The ________ period was a time of major extinctions, including 75 percent of amphibian families.
Permian
During the early Paleozoic era, South America, Africa, Australia, Antarctica, India, and perhaps China comprised the vast southern continent of ________.
Gondwanaland
The most common Precambrian fossils are ________, layered mounds of calcium carbonate.
Stromatolites
During the ________ era, the westward-moving North American plate began to override the Pacific plate, eventually causing the tectonic activity that ultimately formed the mountains of western North America.
Mesozoic
Earth's primitive atmosphere evolved from gases ________.
expelled from within the earth
Mammals became the dominant land animals during the ________ era.
Cenozoic
Which of the following gases was NOT part of Earth's original atmosphere?
oxygen
In the early history of the earth, the solid surface of the earth weathered more rapidly than now because ________.
the early rain was very acidic
The major source of free oxygen in the atmosphere is from ________
green plants
One important reason we do not know much about the geologic history of the Precambrian is that ________.
there are very few fossils preserved in Precambrian rocks