geologic time scale.

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Across
  1. 4. an animal lacking a backbone, such as an arthropod, mollusk, annelid, coelenterate, etc
  2. 5. an expert in or student of geology.
  3. 7. a massive supercontinent that existed approximately 250 million years ago, composed of all the present-day continents joined together.
  4. 10. the action or process of classifying something according to shared qualities or characteristics.
Down
  1. 1. an animal of a large group distinguished by the possession of a backbone or spinal column, including mammals, birds, reptiles, amphibians, and fishes.
  2. 2. a length or porion of time
  3. 3. a person who studies or is an expert in the branch of science concerned with fossil animals and plants.
  4. 6. a major division of time that is a subdivision of an eon and is itself subdivided into periods.
  5. 8. a period of time in history or a person's life, typically one marked by notable events or particular characteristics.
  6. 9. a unit of time equal to a thousand million years.