Loone goes to the Gerudo Desert to find the third leviathan skeleton.
Quest Location
This quest starts after you complete Hebra's Colossal Fossil.
How to Complete the Quest
The Gerudo Great Skeleton cave is southwest of the Lightning Temple, and west of the Irasak Shrine. Go inside and talk to Loone, who points out the baby leviathan skull nearby. She wants to see the child fossil put back together and placed next to the parent fossil.
Find the Fan that is stuck in some sand, and use Ultrahand to pull it up out of the sand. Then you can hit the fan and put it on the ground facing the piles of sand in this area. One of the sand piles has the child fossil's tail in it.
Then climb on top of the parent fossil's skull and look upward to see many broken rocks in the ceiling, with a piece of a fossil sticking out. Throw bombs at these broken rocks or break them some other way, and then take the small torso fossil that falls out of it.
Attach the small skull to the torso near the front flipper (check the parent fossil to see the correct way), and attach the small tail to the other end of the torso fossil. Then place the small skeleton near Loone. Talk to Loone, and if you assembled the fossil correctly, you will complete the quest.
Quest Reward
Loone gives you 50 rupees.