How did geologists decide where one division of the geologic time scale ends and the next begins?

the law of superpostion

events in the history of life on earth, like mass extinctions

they divided the time equally between eras

the placement of fossils throughout the surface layers of earth

segmentation=endoskeleton=backbone=coelom=tissues

enhanced global partnerships and exploration capabilities may advance international preparedness for protecting the earth from catastrophic events such as some asteroid strikes, advancing collaborative research on space weather and protecting spacecraft by developing new means for space debris removal.