Mystic Bone Art allowed Mammoth Clansmen to transform into Pranic Beast that they had fused with at the Body Stage. This was their genetic capability. Even children had this capability. They only had to enter the Body Stage to gain access to the power.
When a Mammoth Clansman transforms into a Pranic Beast, their actual bodies will be housed in a space formed within their Spirit Containers, similar to a biome. Though connected, both their Mammoth Clansman bodies and Pranic Beast bodies were different.
The principle of conservation of mass applied to even Sumatra. When Mammoth Clansmen transformed, they were employing the power of an Empyrean Tusk's biome, which was an effect derived from Internal Inertial Gravity.