index fossil A fossil that identifies and shows the relative age of the rocks in which it is
found. Index fossils come from species that were widespread but existed for a relatively
brief period of time.
living fossil A modern species or genus that has existed on Earth for millions of years
without changing very much.
marine Of or belonging to the sea.
mass extinction A period of time when an unusually high number of species became
extinct.
microfossil A fossil that must be studied with the aid of a microscope.
mold An impression made in sediments by the hard parts of an organism.
permineralization A type of fossilization in which minerals are deposited into the pores
of the original hard parts of an organism.
terrestrial Of or belonging to the land.
trace fossil Evidence of the activity of an ancient organism. Examples include tracks,
trails, burrows, tubes, boreholes, and bite marks.
Points to Consider
- What are some other examples of mythical creatures that may be based on fossils?
- Why is it so rare for an animal to be preserved as a fossil?
- Some organisms are more easily preserved than others. Why is this a problem for
scientists who are studying ancient ecosystems? - Why are examples of amazing fossil preservation so valuable for scientists?
- Many fossils of marine organisms have been found in the middle of continents, far from
any ocean. What conclusion can you draw from this?
11.2 Relative Ages of Rocks
Lesson Objectives
- Explain Steno’s laws of superposition and original horizontality.
- Based on a geological cross-section, identify the oldest and youngest formations.
- Explain what an unconformity represents.
- Use fossils to correlate rock layers.