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Earth's History

Across
Based on types of existing life globally at a particular time
Lava that hardens (igneous rock) on the surface
Must be widely distributed and represent a type of organism that existed only briefly in order to help geologists tell the relative ages of the rock layers in which they occur
Information gathered by paleontologists which provides evidence of: history of life on earth, groups of organisms changing over long periods of time, earth's past climate, and past environments and changes in earth's surface
Break in the earth's crust which is always younger than the rock it cuts through
Scientists who study, collect and classify fossils
Extremely thin coating on rock
In horizontal sedimentary rock layers the oldest layer is at the bottom; each higher layer is younger than the layer below it
Well-tested concept that explains a wide range of observations
Age compared with ages of other rocks
Marked by significant worldwide changes in the types of fossils present in rock
All the atoms of a particular type of matter are the same
Age of the invertebrates
Middle era divided into three periods
Divided periods characterized by differences in life forms
Down
A method geologists use to determine the absolute age of rocks, by first determining the amount of a radioactive element in a rock
Preserved remains or traces of living things and are formed when living things die and are buried by sediments
Youngest era divided into two periods
Process by which one unstable element breaks down into another element that is stable
Age of the fishes
Hollow area in the sediment which shows the shape of an organism or part of an organism
Earliest life form to appear; it was a blue-green algae which added oxygen to the atmosphere through photosynthesis
Age of the Amphibians
Solid copy of the shape of an organism
Age of the Reptiles
When an organism no longer exists and never will again
Longest subdivisions; based on the abundance of fossils