When you are in the temple place by the big Medicine Man tree, work your way through the cave. After getting past the big caterpillar, jump onto the swinging platform and jump to the ledge. Then don't get on the side-ways moving platform, just walk straight off to the left. Once you fall down, keep walking left. The bone is in the jaws of the skull. It is holing the jaw open.
In the Temple
You get the translation paper and go to the cave its on the top left of the cave.
Mask? What mask? There is no mask that you have to find in Shark Tooth Island, Poptropica.
You'll find the bone to the left of the golden statue with the centipede. Underneath the moving platform.
The bone is down in the underground ruins. You will find it at the bottom of the hole with spikes on its sides, just left of the statue with the giant centipede on it.
inside the temple..
The bone is in a tunnel deep in the underground ruins. You will find it when moving left from the door with symbols on it. (see related question)
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Climb back up past the worm statue, go left to find the chamber with the final (key) ingredient,
You have the medicine man mix a sleeping potion, using coconut milk, a creature bone, and a key ingredient. The bone and green ingredient are down in the underground ruins. Once you have it mixed, you fire it from the shark feeding Coconut Cannon at Booga bay. (see related questions)
The green "key ingredient" on Shark Tooth Island is part of the potion to put the giant shark (Booga) to sleep. It is down in the underground ruins, along with another ingredient, the creature bone. Open the underground door and go left to find the bone and then the key ingredient. (You already have the third ingredient, which is coconut milk.) (see related question)