Trilobites were rapidly evolving and widespread marine organisms that spanned a number of geologic time periods. Certain types of trilobites only appeared in specific segments of geologic time; therefore, when one of these trilobites is found, you can be certain that the rock layer it is found in is of a certain geologic age.
People from Asia crossed berengia, a land bridge going into North America, and followed herds of animals since they were hunter/gatherers.
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