Presumably from battle. I thought that I heard somewhere that he had them sliced off in an energy sword duel. I would answer this with my account, but I forgot my password and I'm too lazy to get it back right now. But R'tas likely has it from a battle, seeing as how they are severed rather thank missing like they would be if it were a genetic accident or disorder.
Actually, he received his battle mark after fighting a fellow comrade turned by the flood.