Fossil Fragments are Don't Starve Together exclusive items, introduced in A New Reign. They are dropped by Stalagmites at a 10% rate and by Spilagmites at a 100% rate. One of their uses is to repair an odd skeleton. The other use is for blocking mobs, similar to walls except fossils are unbreakable by them.
In order to build the Odd Skeleton, players have to place a Fossil Fragment on the ground and then repair it using additional Fossil Fragments similar to repairing Walls. The built pile can be Hammered down to yield every single Fossil Fragments used on it, regardless of its size.
There are three Odd Skeleton designs: a complete, correct form and two incorrect forms. It takes eight Fossil Fragments to completely assemble an Odd Skeleton, but the form (Correct, Wrong 1 or Wrong 2) will already be decided when the player places the fifth Fossil Fragments on the Odd Skeleton, at which point the player may want to Hammer it down to start building another one faster if the form is incorrect. It has a 60% chance to become the correct form, and a 20% chance to become one of the two wrong variations each. Only the correct Odd Skeleton form can be resurrected into the Reanimated Skeleton or Ancient Fuelweaver by using a Shadow Atrium.