Why didn't the children of the Rikudo Sennin inherit his surname?


As revealed in Chapter 646, the surname from Rikudo Sennin (Sage of the Six
Paths) is "Oosutsuki."

Why do his two sons have different surnames (Uchiha and Senju), instead of
inheriting their father's surname?




Taken from [the _Naruto_
Wiki](http://naruto.wikia.com/wiki/Sage_of_the_Six_Paths):

> His bloodline was divided into the descendants of his two children, which
> would eventually form the Senju clan — and through distant relations the
> Uzumaki clan — and the Uchiha clan.

