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Chapter 23

Across
Microscopic organisms that are the basis of marine food chains; abundant during the Cretaceous and the remains of their shell-like hard parts are found in chalk deposits worldwide
The ancient geographic setting of an area
Sudden appearance of a diverse collection of organisms in the Cambrian fossil record
Egg with a shell, providing a complete environment for a developing embryo
Down
Edge of a continent along which there is no tectonic activity
Occurs when sea level rises and causes the shoreline to move inland, resulting in deeper-water deposits overlying shallower-water deposits
Species to which modern humans belong
Occurs when sea level falls, causing the shoreline to move seaward, and results in shallower-water deposits overlying deeper-water deposits
Walking upright on two legs
Metal that is rare in rocks at Earth's surface but is relatively common in asteroids