Where are the Shards of Mehrune's Razor?
This seems to be a known bug.
http://en.uesp.net/wiki/Skyrim:Pieces_of_the_Past
Should you discover and clear one of the locations, and kill the "boss" prior to receiving a Museum Pamphlet, the quest will become impossible to complete.
However, you may be able to respawn the whole area. Go away from Cracked Tusk, rest for 30 days, and come back. This should respawn the boss.