Merriam-Webster Collegiate Dictionary - proterozoic

 adjective  Etymology: Greek proteros former, earlier (from pro before) + International Scientific Vocabulary -zoic — more at for  Date: 1899 of, relating to, or being the eon of geologic time or the corresponding segment of rocks that includes the interval between the Archean and Phanerozoic eons, exceeds in length all of subsequent geologic time, and is marked by rocks that contain fossils indicating the first appearance of eukaryotic organisms (as algae) — see geologic time table  • ~ noun