Complex sentence: Although anoxic events have not happened for millions of years, the geological record shows that they happened many times in the past, and may have caused mass extinctions--including some of those which geobiologists employ to serve as a time marker in biostratigraphic dating.
Split into simpler sentences: Although anoxic events have not happened for millions of years, the geological record shows that they happened many times in the past. Anoxic events may have caused mass extinctions--including some of those which geobiologists employ to serve as a time marker in biostratigraphic dating.