Earth and Space Science / Unit 5: Weathering, Erosion and Earth's History
5.7 Fossils and the Geologic Time Scale
Notes
Fossils are preserved remains or traces of ancient life, mostly in sedimentary rock.
- Types: body fossils (bone, shell), molds and casts, petrified wood, trace fossils (footprints), amber.
- Fossils show past environments: marine shells in a desert mean an ancient sea.
- The geologic time scale divides Earth's 4.6-billion-year history into eons, eras and periods.
- Eras: Paleozoic (old life, 541-252 Ma), Mesozoic (age of dinosaurs, 252-66 Ma), Cenozoic (mammals, 66 Ma-present). Precambrian is about 88% of time.
- Mass extinctions mark boundaries, such as the end-Cretaceous 66 million years ago.
- Earth is about 4.6 billion years old.
Practice
Which era is called the age of dinosaurs?
Dinosaurs dominated the Mesozoic.
Shell fossils found high in a desert mountain suggest the area was once:
Marine organisms lived in water.
Earth's approximate age is:
Based on radiometric dating of meteorites and old rocks.
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