What information do scientists infer from fossils?
How organisms and environments have changed over time
Why do geologists use the geologic time scale?
Because the time span of Earth's past is so great
What are the three divisions of time between Precambrian Time and the present called?
Eras
What are eras subdivided into?
Periods
How do scientists think that the Earth formed?
They think it began as a ball of dust, rock ,and ice in space that was pulled together by gravity
During which era did reptiles spread widely?
Mesozoic
During which era did mammals evolve to live in many different environments?
Cenozoic
What is a comet?
A ball of dust, gas, and ice that orbits the sun
What is an invertebrate?
An animal without a backbone
What is a vertebrate
An animal with a back bone
What is an amphibian?
A vertebrate that spends part of its life in the water and part of its life on land
What is a reptile?
A vertebrate that has scaly skin and lays eggs with tough, leathery shells
What is a mass extinction?
When many types of living things become extinct at the same time
What is a mammal?
A vertebrate that can control its body temperature and feeds milk to its young
How did scientists develop the geologic time scale?
By studying rock layers and index fossils
What is the first major division of geologic time?
Precambrian time
