9vQ&feature=related

http://www.youtube.com/watch?v=2Ipa9uhyRoU&feature=related

Thermoluminescence Dating

http://www.youtube.com/watch?v=T6NxycC7Lco

Thermoluminescence dating is the use of heat on archeological or geological samples to produce a light signal that is proportional to an accumulated radiation dose. It is used to date rocks, minerals and ceramics for dates between approximately 300 to 10,000 B.P. (before present). It is usually used in conjunction with other methods of historical dating, such as carbon 14 or stratigraphy (Ref:

http://www.ehow.com/about_5078889_thermoluminescence-dating.html)

Electron Spin Resonance (ESR) Datinghttp://www.youtube.com/watch?v=KOnodnj8a_8

http://www.youtube.com/watch?v=1920gi3swe4&feature=related

http://www.youtube.com/watch?v=EiYYi-QGddE&feature=related

Uranium methods