The correct answer is - extinction of one localized species.
The extinction of one, localized species has no impact of separating the geologic time, as if that was the case, there were going to be eras, periods, and epochs on every few hours, days, and months.
The geologic time is separated into the eras, periods, and epochs because of periods of major extinctions that took place all over the planet. Also, each of them is defined with its own unique flora and fauna, but also with certain geologic activities and changes in the Earth.
