Fossils• Remains of organisms (often partially or wholly replaced by

“rock”)• Bones, teeth, shells,• hard exoskeletons• Usually in sedimentary rocks (water carries and drops small

particles that later formed layered rock) or in volcanic ash sediments

Fossil primates rareHabitat

Dating fossilsRelative dating

• Stratigraphic Correlation – matching strata from different locations based on chemical similarities between layers

• E.g., fluorine dating – vertebrate bones/teeth in fluorine containing soils accumulate fluorine over time. More fluorine = older.

• Law of Superposition• Fossils in a lower stratum (layer) of rock are

older than those above it.• RELATIVE AGE - older younger

Relative Dating

• Cosmic collisions• Evidence of a significant impact, reported in early

1980s• Chemical evidence: iridium

Cretaceous / Tertiary boundary

95 localities Iridium: rare on earthcommon in asteroids& meteorites

Geological evidence: craterAsteroid 10-15 km dia65 mya

Shift from Cretaceous to TertiaryEpochs marked by dinosaur extinctionSignificance to us?

Absolute dating fossils• Radiometric dating• based on rates of radioactive decay• Natural conversion of an element into a different form of

that element (isotope)• 1. Rates constant and independent of environmental

factors• 2. Rates of decay are known• 3. Amount of decay from a parent element into a

daughter isotope = a geologic clock.

1. Determine ratio of parent isotope to daughter isotope.2. Convert ratio to number of half- lives elapsed. 3. Multiply number of elapsed half- lives X number of years it takes for a half-life to elapse 4. This is the age estimate of that rock.

Radiometric Dating

Potassium/Argon clock

• Decay of 40K (potassium) produces 40Ar (argon gas)

• Potassium bearing rocks.

• Igneous rock: heat drives off previously accumulated Argon gas

• Sets the “clock” to zero

• As rock cools and solidifies, 40K continues to decay to 40Ar which is trapped inside the rock.

• To date the rock, it is reheated and the amount of 40Ar given off is measured.

• The ratio of 40K to 40Ar permits dating the rock.

• 40K has a half-life of 1.3 billion years.

• In 1.3 billion years, 1/2 of the original 40K will have been converted to 40Ar

• In 2.6 billion years, 1/4 of the original 40K will remain.

• Radiocarbon Dating: 14C 14N

• 14C has a half life of 5,730 years

• Clock set to zero by death of the individual.

• Examining carbon-14 to nitrogen-14 ratio in organic remains.

• Only if organism-derived carbon is present and only back to 50,000 - 75,000 yBP)

Fossils found in sedimentary rocks, so fossils usually have relative dates

Absolute dating

Dendrochronology – matching tree ring width variationOnly where trees were presentSeasonal climateDates back to 8,000 - 12,000 yBP)

Dating estimates from genetic dataAssumption: average mutation rate is constant.

pod dinosaurs

A bird and some dinosaur relatives