Geologic Time Crossword

"Ancient" time period
"Middle" time period
"Present" time period
Determines the age of a rock or fossil by determining the ration of parent to daughter nuclei
Determines the age of organic objects that were once alive
Establishing the order of past geographic events
Indirect fossil evidence,like footprints,of an organism
Large mat or mound composed of billions of photosynthetic bacteria that dominated shallow oceans
Longest time in the geologic time scale
Method that enables scientists to determine the actual age of rocks
Microscopic,photosynthetic prokaryotes
Occurs when an unusually large number of organisms disappear from the rock record at the same time
Remains of plants or animals that were very abundant and existed briefly
Second longest time in the geologic time scale
Sudden appearance of a diverse collection of organisms in the Cambrian fossil record
The period of time it takes for one half of a radioactive istope to decay
Third longest time unit in the geologic time scale
Time unit in the geologic time scale smaller than a period
Unit of geologic time when Earth formed and became hospitable for life