A Skull Tooth is a red gem which can be obtained by opening a Chest in Rayman Origins. There are a total of ten skull teeth and the player must give them to the Deadman at the Snoring Tree. Once they have all been collected he'll open the entrance to the Land of the Livid Dead. To unlock a Chest Chase level (where the skull teeth can be found), a certain number of Electoons indicated by the Fire Teensie must be collected.
In such a level, the player must chase the chest throughout the level by sprinting non-stop. At the end of the level, the chest will come to a dead end, allowing the player to smash it open and obtain the tooth inside.
The real size of a Skull Tooth is unknown, as on the Chest Challenges they are big enough to be hugged by four characters, but at the Snoring Tree they seem to have the size of a regular tooth.