skulls lists for halo anniversary not all though

Grunt Funeral skull

Once the skull has been activated, every Grunt's methane tank will explode similar to a plasma grenade when killed. This skull will only be available to those who pre-order the game.

Malfunction skull

Once the skull is activated, one random part of the HUD will disappear at every respawn. 

Grunt Birthday skull

Once the skull is activated, the Unggoy's heads will explode in a shower of confetti with a headshot.
Boom skull Once the skull is activated, explosion radius will increase two-fold.

Like in Halo 3, they must be hunted down within the levels to gain access to them. The skulls and the effects will not be visible in Classic Mode.