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Across 5. weathered rock particles 7. sediments that show one year 8. trilobites important fossils in this era 9. the period known as the age of the dinosaurs 17. time periods in Earth's history broken into four segments 18. change over time 19. law that states younger rocks are found on top of older rocks 20. evidence of once living things

Down 1. era known as the period of many changes 2. breakdown of a radioactive element 3. method of determining the age of a fossil 4. the first period in the era known as the age of mammals 6. the period when mammals first appear 10. fossils of organisms that lived during a short period of time 11. period when the ice age begins and ends 12. era known as the dawn of life 13. wide spread flooding and mass extinctions occur 14. mutation that occurs that helps a species to survive 15. subdivisions of time in earth's history 16. era divided into two great periods