You discover tetrapod fossils in layers 3, 4, 5, and 6 You discover tetrapod fossils in layers 3, 4, 5, and 6. Which fossil is the oldest? The youngest? How do you know?

Carbon 14 Normal Carbon Radioactive Carbon Highly unstable Decays to N14

Carbon-14 decays to Nitrogen-14

The time it takes for half of a radioactive element to decay Half-Life The time it takes for half of a radioactive element to decay The time it takes for a radioactive element to fall to half of its original value 50% carbon-14 50% nitrogen-14

Carbon-14 Decays to Nitrogen-14 Half-life- 5730 years

Some Carbon-14 is always present in the atmosphere around us Some Carbon-14 is always present in the atmosphere around us. This gets incorporated into our bodies. When we die, it stays there.

Carbon-14 Decays to Nitrogen-14 Half-life- 5730 years Remains in organisms after they die Scientists can test fossils to see how much C- 14 and N-14 they contain to determine the fossils age Only useful on fossils between 100-50,000 years old (really only used up to 30,000)

Uranium Uranium 238 decays to Lead 206 Half life = 4.5 billion years Half life = 710 million years Combined, can be used to date from 10 million-4.6 billion years Only used on rock!