Fossil Fragments are Don't Starve Together exclusive items, introduced in A New Reign. They are dropped by Stalagmites at 10% rate and by Spilagmites at 100% rate. Their only use is to make an Odd Skeleton.
In order to build the Odd Skeleton, one must keep repairing the pile of fossils first placed on ground much like repairing Walls. It can be hammered down to yield every single fossil used on it, regardless of its size.
There are currently three Odd Skeleton designs: a complete, correct form and two messed up structures. The form will be set when character repairing the fifth fossil on Odd Skeleton. It has 60% probability to become correct form, while 20% probability to become 2 wrong form each. Only the correct Odd Skeleton can be resurrected into the Reanimated Skeleton or Ancient Fuelweaver by using a Shadow Atrium.