geologic time scale

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Across
  1. 2. a major division of time that is a subdivision of an eon and is itself subdivided into periods.
  2. 4. an expert in or student of geology
  3. 7. a unit of time equal to a thousand million years.
  4. 8. the action or process of classifying something according to shared qualities or characteristics.
  5. 9. an animal of a large group distinguished by the possession of a backbone or spinal column, including mammals, birds, reptiles, amphibians, and fishes.\
Down
  1. 1. the remains or impression of a prehistoric organism preserved in petrified form or as a mold or cast in rock.
  2. 2. a period of time in history or a person's life, typically one marked by notable events or particular characteristics.
  3. 3. a length or portion of time.
  4. 5. an animal lacking a backbone, such as an arthropod, mollusk, annelid, coelenterate, etc. The invertebrates constitute an artificial division of the animal kingdom, comprising 95 percent of animal species and about 30 different phyla.
  5. 6. a supercontinent where all of Earth's continents were joined together in one single landmass, existing about 335 million years ago.