The Ghosts of Deep Time
by Frank Landis
This is the big secret: time travel is easy, and people live throughout all of deep time's eras. Of course they live without a trace, for that is the law of deep time. To do otherwise could create paradoxes, bifurcating histories, even time wars and mass extinctions.
Where there is law, there is crime. When crimes span millions of years, law enforcement takes a special kind of officer.