he did invent the anbu but then after 3rd hokage took over, he created roots within anbu. The difference is that roots members grow up as roots members, where anbu members are selected from the konoha shinobi.
Im sure remember it clearly, but i cant tell you where it was stated. But its a too minor detail to look it up...
i just pressed google to save some time, i dont know where they got their informations from (must be the manga, same spot as me though

) but here goes:
"The first ANBU Sotaichou was Danzou of Konohagakure. After being put into place by the Shodai Hokage, he created the original ANBU system which most other villages have modeled themselves after"
Thats from Narutofanonwikia.com, maybe im wrong though
