Bruce R. McConkie teaches that Adam stands as the head of the priesthood (under Christ) as the presiding High Priest.
"Melchizedek Priesthood," in Bruce R. McConkie, Mormon Doctrine (Salt Lake City: Bookcraft, 1958), 433
As pertaining to mortality, the priesthood was first given to Adam. (Moses 6:67-68.) He stands at the head as the presiding high priest (under Christ) over all the earth for all ages. (Teachings, pp. 157-158.)